Internet DRAFT - draft-ietf-cbor-7049bis
draft-ietf-cbor-7049bis
Network Working Group C. Bormann
Internet-Draft Universitaet Bremen TZI
Obsoletes: 7049 (if approved) P. Hoffman
Intended status: Standards Track ICANN
Expires: 3 April 2021 30 September 2020
Concise Binary Object Representation (CBOR)
draft-ietf-cbor-7049bis-16
Abstract
The Concise Binary Object Representation (CBOR) is a data format
whose design goals include the possibility of extremely small code
size, fairly small message size, and extensibility without the need
for version negotiation. These design goals make it different from
earlier binary serializations such as ASN.1 and MessagePack.
This document is a revised edition of RFC 7049, with editorial
improvements, added detail, and fixed errata. This revision formally
obsoletes RFC 7049, while keeping full compatibility of the
interchange format from RFC 7049. It does not create a new version
of the format.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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This Internet-Draft will expire on 3 April 2021.
Copyright Notice
Copyright (c) 2020 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Objectives . . . . . . . . . . . . . . . . . . . . . . . 4
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 6
2. CBOR Data Models . . . . . . . . . . . . . . . . . . . . . . 8
2.1. Extended Generic Data Models . . . . . . . . . . . . . . 9
2.2. Specific Data Models . . . . . . . . . . . . . . . . . . 9
3. Specification of the CBOR Encoding . . . . . . . . . . . . . 10
3.1. Major Types . . . . . . . . . . . . . . . . . . . . . . . 11
3.2. Indefinite Lengths for Some Major Types . . . . . . . . . 14
3.2.1. The "break" Stop Code . . . . . . . . . . . . . . . . 14
3.2.2. Indefinite-Length Arrays and Maps . . . . . . . . . . 14
3.2.3. Indefinite-Length Byte Strings and Text Strings . . . 16
3.2.4. Summary of indefinite-length use of major types . . . 17
3.3. Floating-Point Numbers and Values with No Content . . . . 18
3.4. Tagging of Items . . . . . . . . . . . . . . . . . . . . 20
3.4.1. Standard Date/Time String . . . . . . . . . . . . . . 23
3.4.2. Epoch-based Date/Time . . . . . . . . . . . . . . . . 23
3.4.3. Bignums . . . . . . . . . . . . . . . . . . . . . . . 24
3.4.4. Decimal Fractions and Bigfloats . . . . . . . . . . . 25
3.4.5. Content Hints . . . . . . . . . . . . . . . . . . . . 26
3.4.5.1. Encoded CBOR Data Item . . . . . . . . . . . . . 27
3.4.5.2. Expected Later Encoding for CBOR-to-JSON
Converters . . . . . . . . . . . . . . . . . . . . 27
3.4.5.3. Encoded Text . . . . . . . . . . . . . . . . . . 28
3.4.6. Self-Described CBOR . . . . . . . . . . . . . . . . . 29
4. Serialization Considerations . . . . . . . . . . . . . . . . 29
4.1. Preferred Serialization . . . . . . . . . . . . . . . . . 29
4.2. Deterministically Encoded CBOR . . . . . . . . . . . . . 31
4.2.1. Core Deterministic Encoding Requirements . . . . . . 31
4.2.2. Additional Deterministic Encoding Considerations . . 32
4.2.3. Length-first Map Key Ordering . . . . . . . . . . . . 34
5. Creating CBOR-Based Protocols . . . . . . . . . . . . . . . . 35
5.1. CBOR in Streaming Applications . . . . . . . . . . . . . 35
5.2. Generic Encoders and Decoders . . . . . . . . . . . . . . 36
5.3. Validity of Items . . . . . . . . . . . . . . . . . . . . 37
5.3.1. Basic validity . . . . . . . . . . . . . . . . . . . 37
5.3.2. Tag validity . . . . . . . . . . . . . . . . . . . . 37
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5.4. Validity and Evolution . . . . . . . . . . . . . . . . . 38
5.5. Numbers . . . . . . . . . . . . . . . . . . . . . . . . . 39
5.6. Specifying Keys for Maps . . . . . . . . . . . . . . . . 40
5.6.1. Equivalence of Keys . . . . . . . . . . . . . . . . . 42
5.7. Undefined Values . . . . . . . . . . . . . . . . . . . . 43
6. Converting Data between CBOR and JSON . . . . . . . . . . . . 43
6.1. Converting from CBOR to JSON . . . . . . . . . . . . . . 43
6.2. Converting from JSON to CBOR . . . . . . . . . . . . . . 44
7. Future Evolution of CBOR . . . . . . . . . . . . . . . . . . 46
7.1. Extension Points . . . . . . . . . . . . . . . . . . . . 46
7.2. Curating the Additional Information Space . . . . . . . . 47
8. Diagnostic Notation . . . . . . . . . . . . . . . . . . . . . 47
8.1. Encoding Indicators . . . . . . . . . . . . . . . . . . . 49
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 49
9.1. Simple Values Registry . . . . . . . . . . . . . . . . . 50
9.2. Tags Registry . . . . . . . . . . . . . . . . . . . . . . 50
9.3. Media Type ("MIME Type") . . . . . . . . . . . . . . . . 51
9.4. CoAP Content-Format . . . . . . . . . . . . . . . . . . . 51
9.5. The +cbor Structured Syntax Suffix Registration . . . . . 52
10. Security Considerations . . . . . . . . . . . . . . . . . . . 53
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 55
11.1. Normative References . . . . . . . . . . . . . . . . . . 55
11.2. Informative References . . . . . . . . . . . . . . . . . 57
Appendix A. Examples of Encoded CBOR Data Items . . . . . . . . 59
Appendix B. Jump Table for Initial Byte . . . . . . . . . . . . 63
Appendix C. Pseudocode . . . . . . . . . . . . . . . . . . . . . 66
Appendix D. Half-Precision . . . . . . . . . . . . . . . . . . . 69
Appendix E. Comparison of Other Binary Formats to CBOR's Design
Objectives . . . . . . . . . . . . . . . . . . . . . . . 70
E.1. ASN.1 DER, BER, and PER . . . . . . . . . . . . . . . . . 71
E.2. MessagePack . . . . . . . . . . . . . . . . . . . . . . . 71
E.3. BSON . . . . . . . . . . . . . . . . . . . . . . . . . . 72
E.4. MSDTP: RFC 713 . . . . . . . . . . . . . . . . . . . . . 72
E.5. Conciseness on the Wire . . . . . . . . . . . . . . . . . 72
Appendix F. Well-formedness errors and examples . . . . . . . . 73
F.1. Examples for CBOR data items that are not well-formed . . 74
Appendix G. Changes from RFC 7049 . . . . . . . . . . . . . . . 76
G.1. Errata processing, clerical changes . . . . . . . . . . . 76
G.2. Changes in IANA considerations . . . . . . . . . . . . . 77
G.3. Changes in suggestions and other informational
components . . . . . . . . . . . . . . . . . . . . . . . 77
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 79
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 79
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1. Introduction
There are hundreds of standardized formats for binary representation
of structured data (also known as binary serialization formats). Of
those, some are for specific domains of information, while others are
generalized for arbitrary data. In the IETF, probably the best-known
formats in the latter category are ASN.1's BER and DER [ASN.1].
The format defined here follows some specific design goals that are
not well met by current formats. The underlying data model is an
extended version of the JSON data model [RFC8259]. It is important
to note that this is not a proposal that the grammar in RFC 8259 be
extended in general, since doing so would cause a significant
backwards incompatibility with already deployed JSON documents.
Instead, this document simply defines its own data model that starts
from JSON.
Appendix E lists some existing binary formats and discusses how well
they do or do not fit the design objectives of the Concise Binary
Object Representation (CBOR).
This document is a revised edition of [RFC7049], with editorial
improvements, added detail, and fixed errata. This revision formally
obsoletes RFC 7049, while keeping full compatibility of the
interchange format from RFC 7049. It does not create a new version
of the format.
1.1. Objectives
The objectives of CBOR, roughly in decreasing order of importance,
are:
1. The representation must be able to unambiguously encode most
common data formats used in Internet standards.
* It must represent a reasonable set of basic data types and
structures using binary encoding. "Reasonable" here is
largely influenced by the capabilities of JSON, with the major
addition of binary byte strings. The structures supported are
limited to arrays and trees; loops and lattice-style graphs
are not supported.
* There is no requirement that all data formats be uniquely
encoded; that is, it is acceptable that the number "7" might
be encoded in multiple different ways.
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2. The code for an encoder or decoder must be able to be compact in
order to support systems with very limited memory, processor
power, and instruction sets.
* An encoder and a decoder need to be implementable in a very
small amount of code (for example, in class 1 constrained
nodes as defined in [RFC7228]).
* The format should use contemporary machine representations of
data (for example, not requiring binary-to-decimal
conversion).
3. Data must be able to be decoded without a schema description.
* Similar to JSON, encoded data should be self-describing so
that a generic decoder can be written.
4. The serialization must be reasonably compact, but data
compactness is secondary to code compactness for the encoder and
decoder.
* "Reasonable" here is bounded by JSON as an upper bound in
size, and by the implementation complexity limiting how much
effort can go into achieving that compactness. Using either
general compression schemes or extensive bit-fiddling violates
the complexity goals.
5. The format must be applicable to both constrained nodes and high-
volume applications.
* This means it must be reasonably frugal in CPU usage for both
encoding and decoding. This is relevant both for constrained
nodes and for potential usage in applications with a very high
volume of data.
6. The format must support all JSON data types for conversion to and
from JSON.
* It must support a reasonable level of conversion as long as
the data represented is within the capabilities of JSON. It
must be possible to define a unidirectional mapping towards
JSON for all types of data.
7. The format must be extensible, and the extended data must be
decodable by earlier decoders.
* The format is designed for decades of use.
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* The format must support a form of extensibility that allows
fallback so that a decoder that does not understand an
extension can still decode the message.
* The format must be able to be extended in the future by later
IETF standards.
1.2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
The term "byte" is used in its now-customary sense as a synonym for
"octet". All multi-byte values are encoded in network byte order
(that is, most significant byte first, also known as "big-endian").
This specification makes use of the following terminology:
Data item: A single piece of CBOR data. The structure of a data
item may contain zero, one, or more nested data items. The term
is used both for the data item in representation format and for
the abstract idea that can be derived from that by a decoder; the
former can be addressed specifically by using "encoded data item".
Decoder: A process that decodes a well-formed encoded CBOR data item
and makes it available to an application. Formally speaking, a
decoder contains a parser to break up the input using the syntax
rules of CBOR, as well as a semantic processor to prepare the data
in a form suitable to the application.
Encoder: A process that generates the (well-formed) representation
format of a CBOR data item from application information.
Data Stream: A sequence of zero or more data items, not further
assembled into a larger containing data item (see [RFC8742] for
one application). The independent data items that make up a data
stream are sometimes also referred to as "top-level data items".
Well-formed: A data item that follows the syntactic structure of
CBOR. A well-formed data item uses the initial bytes and the byte
strings and/or data items that are implied by their values as
defined in CBOR and does not include following extraneous data.
CBOR decoders by definition only return contents from well-formed
data items.
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Valid: A data item that is well-formed and also follows the semantic
restrictions that apply to CBOR data items (Section 5.3).
Expected: Besides its normal English meaning, the term "expected" is
used to describe requirements beyond CBOR validity that an
application has on its input data. Well-formed (processable at
all), valid (checked by a validity-checking generic decoder), and
expected (checked by the application) form a hierarchy of layers
of acceptability.
Stream decoder: A process that decodes a data stream and makes each
of the data items in the sequence available to an application as
they are received.
Terms and concepts for floating-point values such as Infinity, NaN
(not a number), negative zero, and subnormal are defined in
[IEEE754].
Where bit arithmetic or data types are explained, this document uses
the notation familiar from the programming language C [C], except
that "**" denotes exponentiation and ".." denotes a range that
includes both ends given. Examples and pseudocode assume that signed
integers use two's complement representation and that right shifts of
signed integers perform sign extension; these assumptions are also
specified in Sections 6.8.2 and 7.6.7 of the 2020 version of C++,
successor of [Cplusplus17].
Similar to the "0x" notation for hexadecimal numbers, numbers in
binary notation are prefixed with "0b". Underscores can be added to
a number solely for readability, so 0b00100001 (0x21) might be
written 0b001_00001 to emphasize the desired interpretation of the
bits in the byte; in this case, it is split into three bits and five
bits. Encoded CBOR data items are sometimes given in the "0x" or
"0b" notation; these values are first interpreted as numbers as in C
and are then interpreted as byte strings in network byte order,
including any leading zero bytes expressed in the notation.
Words may be _italicized_ for emphasis; in the plain text form of
this specification this is indicated by surrounding words with
underscore characters. Verbatim text (e.g., names from a programming
language) may be set in "monospace" type; in plain text this is
approximated somewhat ambiguously by surrounding the text in double
quotes (which also retain their usual meaning).
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2. CBOR Data Models
CBOR is explicit about its generic data model, which defines the set
of all data items that can be represented in CBOR. Its basic generic
data model is extensible by the registration of "simple values" and
tags. Applications can then subset the resulting extended generic
data model to build their specific data models.
Within environments that can represent the data items in the generic
data model, generic CBOR encoders and decoders can be implemented
(which usually involves defining additional implementation data types
for those data items that do not already have a natural
representation in the environment). The ability to provide generic
encoders and decoders is an explicit design goal of CBOR; however
many applications will provide their own application-specific
encoders and/or decoders.
In the basic (un-extended) generic data model defined in Section 3, a
data item is one of:
* an integer in the range -2**64..2**64-1 inclusive
* a simple value, identified by a number between 0 and 255, but
distinct from that number itself
* a floating-point value, distinct from an integer, out of the set
representable by IEEE 754 binary64 (including non-finites)
[IEEE754]
* a sequence of zero or more bytes ("byte string")
* a sequence of zero or more Unicode code points ("text string")
* a sequence of zero or more data items ("array")
* a mapping (mathematical function) from zero or more data items
("keys") each to a data item ("values"), ("map")
* a tagged data item ("tag"), comprising a tag number (an integer in
the range 0..2**64-1) and the tag content (a data item)
Note that integer and floating-point values are distinct in this
model, even if they have the same numeric value.
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Also note that serialization variants are not visible at the generic
data model level, including the number of bytes of the encoded
floating-point value or the choice of one of the ways in which an
integer, the length of a text or byte string, the number of elements
in an array or pairs in a map, or a tag number, (collectively "the
argument", see Section 3) can be encoded.
2.1. Extended Generic Data Models
This basic generic data model comes pre-extended by the registration
of a number of simple values and tag numbers right in this document,
such as:
* "false", "true", "null", and "undefined" (simple values identified
by 20..23)
* integer and floating-point values with a larger range and
precision than the above (tag numbers 2 to 5)
* application data types such as a point in time or an RFC 3339
date/time string (tag numbers 1, 0)
Further elements of the extended generic data model can be (and have
been) defined via the IANA registries created for CBOR. Even if such
an extension is unknown to a generic encoder or decoder, data items
using that extension can be passed to or from the application by
representing them at the interface to the application within the
basic generic data model, i.e., as generic simple values or generic
tags.
In other words, the basic generic data model is stable as defined in
this document, while the extended generic data model expands by the
registration of new simple values or tag numbers, but never shrinks.
While there is a strong expectation that generic encoders and
decoders can represent "false", "true", and "null" ("undefined" is
intentionally omitted) in the form appropriate for their programming
environment, implementation of the data model extensions created by
tags is truly optional and a matter of implementation quality.
2.2. Specific Data Models
The specific data model for a CBOR-based protocol usually subsets the
extended generic data model and assigns application semantics to the
data items within this subset and its components. When documenting
such specific data models, where it is desired to specify the types
of data items, it is preferred to identify the types by the names
they have in the generic data model ("negative integer", "array")
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instead of by referring to aspects of their CBOR representation
("major type 1", "major type 4").
Specific data models can also specify what values (including values
of different types) are equivalent for the purposes of map keys and
encoder freedom. For example, in the generic data model, a valid map
MAY have both "0" and "0.0" as keys, and an encoder MUST NOT encode
"0.0" as an integer (major type 0, Section 3.1). However, if a
specific data model declares that floating-point and integer
representations of integral values are equivalent, using both map
keys "0" and "0.0" in a single map would be considered duplicates,
even while encoded as different major types, and so invalid; and an
encoder could encode integral-valued floats as integers or vice
versa, perhaps to save encoded bytes.
3. Specification of the CBOR Encoding
A CBOR data item (Section 2) is encoded to or decoded from a byte
string carrying a well-formed encoded data item as described in this
section. The encoding is summarized in Table 7 in Appendix B,
indexed by the initial byte. An encoder MUST produce only well-
formed encoded data items. A decoder MUST NOT return a decoded data
item when it encounters input that is not a well-formed encoded CBOR
data item (this does not detract from the usefulness of diagnostic
and recovery tools that might make available some information from a
damaged encoded CBOR data item).
The initial byte of each encoded data item contains both information
about the major type (the high-order 3 bits, described in
Section 3.1) and additional information (the low-order 5 bits). With
a few exceptions, the additional information's value describes how to
load an unsigned integer "argument":
Less than 24: The argument's value is the value of the additional
information.
24, 25, 26, or 27: The argument's value is held in the following 1,
2, 4, or 8 bytes, respectively, in network byte order. For major
type 7 and additional information value 25, 26, 27, these bytes
are not used as an integer argument, but as a floating-point value
(see Section 3.3).
28, 29, 30: These values are reserved for future additions to the
CBOR format. In the present version of CBOR, the encoded item is
not well-formed.
31: No argument value is derived. If the major type is 0, 1, or 6,
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the encoded item is not well-formed. For major types 2 to 5, the
item's length is indefinite, and for major type 7, the byte does
not constitute a data item at all but terminates an indefinite
length item; all are described in Section 3.2.
The initial byte and any additional bytes consumed to construct the
argument are collectively referred to as the "head" of the data item.
The meaning of this argument depends on the major type. For example,
in major type 0, the argument is the value of the data item itself
(and in major type 1 the value of the data item is computed from the
argument); in major type 2 and 3 it gives the length of the string
data in bytes that follows; and in major types 4 and 5 it is used to
determine the number of data items enclosed.
If the encoded sequence of bytes ends before the end of a data item,
that item is not well-formed. If the encoded sequence of bytes still
has bytes remaining after the outermost encoded item is decoded, that
encoding is not a single well-formed CBOR item; depending on the
application, the decoder may either treat the encoding as not well-
formed or just identify the start of the remaining bytes to the
application.
A CBOR decoder implementation can be based on a jump table with all
256 defined values for the initial byte (Table 7). A decoder in a
constrained implementation can instead use the structure of the
initial byte and following bytes for more compact code (see
Appendix C for a rough impression of how this could look).
3.1. Major Types
The following lists the major types and the additional information
and other bytes associated with the type.
Major type 0: an unsigned integer in the range 0..2**64-1 inclusive.
The value of the encoded item is the argument itself. For
example, the integer 10 is denoted as the one byte 0b000_01010
(major type 0, additional information 10). The integer 500 would
be 0b000_11001 (major type 0, additional information 25) followed
by the two bytes 0x01f4, which is 500 in decimal.
Major type 1: a negative integer in the range -2**64..-1 inclusive.
The value of the item is -1 minus the argument. For example, the
integer -500 would be 0b001_11001 (major type 1, additional
information 25) followed by the two bytes 0x01f3, which is 499 in
decimal.
Major type 2: a byte string. The number of bytes in the string is
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equal to the argument. For example, a byte string whose length is
5 would have an initial byte of 0b010_00101 (major type 2,
additional information 5 for the length), followed by 5 bytes of
binary content. A byte string whose length is 500 would have 3
initial bytes of 0b010_11001 (major type 2, additional information
25 to indicate a two-byte length) followed by the two bytes 0x01f4
for a length of 500, followed by 500 bytes of binary content.
Major type 3: a text string (Section 2), encoded as UTF-8
([RFC3629]). The number of bytes in the string is equal to the
argument. A string containing an invalid UTF-8 sequence is well-
formed but invalid (Section 1.2). This type is provided for
systems that need to interpret or display human-readable text, and
allows the differentiation between unstructured bytes and text
that has a specified repertoire (that of Unicode) and encoding
(UTF-8). In contrast to formats such as JSON, the Unicode
characters in this type are never escaped. Thus, a newline
character (U+000A) is always represented in a string as the byte
0x0a, and never as the bytes 0x5c6e (the characters "\" and "n")
nor as 0x5c7530303061 (the characters "\", "u", "0", "0", "0", and
"a").
Major type 4: an array of data items. In other formats, arrays are
also called lists, sequences, or tuples (a "CBOR sequence" is
something slightly different, though [RFC8742]). The argument is
the number of data items in the array. Items in an array do not
need to all be of the same type. For example, an array that
contains 10 items of any type would have an initial byte of
0b100_01010 (major type 4, additional information 10 for the
length) followed by the 10 remaining items.
Major type 5: a map of pairs of data items. Maps are also called
tables, dictionaries, hashes, or objects (in JSON). A map is
comprised of pairs of data items, each pair consisting of a key
that is immediately followed by a value. The argument is the
number of _pairs_ of data items in the map. For example, a map
that contains 9 pairs would have an initial byte of 0b101_01001
(major type 5, additional information 9 for the number of pairs)
followed by the 18 remaining items. The first item is the first
key, the second item is the first value, the third item is the
second key, and so on. Because items in a map come in pairs,
their total number is always even: A map that contains an odd
number of items (no value data present after the last key data
item) is not well-formed. A map that has duplicate keys may be
well-formed, but it is not valid, and thus it causes indeterminate
decoding; see also Section 5.6.
Major type 6: a tagged data item ("tag") whose tag number, an
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integer in the range 0..2**64-1 inclusive, is the argument and
whose enclosed data item ("tag content") is the single encoded
data item that follows the head. See Section 3.4.
Major type 7: floating-point numbers and simple values, as well as
the "break" stop code. See Section 3.3.
These eight major types lead to a simple table showing which of the
256 possible values for the initial byte of a data item are used
(Table 7).
In major types 6 and 7, many of the possible values are reserved for
future specification. See Section 9 for more information on these
values.
Table 1 summarizes the major types defined by CBOR, ignoring the next
section for now. The number N in this table stands for the argument,
mt for the major type.
+====+=======================+=================================+
| mt | Meaning | Content |
+====+=======================+=================================+
| 0 | unsigned integer N | - |
+----+-----------------------+---------------------------------+
| 1 | negative integer -1-N | - |
+----+-----------------------+---------------------------------+
| 2 | byte string | N bytes |
+----+-----------------------+---------------------------------+
| 3 | text string | N bytes (UTF-8 text) |
+----+-----------------------+---------------------------------+
| 4 | array | N data items (elements) |
+----+-----------------------+---------------------------------+
| 5 | map | 2N data items (key/value pairs) |
+----+-----------------------+---------------------------------+
| 6 | tag of number N | 1 data item |
+----+-----------------------+---------------------------------+
| 7 | simple/float | - |
+----+-----------------------+---------------------------------+
Table 1: Overview over the definite-length use of CBOR major
types (mt = major type, N = argument)
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3.2. Indefinite Lengths for Some Major Types
Four CBOR items (arrays, maps, byte strings, and text strings) can be
encoded with an indefinite length using additional information value
31. This is useful if the encoding of the item needs to begin before
the number of items inside the array or map, or the total length of
the string, is known. (The ability to start sending a data item
before all of it is known is often referred to as "streaming" within
that data item.)
Indefinite-length arrays and maps are dealt with differently than
indefinite-length strings (byte strings and text strings).
3.2.1. The "break" Stop Code
The "break" stop code is encoded with major type 7 and additional
information value 31 (0b111_11111). It is not itself a data item: it
is just a syntactic feature to close an indefinite-length item.
If the "break" stop code appears anywhere where a data item is
expected, other than directly inside an indefinite-length string,
array, or map -- for example directly inside a definite-length array
or map -- the enclosing item is not well-formed.
3.2.2. Indefinite-Length Arrays and Maps
Indefinite-length arrays and maps are represented using their major
type with the additional information value of 31, followed by an
arbitrary-length sequence of zero or more items for an array or key/
value pairs for a map, followed by the "break" stop code
(Section 3.2.1). In other words, indefinite-length arrays and maps
look identical to other arrays and maps except for beginning with the
additional information value of 31 and ending with the "break" stop
code.
If the "break" stop code appears after a key in a map, in place of
that key's value, the map is not well-formed.
There is no restriction against nesting indefinite-length array or
map items. A "break" only terminates a single item, so nested
indefinite-length items need exactly as many "break" stop codes as
there are type bytes starting an indefinite-length item.
For example, assume an encoder wants to represent the abstract array
[1, [2, 3], [4, 5]]. The definite-length encoding would be
0x8301820203820405:
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83 -- Array of length 3
01 -- 1
82 -- Array of length 2
02 -- 2
03 -- 3
82 -- Array of length 2
04 -- 4
05 -- 5
Indefinite-length encoding could be applied independently to each of
the three arrays encoded in this data item, as required, leading to
representations such as:
0x9f018202039f0405ffff
9F -- Start indefinite-length array
01 -- 1
82 -- Array of length 2
02 -- 2
03 -- 3
9F -- Start indefinite-length array
04 -- 4
05 -- 5
FF -- "break" (inner array)
FF -- "break" (outer array)
0x9f01820203820405ff
9F -- Start indefinite-length array
01 -- 1
82 -- Array of length 2
02 -- 2
03 -- 3
82 -- Array of length 2
04 -- 4
05 -- 5
FF -- "break"
0x83018202039f0405ff
83 -- Array of length 3
01 -- 1
82 -- Array of length 2
02 -- 2
03 -- 3
9F -- Start indefinite-length array
04 -- 4
05 -- 5
FF -- "break"
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0x83019f0203ff820405
83 -- Array of length 3
01 -- 1
9F -- Start indefinite-length array
02 -- 2
03 -- 3
FF -- "break"
82 -- Array of length 2
04 -- 4
05 -- 5
An example of an indefinite-length map (that happens to have two key/
value pairs) might be:
0xbf6346756ef563416d7421ff
BF -- Start indefinite-length map
63 -- First key, UTF-8 string length 3
46756e -- "Fun"
F5 -- First value, true
63 -- Second key, UTF-8 string length 3
416d74 -- "Amt"
21 -- Second value, -2
FF -- "break"
3.2.3. Indefinite-Length Byte Strings and Text Strings
Indefinite-length strings are represented by a byte containing the
major type for byte string or text string with an additional
information value of 31, followed by a series of zero or more strings
of the specified type ("chunks") that have definite lengths, and
finished by the "break" stop code (Section 3.2.1). The data item
represented by the indefinite-length string is the concatenation of
the chunks. If no chunks are present, the data item is an empty
string of the specified type. Zero-length chunks, while not
particularly useful, are permitted.
If any item between the indefinite-length string indicator
(0b010_11111 or 0b011_11111) and the "break" stop code is not a
definite-length string item of the same major type, the string is not
well-formed.
The design does not allow nesting indefinite-length strings as chunks
into indefinite-length strings. If it were allowed, it would require
decoder implementations to keep a stack, or at least a count, of
nesting levels. It is unnecessary on the encoder side because the
inner indefinite-length string would consist of chunks, and these
could instead be put directly into the outer indefinite-length
string.
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If any definite-length text string inside an indefinite-length text
string is invalid, the indefinite-length text string is invalid.
Note that this implies that the UTF-8 bytes of a single Unicode code
point (scalar value) cannot be spread between chunks: a new chunk of
a text string can only be started at a code point boundary.
For example, assume an encoded data item consisting of the bytes:
0b010_11111 0b010_00100 0xaabbccdd 0b010_00011 0xeeff99 0b111_11111
5F -- Start indefinite-length byte string
44 -- Byte string of length 4
aabbccdd -- Bytes content
43 -- Byte string of length 3
eeff99 -- Bytes content
FF -- "break"
After decoding, this results in a single byte string with seven
bytes: 0xaabbccddeeff99.
3.2.4. Summary of indefinite-length use of major types
Table 2 summarizes the major types defined by CBOR as used for
indefinite length encoding (with additional information set to 31).
mt stands for the major type.
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+====+===================+==================================+
| mt | Meaning | enclosed up to "break" stop code |
+====+===================+==================================+
| 0 | (not well-formed) | - |
+----+-------------------+----------------------------------+
| 1 | (not well-formed) | - |
+----+-------------------+----------------------------------+
| 2 | byte string | definite-length byte strings |
+----+-------------------+----------------------------------+
| 3 | text string | definite-length text strings |
+----+-------------------+----------------------------------+
| 4 | array | data items (elements) |
+----+-------------------+----------------------------------+
| 5 | map | data items (key/value pairs) |
+----+-------------------+----------------------------------+
| 6 | (not well-formed) | - |
+----+-------------------+----------------------------------+
| 7 | "break" stop code | - |
+----+-------------------+----------------------------------+
Table 2: Overview over the indefinite-length use of CBOR
major types (mt = major type, additional information =
31)
3.3. Floating-Point Numbers and Values with No Content
Major type 7 is for two types of data: floating-point numbers and
"simple values" that do not need any content. Each value of the
5-bit additional information in the initial byte has its own separate
meaning, as defined in Table 3. Like the major types for integers,
items of this major type do not carry content data; all the
information is in the initial bytes (the head).
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+=============+===================================================+
| 5-Bit Value | Semantics |
+=============+===================================================+
| 0..23 | Simple value (value 0..23) |
+-------------+---------------------------------------------------+
| 24 | Simple value (value 32..255 in following byte) |
+-------------+---------------------------------------------------+
| 25 | IEEE 754 Half-Precision Float (16 bits follow) |
+-------------+---------------------------------------------------+
| 26 | IEEE 754 Single-Precision Float (32 bits follow) |
+-------------+---------------------------------------------------+
| 27 | IEEE 754 Double-Precision Float (64 bits follow) |
+-------------+---------------------------------------------------+
| 28-30 | Reserved, not well-formed in the present document |
+-------------+---------------------------------------------------+
| 31 | "break" stop code for indefinite-length items |
| | (Section 3.2.1) |
+-------------+---------------------------------------------------+
Table 3: Values for Additional Information in Major Type 7
As with all other major types, the 5-bit value 24 signifies a single-
byte extension: it is followed by an additional byte to represent the
simple value. (To minimize confusion, only the values 32 to 255 are
used.) This maintains the structure of the initial bytes: as for the
other major types, the length of these always depends on the
additional information in the first byte. Table 4 lists the numeric
values assigned and available for simple values.
+=========+==============+
| Value | Semantics |
+=========+==============+
| 0..19 | (Unassigned) |
+---------+--------------+
| 20 | False |
+---------+--------------+
| 21 | True |
+---------+--------------+
| 22 | Null |
+---------+--------------+
| 23 | Undefined |
+---------+--------------+
| 24..31 | (Reserved) |
+---------+--------------+
| 32..255 | (Unassigned) |
+---------+--------------+
Table 4: Simple Values
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An encoder MUST NOT issue two-byte sequences that start with 0xf8
(major type 7, additional information 24) and continue with a byte
less than 0x20 (32 decimal). Such sequences are not well-formed.
(This implies that an encoder cannot encode false, true, null, or
undefined in two-byte sequences, and that only the one-byte variants
of these are well-formed; more generally speaking, each simple value
only has a single representation variant).
The 5-bit values of 25, 26, and 27 are for 16-bit, 32-bit, and 64-bit
IEEE 754 binary floating-point values [IEEE754]. These floating-
point values are encoded in the additional bytes of the appropriate
size. (See Appendix D for some information about 16-bit floating-
point numbers.)
3.4. Tagging of Items
In CBOR, a data item can be enclosed by a tag to give it some
additional semantics, as uniquely identified by a "tag number". The
tag is major type 6, its argument (Section 3) indicates the tag
number, and it contains a single enclosed data item, the "tag
content". (If a tag requires further structure to its content, this
structure is provided by the enclosed data item.) We use the term
"tag" for the entire data item consisting of both a tag number and
the tag content: the tag content is the data item that is being
tagged.
For example, assume that a byte string of length 12 is marked with a
tag of number 2 to indicate it is a positive "bignum"
(Section 3.4.3). The encoded data item would start with a byte
0b110_00010 (major type 6, additional information 2 for the tag
number) followed by the encoded tag content: 0b010_01100 (major type
2, additional information of 12 for the length) followed by the 12
bytes of the bignum.
The definition of a tag number describes the additional semantics
conveyed for tags with this tag number in the extended generic data
model. These semantics may include equivalence of some tagged data
items with other data items, including some that can already be
represented in the basic generic data model. For instance, 0xc24101,
a bignum the tag content of which is the byte string with the single
byte 0x01, is equivalent to an integer 1, which could also be encoded
for instance as 0x01, 0x1801, or 0x190001. The tag definition may
include the definition of a preferred serialization (Section 4.1)
that is recommended for generic encoders; this may prefer basic
generic data model representations over ones that employ a tag.
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The tag definition usually restricts what kinds of nested data item
or items are valid for such tags. Tag definitions may restrict their
content to a very specific syntactic structure, as the tags defined
in this document do, or they may aim at a more semantically defined
definition of their content, as for instance tags 40 and 1040 do
[RFC8746]: These accept a number of different ways of representing
arrays.
As a matter of convention, many tags do not accept null or undefined
values as tag content; instead, the expectation is that a null or
undefined value can be used in place of the entire tag; Section 3.4.2
provides some further considerations for one specific tag about the
handling of this convention in application protocols and in mapping
to platform types.
Decoders do not need to understand tags of every tag number, and tags
may be of little value in applications where the implementation
creating a particular CBOR data item and the implementation decoding
that stream know the semantic meaning of each item in the data flow.
Their primary purpose in this specification is to define common data
types such as dates. A secondary purpose is to provide conversion
hints when it is foreseen that the CBOR data item needs to be
translated into a different format, requiring hints about the content
of items. Understanding the semantics of tags is optional for a
decoder; it can simply present both the tag number and the tag
content to the application, without interpreting the additional
semantics of the tag.
A tag applies semantics to the data item it encloses. Tags can nest:
If tag A encloses tag B, which encloses data item C, tag A applies to
the result of applying tag B on data item C.
IANA maintains a registry of tag numbers as described in Section 9.2.
Table 5 provides a list of tag numbers that were defined in
[RFC7049], with definitions in the rest of this section. (Tag number
35 was also defined in [RFC7049]; a discussion of this tag number
follows in Section 3.4.5.3.) Note that many other tag numbers have
been defined since the publication of [RFC7049]; see the registry
described at Section 9.2 for the complete list.
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+============+=============+==================================+
| Tag Number | Data Item | Tag Content Semantics |
+============+=============+==================================+
| 0 | text string | Standard date/time string; see |
| | | Section 3.4.1 |
+------------+-------------+----------------------------------+
| 1 | integer or | Epoch-based date/time; see |
| | float | Section 3.4.2 |
+------------+-------------+----------------------------------+
| 2 | byte string | Positive bignum; see |
| | | Section 3.4.3 |
+------------+-------------+----------------------------------+
| 3 | byte string | Negative bignum; see |
| | | Section 3.4.3 |
+------------+-------------+----------------------------------+
| 4 | array | Decimal fraction; see |
| | | Section 3.4.4 |
+------------+-------------+----------------------------------+
| 5 | array | Bigfloat; see Section 3.4.4 |
+------------+-------------+----------------------------------+
| 21 | (any) | Expected conversion to base64url |
| | | encoding; see Section 3.4.5.2 |
+------------+-------------+----------------------------------+
| 22 | (any) | Expected conversion to base64 |
| | | encoding; see Section 3.4.5.2 |
+------------+-------------+----------------------------------+
| 23 | (any) | Expected conversion to base16 |
| | | encoding; see Section 3.4.5.2 |
+------------+-------------+----------------------------------+
| 24 | byte string | Encoded CBOR data item; see |
| | | Section 3.4.5.1 |
+------------+-------------+----------------------------------+
| 32 | text string | URI; see Section 3.4.5.3 |
+------------+-------------+----------------------------------+
| 33 | text string | base64url; see Section 3.4.5.3 |
+------------+-------------+----------------------------------+
| 34 | text string | base64; see Section 3.4.5.3 |
+------------+-------------+----------------------------------+
| 36 | text string | MIME message; see |
| | | Section 3.4.5.3 |
+------------+-------------+----------------------------------+
| 55799 | (any) | Self-described CBOR; see |
| | | Section 3.4.6 |
+------------+-------------+----------------------------------+
Table 5: Tag numbers defined in RFC 7049
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Conceptually, tags are interpreted in the generic data model, not at
(de-)serialization time. A small number of tags (at this time, tag
number 25 and tag number 29 [IANA.cbor-tags]) have been registered
with semantics that may require processing at (de-)serialization
time: The decoder needs to be aware and the encoder needs to be in
control of the exact sequence in which data items are encoded into
the CBOR data item. This means these tags cannot be implemented on
top of an arbitrary generic CBOR encoder/decoder (which might not
reflect the serialization order for entries in a map at the data
model level and vice versa); their implementation therefore typically
needs to be integrated into the generic encoder/decoder. The
definition of new tags with this property is NOT RECOMMENDED.
IANA allocated tag numbers 65535, 4294967295, and
18446744073709551615 (binary all-ones in 16-bit, 32-bit, and 64-bit).
These can be used as a convenience for implementers that want a
single integer data structure to indicate either that a specific tag
is present, or the absence of a tag. That allocation is described in
Section 10 of [I-D.bormann-cbor-notable-tags]. These tags are not
intended to occur in actual CBOR data items; implementations MAY flag
such an occurrence as an error.
Protocols using tag numbers 0 and 1 extend the generic data model
(Section 2) with data items representing points in time; tag numbers
2 and 3, with arbitrarily sized integers; and tag numbers 4 and 5,
with floating-point values of arbitrary size and precision.
3.4.1. Standard Date/Time String
Tag number 0 contains a text string in the standard format described
by the "date-time" production in [RFC3339], as refined by Section 3.3
of [RFC4287], representing the point in time described there. A
nested item of another type or a text string that doesn't match the
[RFC4287] format is invalid.
3.4.2. Epoch-based Date/Time
Tag number 1 contains a numerical value counting the number of
seconds from 1970-01-01T00:00Z in UTC time to the represented point
in civil time.
The tag content MUST be an unsigned or negative integer (major types
0 and 1), or a floating-point number (major type 7 with additional
information 25, 26, or 27). Other contained types are invalid.
Non-negative values (major type 0 and non-negative floating-point
numbers) stand for time values on or after 1970-01-01T00:00Z UTC and
are interpreted according to POSIX [TIME_T]. (POSIX time is also
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known as "UNIX Epoch time".) Leap seconds are handled specially by
POSIX time and this results in a 1 second discontinuity several times
per decade. Note that applications that require the expression of
times beyond early 2106 cannot leave out support of 64-bit integers
for the tag content.
Negative values (major type 1 and negative floating-point numbers)
are interpreted as determined by the application requirements as
there is no universal standard for UTC count-of-seconds time before
1970-01-01T00:00Z (this is particularly true for points in time that
precede discontinuities in national calendars). The same applies to
non-finite values.
To indicate fractional seconds, floating-point values can be used
within tag number 1 instead of integer values. Note that this
generally requires binary64 support, as binary16 and binary32 provide
non-zero fractions of seconds only for a short period of time around
early 1970. An application that requires tag number 1 support may
restrict the tag content to be an integer (or a floating-point value)
only.
Note that platform types for date/time may include null or undefined
values, which may also be desirable at an application protocol level.
While emitting tag number 1 values with non-finite tag content values
(e.g., with NaN for undefined date/time values or with Infinite for
an expiry date that is not set) may seem an obvious way to handle
this, using untagged null or undefined avoids the use of non-finites
and results in a shorter encoding. Application protocol designers
are encouraged to consider these cases and include clear guidelines
for handling them.
3.4.3. Bignums
Protocols using tag numbers 2 and 3 extend the generic data model
(Section 2) with "bignums" representing arbitrarily sized integers.
In the basic generic data model, bignum values are not equal to
integers from the same model, but the extended generic data model
created by this tag definition defines equivalence based on numeric
value, and preferred serialization (Section 4.1) never makes use of
bignums that also can be expressed as basic integers (see below).
Bignums are encoded as a byte string data item, which is interpreted
as an unsigned integer n in network byte order. Contained items of
other types are invalid. For tag number 2, the value of the bignum
is n. For tag number 3, the value of the bignum is -1 - n. The
preferred serialization of the byte string is to leave out any
leading zeroes (note that this means the preferred serialization for
n = 0 is the empty byte string, but see below). Decoders that
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understand these tags MUST be able to decode bignums that do have
leading zeroes. The preferred serialization of an integer that can
be represented using major type 0 or 1 is to encode it this way
instead of as a bignum (which means that the empty string never
occurs in a bignum when using preferred serialization). Note that
this means the non-preferred choice of a bignum representation
instead of a basic integer for encoding a number is not intended to
have application semantics (just as the choice of a longer basic
integer representation than needed, such as 0x1800 for 0x00 does
not).
For example, the number 18446744073709551616 (2**64) is represented
as 0b110_00010 (major type 6, tag number 2), followed by 0b010_01001
(major type 2, length 9), followed by 0x010000000000000000 (one byte
0x01 and eight bytes 0x00). In hexadecimal:
C2 -- Tag 2
49 -- Byte string of length 9
010000000000000000 -- Bytes content
3.4.4. Decimal Fractions and Bigfloats
Protocols using tag number 4 extend the generic data model with data
items representing arbitrary-length decimal fractions of the form
m*(10**e). Protocols using tag number 5 extend the generic data
model with data items representing arbitrary-length binary fractions
of the form m*(2**e). As with bignums, values of different types are
not equal in the generic data model.
Decimal fractions combine an integer mantissa with a base-10 scaling
factor. They are most useful if an application needs the exact
representation of a decimal fraction such as 1.1 because there is no
exact representation for many decimal fractions in binary floating-
point representations.
"Bigfloats" combine an integer mantissa with a base-2 scaling factor.
They are binary floating-point values that can exceed the range or
the precision of the three IEEE 754 formats supported by CBOR
(Section 3.3). Bigfloats may also be used by constrained
applications that need some basic binary floating-point capability
without the need for supporting IEEE 754.
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A decimal fraction or a bigfloat is represented as a tagged array
that contains exactly two integer numbers: an exponent e and a
mantissa m. Decimal fractions (tag number 4) use base-10 exponents;
the value of a decimal fraction data item is m*(10**e). Bigfloats
(tag number 5) use base-2 exponents; the value of a bigfloat data
item is m*(2**e). The exponent e MUST be represented in an integer
of major type 0 or 1, while the mantissa can also be a bignum
(Section 3.4.3). Contained items with other structures are invalid.
An example of a decimal fraction is that the number 273.15 could be
represented as 0b110_00100 (major type 6 for tag, additional
information 4 for the tag number), followed by 0b100_00010 (major
type 4 for the array, additional information 2 for the length of the
array), followed by 0b001_00001 (major type 1 for the first integer,
additional information 1 for the value of -2), followed by
0b000_11001 (major type 0 for the second integer, additional
information 25 for a two-byte value), followed by 0b0110101010110011
(27315 in two bytes). In hexadecimal:
C4 -- Tag 4
82 -- Array of length 2
21 -- -2
19 6ab3 -- 27315
An example of a bigfloat is that the number 1.5 could be represented
as 0b110_00101 (major type 6 for tag, additional information 5 for
the tag number), followed by 0b100_00010 (major type 4 for the array,
additional information 2 for the length of the array), followed by
0b001_00000 (major type 1 for the first integer, additional
information 0 for the value of -1), followed by 0b000_00011 (major
type 0 for the second integer, additional information 3 for the value
of 3). In hexadecimal:
C5 -- Tag 5
82 -- Array of length 2
20 -- -1
03 -- 3
Decimal fractions and bigfloats provide no representation of
Infinity, -Infinity, or NaN; if these are needed in place of a
decimal fraction or bigfloat, the IEEE 754 half-precision
representations from Section 3.3 can be used.
3.4.5. Content Hints
The tags in this section are for content hints that might be used by
generic CBOR processors. These content hints do not extend the
generic data model.
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3.4.5.1. Encoded CBOR Data Item
Sometimes it is beneficial to carry an embedded CBOR data item that
is not meant to be decoded immediately at the time the enclosing data
item is being decoded. Tag number 24 (CBOR data item) can be used to
tag the embedded byte string as a single data item encoded in CBOR
format. Contained items that aren't byte strings are invalid. A
contained byte string is valid if it encodes a well-formed CBOR data
item; validity checking of the decoded CBOR item is not required for
tag validity (but could be offered by a generic decoder as a special
option).
3.4.5.2. Expected Later Encoding for CBOR-to-JSON Converters
Tag numbers 21 to 23 indicate that a byte string might require a
specific encoding when interoperating with a text-based
representation. These tags are useful when an encoder knows that the
byte string data it is writing is likely to be later converted to a
particular JSON-based usage. That usage specifies that some strings
are encoded as base64, base64url, and so on. The encoder uses byte
strings instead of doing the encoding itself to reduce the message
size, to reduce the code size of the encoder, or both. The encoder
does not know whether or not the converter will be generic, and
therefore wants to say what it believes is the proper way to convert
binary strings to JSON.
The data item tagged can be a byte string or any other data item. In
the latter case, the tag applies to all of the byte string data items
contained in the data item, except for those contained in a nested
data item tagged with an expected conversion.
These three tag numbers suggest conversions to three of the base data
encodings defined in [RFC4648]. Tag number 21 suggests conversion to
base64url encoding (Section 5 of RFC 4648), where padding is not used
(see Section 3.2 of RFC 4648); that is, all trailing equals signs
("=") are removed from the encoded string. Tag number 22 suggests
conversion to classical base64 encoding (Section 4 of RFC 4648), with
padding as defined in RFC 4648. For both base64url and base64,
padding bits are set to zero (see Section 3.5 of RFC 4648), and the
conversion to alternate encoding is performed on the contents of the
byte string (that is, without adding any line breaks, whitespace, or
other additional characters). Tag number 23 suggests conversion to
base16 (hex) encoding, with uppercase alphabetics (see Section 8 of
RFC 4648). Note that, for all three tag numbers, the encoding of the
empty byte string is the empty text string.
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3.4.5.3. Encoded Text
Some text strings hold data that have formats widely used on the
Internet, and sometimes those formats can be validated and presented
to the application in appropriate form by the decoder. There are
tags for some of these formats.
* Tag number 32 is for URIs, as defined in [RFC3986]. If the text
string doesn't match the "URI-reference" production, the string is
invalid.
* Tag numbers 33 and 34 are for base64url- and base64-encoded text
strings, respectively, as defined in [RFC4648]. If any of:
- the encoded text string contains non-alphabet characters or
only 1 alphabet character in the last block of 4 (where
alphabet is defined by Section 5 of [RFC4648] for tag number 33
and Section 4 of [RFC4648] for tag number 34), or
- the padding bits in a 2- or 3-character block are not 0, or
- the base64 encoding has the wrong number of padding characters,
or
- the base64url encoding has padding characters,
the string is invalid.
* Tag number 36 is for MIME messages (including all headers), as
defined in [RFC2045]. A text string that isn't a valid MIME
message is invalid. (For this tag, validity checking may be
particularly onerous for a generic decoder and might therefore not
be offered. Note that many MIME messages are general binary data
and can therefore not be represented in a text string;
[IANA.cbor-tags] lists a registration for tag number 257 that is
similar to tag number 36 but uses a byte string as its tag
content.)
Note that tag numbers 33 and 34 differ from 21 and 22 in that the
data is transported in base-encoded form for the former and in raw
byte string form for the latter.
[RFC7049] also defined a tag number 35, for regular expressions that
are in Perl Compatible Regular Expressions (PCRE/PCRE2) form [PCRE]
or in JavaScript regular expression syntax [ECMA262]. The state of
the art in these regular expression specifications has since advanced
and is continually advancing, so the present specification does not
attempt to update the references to a snapshot that is current at the
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time of writing. Instead, this tag remains available (as registered
in [RFC7049]) for applications that specify the particular regular
expression variant they use out-of-band (possibly by limiting the
usage to a defined common subset of both PCRE and ECMA262). As the
present specification clarifies tag validity beyond [RFC7049], we
note that due to the open way the tag was defined in [RFC7049], any
contained string value needs to be valid at the CBOR tag level (but
may then not be "expected" at the application level).
3.4.6. Self-Described CBOR
In many applications, it will be clear from the context that CBOR is
being employed for encoding a data item. For instance, a specific
protocol might specify the use of CBOR, or a media type is indicated
that specifies its use. However, there may be applications where
such context information is not available, such as when CBOR data is
stored in a file that does not have disambiguating metadata. Here,
it may help to have some distinguishing characteristics for the data
itself.
Tag number 55799 is defined for this purpose, specifically for use at
the start of a stored encoded CBOR data item as specified by an
application. It does not impart any special semantics on the data
item that it encloses; that is, the semantics of the tag content
enclosed in tag number 55799 is exactly identical to the semantics of
the tag content itself.
The serialization of this tag's head is 0xd9d9f7, which does not
appear to be in use as a distinguishing mark for any frequently used
file types. In particular, 0xd9d9f7 is not a valid start of a
Unicode text in any Unicode encoding if it is followed by a valid
CBOR data item.
For instance, a decoder might be able to decode both CBOR and JSON.
Such a decoder would need to mechanically distinguish the two
formats. An easy way for an encoder to help the decoder would be to
tag the entire CBOR item with tag number 55799, the serialization of
which will never be found at the beginning of a JSON text.
4. Serialization Considerations
4.1. Preferred Serialization
For some values at the data model level, CBOR provides multiple
serializations. For many applications, it is desirable that an
encoder always chooses a preferred serialization (preferred
encoding); however, the present specification does not put the burden
of enforcing this preference on either encoder or decoder.
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Some constrained decoders may be limited in their ability to decode
non-preferred serializations: For example, if only integers below
1_000_000_000 (one billion) are expected in an application, the
decoder may leave out the code that would be needed to decode 64-bit
arguments in integers. An encoder that always uses preferred
serialization ("preferred encoder") interoperates with this decoder
for the numbers that can occur in this application. More generally
speaking, it therefore can be said that a preferred encoder is more
universally interoperable (and also less wasteful) than one that,
say, always uses 64-bit integers.
Similarly, a constrained encoder may be limited in the variety of
representation variants it supports in such a way that it does not
emit preferred serializations ("variant encoder"): Say, it could be
designed to always use the 32-bit variant for an integer that it
encodes even if a short representation is available (again, assuming
that there is no application need for integers that can only be
represented with the 64-bit variant). A decoder that does not rely
on only ever receiving preferred serializations ("variation-tolerant
decoder") can therefore be said to be more universally interoperable
(it might very well optimize for the case of receiving preferred
serializations, though). Full implementations of CBOR decoders are
by definition variation-tolerant; the distinction is only relevant if
a constrained implementation of a CBOR decoder meets a variant
encoder.
The preferred serialization always uses the shortest form of
representing the argument (Section 3); it also uses the shortest
floating-point encoding that preserves the value being encoded.
The preferred serialization for a floating-point value is the
shortest floating-point encoding that preserves its value, e.g.,
0xf94580 for the number 5.5, and 0xfa45ad9c00 for the number 5555.5.
For NaN values, a shorter encoding is preferred if zero-padding the
shorter significand towards the right reconstitutes the original NaN
value (for many applications, the single NaN encoding 0xf97e00 will
suffice).
Definite length encoding is preferred whenever the length is known at
the time the serialization of the item starts.
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4.2. Deterministically Encoded CBOR
Some protocols may want encoders to only emit CBOR in a particular
deterministic format; those protocols might also have the decoders
check that their input is in that deterministic format. Those
protocols are free to define what they mean by a "deterministic
format" and what encoders and decoders are expected to do. This
section defines a set of restrictions that can serve as the base of
such a deterministic format.
4.2.1. Core Deterministic Encoding Requirements
A CBOR encoding satisfies the "core deterministic encoding
requirements" if it satisfies the following restrictions:
* Preferred serialization MUST be used. In particular, this means
that arguments (see Section 3) for integers, lengths in major
types 2 through 5, and tags MUST be as short as possible, for
instance:
- 0 to 23 and -1 to -24 MUST be expressed in the same byte as the
major type;
- 24 to 255 and -25 to -256 MUST be expressed only with an
additional uint8_t;
- 256 to 65535 and -257 to -65536 MUST be expressed only with an
additional uint16_t;
- 65536 to 4294967295 and -65537 to -4294967296 MUST be expressed
only with an additional uint32_t.
Floating-point values also MUST use the shortest form that
preserves the value, e.g. 1.5 is encoded as 0xf93e00 (binary16)
and 1000000.5 as 0xfa49742408 (binary32). (One implementation of
this is to have all floats start as a 64-bit float, then do a test
conversion to a 32-bit float; if the result is the same numeric
value, use the shorter form and repeat the process with a test
conversion to a 16-bit float. This also works to select 16-bit
float for positive and negative Infinity as well.)
* Indefinite-length items MUST NOT appear. They can be encoded as
definite-length items instead.
* The keys in every map MUST be sorted in the bytewise lexicographic
order of their deterministic encodings. For example, the
following keys are sorted correctly:
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1. 10, encoded as 0x0a.
2. 100, encoded as 0x1864.
3. -1, encoded as 0x20.
4. "z", encoded as 0x617a.
5. "aa", encoded as 0x626161.
6. [100], encoded as 0x811864.
7. [-1], encoded as 0x8120.
8. false, encoded as 0xf4.
(Implementation note: the self-delimiting nature of the CBOR
encoding means that there are no two well-formed CBOR encoded data
items where one is a prefix of the other. The bytewise
lexicographic comparison of deterministic encodings of different
map keys therefore always ends in a position where the byte
differs between the keys, before the end of a key is reached.)
4.2.2. Additional Deterministic Encoding Considerations
CBOR tags present additional considerations for deterministic
encoding. If a CBOR-based protocol were to provide the same
semantics for the presence and absence of a specific tag (e.g., by
allowing both tag 1 data items and raw numbers in a date/time
position, treating the latter as if they were tagged), the
deterministic format would not allow the presence of the tag, based
on the "shortest form" principle. For example, a protocol might give
encoders the choice of representing a URL as either a text string or,
using Section 3.4.5.3, tag number 32 containing a text string. This
protocol's deterministic encoding needs to either require that the
tag is present or require that it is absent, not allow either one.
In a protocol that does require tags in certain places to obtain
specific semantics, the tag needs to appear in the deterministic
format as well. Deterministic encoding considerations also apply to
the content of tags.
If a protocol includes a field that can express integers with an
absolute value of 2^64 or larger using tag numbers 2 or 3
(Section 3.4.3), the protocol's deterministic encoding needs to
specify whether smaller integers are also expressed using these tags
or using major types 0 and 1. Preferred serialization uses the
latter choice, which is therefore recommended.
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Protocols that include floating-point values, whether represented
using basic floating-point values (Section 3.3) or using tags (or
both), may need to define extra requirements on their deterministic
encodings, such as:
* Although IEEE floating-point values can represent both positive
and negative zero as distinct values, the application might not
distinguish these and might decide to represent all zero values
with a positive sign, disallowing negative zero. (The application
may also want to restrict the precision of floating-point values
in such a way that there is never a need to represent 64-bit -- or
even 32-bit -- floating-point values.)
* If a protocol includes a field that can express floating-point
values, with a specific data model that declares integer and
floating-point values to be interchangeable, the protocol's
deterministic encoding needs to specify whether (for example) the
integer 1.0 is encoded as 0x01 (unsigned integer), 0xf93c00
(binary16), 0xfa3f800000 (binary32), or 0xfb3ff0000000000000
(binary64). Example rules for this are:
1. Encode integral values that fit in 64 bits as values from
major types 0 and 1, and other values as the preferred
(smallest of 16-, 32-, or 64-bit) floating-point
representation that accurately represents the value,
2. Encode all values as the preferred floating-point
representation that accurately represents the value, even for
integral values, or
3. Encode all values as 64-bit floating-point representations.
Rule 1 straddles the boundaries between integers and floating-
point values, and Rule 3 does not use preferred serialization, so
Rule 2 may be a good choice in many cases.
* If NaN is an allowed value and there is no intent to support NaN
payloads or signaling NaNs, the protocol needs to pick a single
representation, typically 0xf97e00. If that simple choice is not
possible, specific attention will be needed for NaN handling.
* Subnormal numbers (nonzero numbers with the lowest possible
exponent of a given IEEE 754 number format) may be flushed to zero
outputs or be treated as zero inputs in some floating-point
implementations. A protocol's deterministic encoding may want to
specifically accommodate such implementations while creating an
onus on other implementations, by excluding subnormal numbers from
interchange, interchanging zero instead.
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* The same number can be represented by different decimal fractions,
by different bigfloats, and by different forms under other tags
that may be defined to express numeric values. Depending on the
implementation, it may not always be practical to determine
whether any of these forms (or forms in the basic generic data
model) are equivalent. An application protocol that presents
choices of this kind for the representation format of numbers
needs to be explicit in how the formats are to be chosen for
deterministic encoding.
4.2.3. Length-first Map Key Ordering
The core deterministic encoding requirements (Section 4.2.1) sort map
keys in a different order from the one suggested by Section 3.9 of
[RFC7049] (called "Canonical CBOR" there). Protocols that need to be
compatible with [RFC7049]'s order can instead be specified in terms
of this specification's "length-first core deterministic encoding
requirements":
A CBOR encoding satisfies the "length-first core deterministic
encoding requirements" if it satisfies the core deterministic
encoding requirements except that the keys in every map MUST be
sorted such that:
1. If two keys have different lengths, the shorter one sorts
earlier;
2. If two keys have the same length, the one with the lower value in
(byte-wise) lexical order sorts earlier.
For example, under the length-first core deterministic encoding
requirements, the following keys are sorted correctly:
1. 10, encoded as 0x0a.
2. -1, encoded as 0x20.
3. false, encoded as 0xf4.
4. 100, encoded as 0x1864.
5. "z", encoded as 0x617a.
6. [-1], encoded as 0x8120.
7. "aa", encoded as 0x626161.
8. [100], encoded as 0x811864.
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(Although [RFC7049] used the term "Canonical CBOR" for its form of
requirements on deterministic encoding, this document avoids this
term because "canonicalization" is often associated with specific
uses of deterministic encoding only. The terms are essentially
interchangeable, however, and the set of core requirements in this
document could also be called "Canonical CBOR", while the length-
first-ordered version of that could be called "Old Canonical CBOR".)
5. Creating CBOR-Based Protocols
Data formats such as CBOR are often used in environments where there
is no format negotiation. A specific design goal of CBOR is to not
need any included or assumed schema: a decoder can take a CBOR item
and decode it with no other knowledge.
Of course, in real-world implementations, the encoder and the decoder
will have a shared view of what should be in a CBOR data item. For
example, an agreed-to format might be "the item is an array whose
first value is a UTF-8 string, second value is an integer, and
subsequent values are zero or more floating-point numbers" or "the
item is a map that has byte strings for keys and contains a pair
whose key is 0xab01".
CBOR-based protocols MUST specify how their decoders handle invalid
and other unexpected data. CBOR-based protocols MAY specify that
they treat arbitrary valid data as unexpected. Encoders for CBOR-
based protocols MUST produce only valid items, that is, the protocol
cannot be designed to make use of invalid items. An encoder can be
capable of encoding as many or as few types of values as is required
by the protocol in which it is used; a decoder can be capable of
understanding as many or as few types of values as is required by the
protocols in which it is used. This lack of restrictions allows CBOR
to be used in extremely constrained environments.
The rest of this section discusses some considerations in creating
CBOR-based protocols. With few exceptions, it is advisory only and
explicitly excludes any language from BCP 14 other than words that
could be interpreted as "MAY" in the sense of BCP 14. The exceptions
aim at facilitating interoperability of CBOR-based protocols while
making use of a wide variety of both generic and application-specific
encoders and decoders.
5.1. CBOR in Streaming Applications
In a streaming application, a data stream may be composed of a
sequence of CBOR data items concatenated back-to-back. In such an
environment, the decoder immediately begins decoding a new data item
if data is found after the end of a previous data item.
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Not all of the bytes making up a data item may be immediately
available to the decoder; some decoders will buffer additional data
until a complete data item can be presented to the application.
Other decoders can present partial information about a top-level data
item to an application, such as the nested data items that could
already be decoded, or even parts of a byte string that hasn't
completely arrived yet. Such an application also MUST have a
matching streaming security mechanism, where the desired protection
is available for incremental data presented to the application.
Note that some applications and protocols will not want to use
indefinite-length encoding. Using indefinite-length encoding allows
an encoder to not need to marshal all the data for counting, but it
requires a decoder to allocate increasing amounts of memory while
waiting for the end of the item. This might be fine for some
applications but not others.
5.2. Generic Encoders and Decoders
A generic CBOR decoder can decode all well-formed encoded CBOR data
items and present the data items to an application. See Appendix C.
(The diagnostic notation, Section 8, may be used to present well-
formed CBOR values to humans.)
Generic CBOR encoders provide an application interface that allows
the application to specify any well-formed value to be encoded as a
CBOR data item, including simple values and tags unknown to the
encoder.
Even though CBOR attempts to minimize these cases, not all well-
formed CBOR data is valid: for example, the encoded text string
"0x62c0ae" does not contain valid UTF-8 (because [RFC3629] requires
always using the shortest form) and so is not a valid CBOR item.
Also, specific tags may make semantic constraints that may be
violated, for instance by a bignum tag enclosing another tag, or by
an instance of tag number 0 containing a byte string, or containing a
text string with contents that do not match [RFC3339]'s "date-time"
production. There is no requirement that generic encoders and
decoders make unnatural choices for their application interface to
enable the processing of invalid data. Generic encoders and decoders
are expected to forward simple values and tags even if their specific
codepoints are not registered at the time the encoder/decoder is
written (Section 5.4).
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5.3. Validity of Items
A well-formed but invalid CBOR data item (Section 1.2) presents a
problem with interpreting the data encoded in it in the CBOR data
model. A CBOR-based protocol could be specified in several layers,
in which the lower layers don't process the semantics of some of the
CBOR data they forward. These layers can't notice any validity
errors in data they don't process and MUST forward that data as-is.
The first layer that does process the semantics of an invalid CBOR
item MUST take one of two choices:
1. Replace the problematic item with an error marker and continue
with the next item, or
2. Issue an error and stop processing altogether.
A CBOR-based protocol MUST specify which of these options its
decoders take, for each kind of invalid item they might encounter.
Such problems might occur at the basic validity level of CBOR or in
the context of tags (tag validity).
5.3.1. Basic validity
Two kinds of validity errors can occur in the basic generic data
model:
Duplicate keys in a map: Generic decoders (Section 5.2) make data
available to applications using the native CBOR data model. That
data model includes maps (key-value mappings with unique keys),
not multimaps (key-value mappings where multiple entries can have
the same key). Thus, a generic decoder that gets a CBOR map item
that has duplicate keys will decode to a map with only one
instance of that key, or it might stop processing altogether. On
the other hand, a "streaming decoder" may not even be able to
notice. See Section 5.6 for more discussion of keys in maps.
Invalid UTF-8 string: A decoder might or might not want to verify
that the sequence of bytes in a UTF-8 string (major type 3) is
actually valid UTF-8 and react appropriately.
5.3.2. Tag validity
Two additional kinds of validity errors are introduced by adding tags
to the basic generic data model:
Inadmissible type for tag content: Tag numbers (Section 3.4) specify
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what type of data item is supposed to be used as their tag
content; for example, the tag numbers for positive or negative
bignums are supposed to be put on byte strings. A decoder that
decodes the tagged data item into a native representation (a
native big integer in this example) is expected to check the type
of the data item being tagged. Even decoders that don't have such
native representations available in their environment may perform
the check on those tags known to them and react appropriately.
Inadmissible value for tag content: The type of data item may be
admissible for a tag's content, but the specific value may not be;
e.g., a value of "yesterday" is not acceptable for the content of
tag 0, even though it properly is a text string. A decoder that
normally ingests such tags into equivalent platform types might
present this tag to the application in a similar way to how it
would present a tag with an unknown tag number (Section 5.4).
5.4. Validity and Evolution
A decoder with validity checking will expend the effort to reliably
detect data items with validity errors. For example, such a decoder
needs to have an API that reports an error (and does not return data)
for a CBOR data item that contains any of the validity errors listed
in the previous subsection.
The set of tags defined in the tag registry (Section 9.2), as well as
the set of simple values defined in the simple values registry
(Section 9.1), can grow at any time beyond the set understood by a
generic decoder. A validity-checking decoder can do one of two
things when it encounters such a case that it does not recognize:
* It can report an error (and not return data). Note that treating
this case as an error can cause ossification, and is thus not
encouraged. This error is not a validity error per se. This kind
of error is more likely to be raised by a decoder that would be
performing validity checking if this were a known case.
* It can emit the unknown item (type, value, and, for tags, the
decoded tagged data item) to the application calling the decoder,
with an indication that the decoder did not recognize that tag
number or simple value.
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The latter approach, which is also appropriate for decoders that do
not support validity checking, provides forward compatibility with
newly registered tags and simple values without the requirement to
update the encoder at the same time as the calling application. (For
this, the API for the decoder needs to have a way to mark unknown
items so that the calling application can handle them in a manner
appropriate for the program.)
Since some of the processing needed for validity checking may have an
appreciable cost (in particular with duplicate detection for maps),
support of validity checking is not a requirement placed on all CBOR
decoders.
Some encoders will rely on their applications to provide input data
in such a way that valid CBOR results from the encoder. A generic
encoder may also want to provide a validity-checking mode where it
reliably limits its output to valid CBOR, independent of whether or
not its application is indeed providing API-conformant data.
5.5. Numbers
CBOR-based protocols should take into account that different language
environments pose different restrictions on the range and precision
of numbers that are representable. For example, the basic JavaScript
number system treats all numbers as floating-point values, which may
result in silent loss of precision in decoding integers with more
than 53 significant bits. Another example is that, since CBOR keeps
the sign bit for its integer representation in the major type, it has
one bit more for signed numbers of a certain length (e.g.,
-2**64..2**64-1 for 1+8-byte integers) than the typical platform
signed integer representation of the same length (-2**63..2**63-1 for
8-byte int64_t). A protocol that uses numbers should define its
expectations on the handling of non-trivial numbers in decoders and
receiving applications.
A CBOR-based protocol that includes floating-point numbers can
restrict which of the three formats (half-precision, single-
precision, and double-precision) are to be supported. For an
integer-only application, a protocol may want to completely exclude
the use of floating-point values.
A CBOR-based protocol designed for compactness may want to exclude
specific integer encodings that are longer than necessary for the
application, such as to save the need to implement 64-bit integers.
There is an expectation that encoders will use the most compact
integer representation that can represent a given value. However, a
compact application that does not require deterministic encoding
should accept values that use a longer-than-needed encoding (such as
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encoding "0" as 0b000_11001 followed by two bytes of 0x00) as long as
the application can decode an integer of the given size. Similar
considerations apply to floating-point values; decoding both
preferred serializations and longer-than-needed ones is recommended.
CBOR-based protocols for constrained applications that provide a
choice between representing a specific number as an integer and as a
decimal fraction or bigfloat (such as when the exponent is small and
non-negative), might express a quality-of-implementation expectation
that the integer representation is used directly.
5.6. Specifying Keys for Maps
The encoding and decoding applications need to agree on what types of
keys are going to be used in maps. In applications that need to
interwork with JSON-based applications, conversion is simplified by
limiting keys to text strings only; otherwise, there has to be a
specified mapping from the other CBOR types to text strings, and this
often leads to implementation errors. In applications where keys are
numeric in nature and numeric ordering of keys is important to the
application, directly using the numbers for the keys is useful.
If multiple types of keys are to be used, consideration should be
given to how these types would be represented in the specific
programming environments that are to be used. For example, in
JavaScript Maps [ECMA262], a key of integer 1 cannot be distinguished
from a key of floating-point 1.0. This means that, if integer keys
are used, the protocol needs to avoid use of floating-point keys the
values of which happen to be integer numbers in the same map.
Decoders that deliver data items nested within a CBOR data item
immediately on decoding them ("streaming decoders") often do not keep
the state that is necessary to ascertain uniqueness of a key in a
map. Similarly, an encoder that can start encoding data items before
the enclosing data item is completely available ("streaming encoder")
may want to reduce its overhead significantly by relying on its data
source to maintain uniqueness.
A CBOR-based protocol MUST define what to do when a receiving
application does see multiple identical keys in a map. The resulting
rule in the protocol MUST respect the CBOR data model: it cannot
prescribe a specific handling of the entries with the identical keys,
except that it might have a rule that having identical keys in a map
indicates a malformed map and that the decoder has to stop with an
error. When processing maps that exhibit entries with duplicate
keys, a generic decoder might do one of the following:
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* Not accept maps with duplicate keys (that is, enforce validity for
maps, see also Section 5.4). These generic decoders are
universally useful. An application may still need to do perform
its own duplicate checking based on application rules (for
instance if the application equates integers and floating-point
values in map key positions for specific maps).
* Pass all map entries to the application, including ones with
duplicate keys. This requires the application to handle (check
against) duplicate keys, even if the application rules are
identical to the generic data model rules.
* Lose some entries with duplicate keys, e.g. by only delivering the
final (or first) entry out of the entries with the same key. With
such a generic decoder, applications may get different results for
a specific key on different runs and with different generic
decoders as which value is returned is based on generic decoder
implementation and the actual order of keys in the map. In
particular, applications cannot validate key uniqueness on their
own as they do not necessarily see all entries; they may not be
able to use such a generic decoder if they do need to validate key
uniqueness. These generic decoders can only be used in situations
where the data source and transfer can be relied upon to always
provide valid maps; this is not possible if the data source and
transfer can be attacked.
Generic decoders need to document which of these three approaches
they implement.
The CBOR data model for maps does not allow ascribing semantics to
the order of the key/value pairs in the map representation. Thus, a
CBOR-based protocol MUST NOT specify that changing the key/value pair
order in a map would change the semantics, except to specify that
some orders are disallowed, for example where they would not meet the
requirements of a deterministic encoding (Section 4.2). (Any
secondary effects of map ordering such as on timing, cache usage, and
other potential side channels are not considered part of the
semantics but may be enough reason on their own for a protocol to
require a deterministic encoding format.)
Applications for constrained devices that have maps where a small
number of frequently used keys can be identified should consider
using small integers as keys; for instance, a set of 24 or fewer
frequent keys can be encoded in a single byte as unsigned integers,
up to 48 if negative integers are also used. Less frequently
occurring keys can then use integers with longer encodings.
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5.6.1. Equivalence of Keys
The specific data model applying to a CBOR data item is used to
determine whether keys occurring in maps are duplicates or distinct.
At the generic data model level, numerically equivalent integer and
floating-point values are distinct from each other, as they are from
the various big numbers (Tags 2 to 5). Similarly, text strings are
distinct from byte strings, even if composed of the same bytes. A
tagged value is distinct from an untagged value or from a value
tagged with a different tag number.
Within each of these groups, numeric values are distinct unless they
are numerically equal (specifically, -0.0 is equal to 0.0); for the
purpose of map key equivalence, NaN (not a number) values are
equivalent if they have the same significand after zero-extending
both significands at the right to 64 bits.
(Byte and text) strings are compared byte by byte, arrays element by
element, and are equal if they have the same number of bytes/elements
and the same values at the same positions. Two maps are equal if
they have the same set of pairs regardless of their order; pairs are
equal if both the key and value are equal.
Tagged values are equal if both the tag number and the tag content
are equal. (Note that a generic decoder that provides processing for
a specific tag may not be able to distinguish some semantically
equivalent values, e.g. if leading zeroes occur in the content of tag
2/3 (Section 3.4.3).) Simple values are equal if they simply have
the same value. Nothing else is equal in the generic data model; a
simple value 2 is not equivalent to an integer 2 and an array is
never equivalent to a map.
As discussed in Section 2.2, specific data models can make values
equivalent for the purpose of comparing map keys that are distinct in
the generic data model. Note that this implies that a generic
decoder may deliver a decoded map to an application that needs to be
checked for duplicate map keys by that application (alternatively,
the decoder may provide a programming interface to perform this
service for the application). Specific data models are not able to
distinguish values for map keys that are equal for this purpose at
the generic data model level.
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5.7. Undefined Values
In some CBOR-based protocols, the simple value (Section 3.3) of
Undefined might be used by an encoder as a substitute for a data item
with an encoding problem, in order to allow the rest of the enclosing
data items to be encoded without harm.
6. Converting Data between CBOR and JSON
This section gives non-normative advice about converting between CBOR
and JSON. Implementations of converters MAY use whichever advice
here they want.
It is worth noting that a JSON text is a sequence of characters, not
an encoded sequence of bytes, while a CBOR data item consists of
bytes, not characters.
6.1. Converting from CBOR to JSON
Most of the types in CBOR have direct analogs in JSON. However, some
do not, and someone implementing a CBOR-to-JSON converter has to
consider what to do in those cases. The following non-normative
advice deals with these by converting them to a single substitute
value, such as a JSON null.
* An integer (major type 0 or 1) becomes a JSON number.
* A byte string (major type 2) that is not embedded in a tag that
specifies a proposed encoding is encoded in base64url without
padding and becomes a JSON string.
* A UTF-8 string (major type 3) becomes a JSON string. Note that
JSON requires escaping certain characters ([RFC8259], Section 7):
quotation mark (U+0022), reverse solidus (U+005C), and the "C0
control characters" (U+0000 through U+001F). All other characters
are copied unchanged into the JSON UTF-8 string.
* An array (major type 4) becomes a JSON array.
* A map (major type 5) becomes a JSON object. This is possible
directly only if all keys are UTF-8 strings. A converter might
also convert other keys into UTF-8 strings (such as by converting
integers into strings containing their decimal representation);
however, doing so introduces a danger of key collision. Note also
that, if tags on UTF-8 strings are ignored as proposed below, this
will cause a key collision if the tags are different but the
strings are the same.
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* False (major type 7, additional information 20) becomes a JSON
false.
* True (major type 7, additional information 21) becomes a JSON
true.
* Null (major type 7, additional information 22) becomes a JSON
null.
* A floating-point value (major type 7, additional information 25
through 27) becomes a JSON number if it is finite (that is, it can
be represented in a JSON number); if the value is non-finite (NaN,
or positive or negative Infinity), it is represented by the
substitute value.
* Any other simple value (major type 7, any additional information
value not yet discussed) is represented by the substitute value.
* A bignum (major type 6, tag number 2 or 3) is represented by
encoding its byte string in base64url without padding and becomes
a JSON string. For tag number 3 (negative bignum), a "~" (ASCII
tilde) is inserted before the base-encoded value. (The conversion
to a binary blob instead of a number is to prevent a likely
numeric overflow for the JSON decoder.)
* A byte string with an encoding hint (major type 6, tag number 21
through 23) is encoded as described by the hint and becomes a JSON
string.
* For all other tags (major type 6, any other tag number), the tag
content is represented as a JSON value; the tag number is ignored.
* Indefinite-length items are made definite before conversion.
A CBOR-to-JSON converter may want to keep to the JSON profile I-JSON
[RFC7493], to maximize interoperability and increase confidence that
the JSON output can be processed with predictable results. For
example, this has implications on the range of integers that can be
represented reliably, as well as on the top-level items that may be
supported by older JSON implementations.
6.2. Converting from JSON to CBOR
All JSON values, once decoded, directly map into one or more CBOR
values. As with any kind of CBOR generation, decisions have to be
made with respect to number representation. In a suggested
conversion:
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* JSON numbers without fractional parts (integer numbers) are
represented as integers (major types 0 and 1, possibly major type
6 tag number 2 and 3), choosing the shortest form; integers longer
than an implementation-defined threshold may instead be
represented as floating-point values. The default range that is
represented as integer is -2**53+1..2**53-1 (fully exploiting the
range for exact integers in the binary64 representation often used
for decoding JSON [RFC7493]). A CBOR-based protocol, or a generic
converter implementation, may choose -2**32..2**32-1 or
-2**64..2**64-1 (fully using the integer ranges available in CBOR
with uint32_t or uint64_t, respectively) or even -2**31..2**31-1
or -2**63..2**63-1 (using popular ranges for two's complement
signed integers). (If the JSON was generated from a JavaScript
implementation, its precision is already limited to 53 bits
maximum.)
* Numbers with fractional parts are represented as floating-point
values, performing the decimal-to-binary conversion based on the
precision provided by IEEE 754 binary64. The mathematical value
of the JSON number is converted to binary64 using the
roundTiesToEven procedure in Section 4.3.1 of [IEEE754]. Then,
when encoding in CBOR, the preferred serialization uses the
shortest floating-point representation exactly representing this
conversion result; for instance, 1.5 is represented in a 16-bit
floating-point value (not all implementations will be capable of
efficiently finding the minimum form, though). Instead of using
the default binary64 precision, there may be an implementation-
defined limit to the precision of the conversion that will affect
the precision of the represented values. Decimal representation
should only be used on the CBOR side if that is specified in a
protocol.
CBOR has been designed to generally provide a more compact encoding
than JSON. One implementation strategy that might come to mind is to
perform a JSON-to-CBOR encoding in place in a single buffer. This
strategy would need to carefully consider a number of pathological
cases, such as that some strings represented with no or very few
escapes and longer (or much longer) than 255 bytes may expand when
encoded as UTF-8 strings in CBOR. Similarly, a few of the binary
floating-point representations might cause expansion from some short
decimal representations (1.1, 1e9) in JSON. This may be hard to get
right, and any ensuing vulnerabilities may be exploited by an
attacker.
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7. Future Evolution of CBOR
Successful protocols evolve over time. New ideas appear,
implementation platforms improve, related protocols are developed and
evolve, and new requirements from applications and protocols are
added. Facilitating protocol evolution is therefore an important
design consideration for any protocol development.
For protocols that will use CBOR, CBOR provides some useful
mechanisms to facilitate their evolution. Best practices for this
are well known, particularly from JSON format development of JSON-
based protocols. Therefore, such best practices are outside the
scope of this specification.
However, facilitating the evolution of CBOR itself is very well
within its scope. CBOR is designed to both provide a stable basis
for development of CBOR-based protocols and to be able to evolve.
Since a successful protocol may live for decades, CBOR needs to be
designed for decades of use and evolution. This section provides
some guidance for the evolution of CBOR. It is necessarily more
subjective than other parts of this document. It is also necessarily
incomplete, lest it turn into a textbook on protocol development.
7.1. Extension Points
In a protocol design, opportunities for evolution are often included
in the form of extension points. For example, there may be a
codepoint space that is not fully allocated from the outset, and the
protocol is designed to tolerate and embrace implementations that
start using more codepoints than initially allocated.
Sizing the codepoint space may be difficult because the range
required may be hard to predict. Protocol designs should attempt to
make the codepoint space large enough so that it can slowly be filled
over the intended lifetime of the protocol.
CBOR has three major extension points:
* the "simple" space (values in major type 7). Of the 24 efficient
(and 224 slightly less efficient) values, only a small number have
been allocated. Implementations receiving an unknown simple data
item may easily be able to process it as such, given that the
structure of the value is indeed simple. The IANA registry in
Section 9.1 is the appropriate way to address the extensibility of
this codepoint space.
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* the "tag" space (values in major type 6). The total codepoint
space is abundant; only a tiny part of it has been allocated.
However, not all of these codepoints are equally efficient: the
first 24 only consume a single ("1+0") byte, and half of them have
already been allocated. The next 232 values only consume two
("1+1") bytes, with nearly a quarter already allocated. These
subspaces need some curation to last for a few more decades.
Implementations receiving an unknown tag number can choose to
process just the enclosed tag content or, preferably, to process
the tag as an unknown tag number wrapping the tag content. The
IANA registry in Section 9.2 is the appropriate way to address the
extensibility of this codepoint space.
* the "additional information" space. An implementation receiving
an unknown additional information value has no way to continue
decoding, so allocating codepoints in this space is a major step
beyond just exercising an extension point. There are also very
few codepoints left. See also Section 7.2.
7.2. Curating the Additional Information Space
The human mind is sometimes drawn to filling in little perceived gaps
to make something neat. We expect the remaining gaps in the
codepoint space for the additional information values to be an
attractor for new ideas, just because they are there.
The present specification does not manage the additional information
codepoint space by an IANA registry. Instead, allocations out of
this space can only be done by updating this specification.
For an additional information value of n >= 24, the size of the
additional data typically is 2**(n-24) bytes. Therefore, additional
information values 28 and 29 should be viewed as candidates for
128-bit and 256-bit quantities, in case a need arises to add them to
the protocol. Additional information value 30 is then the only
additional information value available for general allocation, and
there should be a very good reason for allocating it before assigning
it through an update of the present specification.
8. Diagnostic Notation
CBOR is a binary interchange format. To facilitate documentation and
debugging, and in particular to facilitate communication between
entities cooperating in debugging, this section defines a simple
human-readable diagnostic notation. All actual interchange always
happens in the binary format.
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Note that this truly is a diagnostic format; it is not meant to be
parsed. Therefore, no formal definition (as in ABNF) is given in
this document. (Implementers looking for a text-based format for
representing CBOR data items in configuration files may also want to
consider YAML [YAML].)
The diagnostic notation is loosely based on JSON as it is defined in
RFC 8259, extending it where needed.
The notation borrows the JSON syntax for numbers (integer and
floating-point), True (>true<), False (>false<), Null (>null<), UTF-8
strings, arrays, and maps (maps are called objects in JSON; the
diagnostic notation extends JSON here by allowing any data item in
the key position). Undefined is written >undefined< as in
JavaScript. The non-finite floating-point numbers Infinity,
-Infinity, and NaN are written exactly as in this sentence (this is
also a way they can be written in JavaScript, although JSON does not
allow them). A tag is written as an integer number for the tag
number, followed by the tag content in parentheses; for instance, an
RFC 3339 (ISO 8601) date could be notated as:
0("2013-03-21T20:04:00Z")
or the equivalent relative time as
1(1363896240)
Byte strings are notated in one of the base encodings, without
padding, enclosed in single quotes, prefixed by >h< for base16, >b32<
for base32, >h32< for base32hex, >b64< for base64 or base64url (the
actual encodings do not overlap, so the string remains unambiguous).
For example, the byte string 0x12345678 could be written h'12345678',
b32'CI2FM6A', or b64'EjRWeA'.
Unassigned simple values are given as "simple()" with the appropriate
integer in the parentheses. For example, "simple(42)" indicates
major type 7, value 42.
A number of useful extensions to the diagnostic notation defined here
are provided in Appendix G of [RFC8610], "Extended Diagnostic
Notation" (EDN). Similarly, an extension of this notation could be
provided in a separate document to provide for the documentation of
NaN payloads, which are not covered in the present document.
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8.1. Encoding Indicators
Sometimes it is useful to indicate in the diagnostic notation which
of several alternative representations were actually used; for
example, a data item written >1.5< by a diagnostic decoder might have
been encoded as a half-, single-, or double-precision float.
The convention for encoding indicators is that anything starting with
an underscore and all following characters that are alphanumeric or
underscore, is an encoding indicator, and can be ignored by anyone
not interested in this information. For example, "_" or "_3".
Encoding indicators are always optional.
A single underscore can be written after the opening brace of a map
or the opening bracket of an array to indicate that the data item was
represented in indefinite-length format. For example, [_ 1, 2]
contains an indicator that an indefinite-length representation was
used to represent the data item [1, 2].
An underscore followed by a decimal digit n indicates that the
preceding item (or, for arrays and maps, the item starting with the
preceding bracket or brace) was encoded with an additional
information value of 24+n. For example, 1.5_1 is a half-precision
floating-point number, while 1.5_3 is encoded as double precision.
This encoding indicator is not shown in Appendix A. (Note that the
encoding indicator "_" is thus an abbreviation of the full form "_7",
which is not used.)
The detailed chunk structure of byte and text strings of indefinite
length can be notated in the form (_ h'0123', h'4567') and (_ "foo",
"bar"). However, for an indefinite length string with no chunks
inside, (_ ) would be ambiguous whether a byte string (0x5fff) or a
text string (0x7fff) is meant and is therefore not used. The basic
forms ''_ and ""_ can be used instead and are reserved for the case
with no chunks only -- not as short forms for the (permitted, but not
really useful) encodings with only empty chunks, which to preserve
the chunk structure need to be notated as (_ ''), (_ ""), etc.
9. IANA Considerations
IANA has created two registries for new CBOR values. The registries
are separate, that is, not under an umbrella registry, and follow the
rules in [RFC8126]. IANA has also assigned a new MIME media type and
an associated Constrained Application Protocol (CoAP) Content-Format
entry.
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9.1. Simple Values Registry
IANA has created the "Concise Binary Object Representation (CBOR)
Simple Values" registry at [IANA.cbor-simple-values]. The initial
values are shown in Table 4.
New entries in the range 0 to 19 are assigned by Standards Action.
It is suggested that these Standards Actions allocate values starting
with the number 16 in order to reserve the lower numbers for
contiguous blocks (if any).
New entries in the range 32 to 255 are assigned by Specification
Required.
9.2. Tags Registry
IANA has created the "Concise Binary Object Representation (CBOR)
Tags" registry at [IANA.cbor-tags]. The tags that were defined in
[RFC7049] are described in detail in Section 3.4, and other tags have
already been defined since then.
New entries in the range 0 to 23 ("1+0") are assigned by Standards
Action. New entries in the ranges 24 to 255 ("1+1") and 256 to 32767
(lower half of "1+2") are assigned by Specification Required. New
entries in the range 32768 to 18446744073709551615 (upper half of
"1+2", "1+4", and "1+8") are assigned by First Come First Served.
The template for registration requests is:
* Data item
* Semantics (short form)
In addition, First Come First Served requests should include:
* Point of contact
* Description of semantics (URL) -- This description is optional;
the URL can point to something like an Internet-Draft or a web
page.
Applicants exercising the First Come First Served range and making a
suggestion for a tag number that is not representable in 32 bits
(i.e., larger than 4294967295) should be aware that this could reduce
interoperability with implementations that do not support 64-bit
numbers.
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9.3. Media Type ("MIME Type")
The Internet media type [RFC6838] for a single encoded CBOR data item
is application/cbor, as defined in [IANA.media-types]:
Type name: application
Subtype name: cbor
Required parameters: n/a
Optional parameters: n/a
Encoding considerations: Binary
Security considerations: See Section 10 of this document
Interoperability considerations: n/a
Published specification: This document
Applications that use this media type: Many
Additional information:
* Magic number(s): n/a
* File extension(s): .cbor
* Macintosh file type code(s): n/a
Person & email address to contact for further information: IETF CBOR
Working Group cbor@ietf.org (mailto:cbor@ietf.org) or IETF
Applications and Real-Time Area art@ietf.org (mailto:art@ietf.org)
Intended usage: COMMON
Restrictions on usage: none
Author: IETF CBOR Working Group cbor@ietf.org (mailto:cbor@ietf.org)
Change controller: The IESG iesg@ietf.org (mailto:iesg@ietf.org)
9.4. CoAP Content-Format
The CoAP Content-Format for CBOR is registered in
[IANA.core-parameters]:
Media Type: application/cbor
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Encoding: -
Id: 60
Reference: [RFCthis]
9.5. The +cbor Structured Syntax Suffix Registration
The Structured Syntax Suffix [RFC6838] for media types based on a
single encoded CBOR data item is +cbor, as defined in
[IANA.media-type-structured-suffix]:
Name: Concise Binary Object Representation (CBOR)
+suffix: +cbor
References: [RFCthis]
Encoding Considerations: CBOR is a binary format.
Interoperability Considerations: n/a
Fragment Identifier Considerations: The syntax and semantics of
fragment identifiers specified for +cbor SHOULD be as specified
for "application/cbor". (At publication of this document, there
is no fragment identification syntax defined for "application/
cbor".)
The syntax and semantics for fragment identifiers for a specific
"xxx/yyy+cbor" SHOULD be processed as follows:
* For cases defined in +cbor, where the fragment identifier
resolves per the +cbor rules, then process as specified in
+cbor.
* For cases defined in +cbor, where the fragment identifier does
not resolve per the +cbor rules, then process as specified in
"xxx/yyy+cbor".
* For cases not defined in +cbor, then process as specified in
"xxx/yyy+cbor".
Security Considerations: See Section 10 of this document
Contact: IETF CBOR Working Group cbor@ietf.org
(mailto:cbor@ietf.org) or IETF Applications and Real-Time Area
art@ietf.org (mailto:art@ietf.org)
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Author/Change Controller: The IESG iesg@ietf.org
(mailto:iesg@ietf.org)
10. Security Considerations
A network-facing application can exhibit vulnerabilities in its
processing logic for incoming data. Complex parsers are well known
as a likely source of such vulnerabilities, such as the ability to
remotely crash a node, or even remotely execute arbitrary code on it.
CBOR attempts to narrow the opportunities for introducing such
vulnerabilities by reducing parser complexity, by giving the entire
range of encodable values a meaning where possible.
Because CBOR decoders are often used as a first step in processing
unvalidated input, they need to be fully prepared for all types of
hostile input that may be designed to corrupt, overrun, or achieve
control of the system decoding the CBOR data item. A CBOR decoder
needs to assume that all input may be hostile even if it has been
checked by a firewall, has come over a secure channel such as TLS, is
encrypted or signed, or has come from some other source that is
presumed trusted.
Section 4.1 gives examples of limitations in interoperability when
using a constrained CBOR decoder with input from a CBOR encoder that
uses a non-preferred serialization. When a single data item is
consumed both by such a constrained decoder and a full decoder, it
can lead to security issues that can be exploited by an attacker who
can inject or manipulate content.
As discussed throughout this document, there are many values that can
be considered "equivalent" in some circumstances and "not equivalent"
in others. As just one example, the numeric value for the number
"one" might be expressed as an integer or a bignum. A system
interpreting CBOR input might accept either form for the number
"one", or might reject one (or both) forms. Such acceptance or
rejection can have security implications in the program that is using
the interpreted input.
Hostile input may be constructed to overrun buffers, overflow or
underflow integer arithmetic, or cause other decoding disruption.
CBOR data items might have lengths or sizes that are intentionally
extremely large or too short. Resource exhaustion attacks might
attempt to lure a decoder into allocating very big data items
(strings, arrays, maps, or even arbitrary precision numbers) or
exhaust the stack depth by setting up deeply nested items. Decoders
need to have appropriate resource management to mitigate these
attacks. (Items for which very large sizes are given can also
attempt to exploit integer overflow vulnerabilities.)
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A CBOR decoder, by definition, only accepts well-formed CBOR; this is
the first step to its robustness. Input that is not well-formed CBOR
causes no further processing from the point where the lack of well-
formedness was detected. If possible, any data decoded up to this
point should have no impact on the application using the CBOR
decoder.
In addition to ascertaining well-formedness, a CBOR decoder might
also perform validity checks on the CBOR data. Alternatively, it can
leave those checks to the application using the decoder. This choice
needs to be clearly documented in the decoder. Beyond the validity
at the CBOR level, an application also needs to ascertain that the
input is in alignment with the application protocol that is
serialized in CBOR.
The input check itself may consume resources. This is usually linear
in the size of the input, which means that an attacker has to spend
resources that are commensurate to the resources spent by the
defender on input validation. However, an attacker might be able to
craft inputs that will take longer for a target decoder to process
than for the attacker to produce. Processing for arbitrary-precision
numbers may exceed linear effort. Also, some hash-table
implementations that are used by decoders to build in-memory
representations of maps can be attacked to spend quadratic effort,
unless a secret key (see Section 7 of [SIPHASH_LNCS], also
[SIPHASH_OPEN]) or some other mitigation is employed. Such
superlinear efforts can be exploited by an attacker to exhaust
resources at or before the input validator; they therefore need to be
avoided in a CBOR decoder implementation. Note that tag number
definitions and their implementations can add security considerations
of this kind; this should then be discussed in the security
considerations of the tag number definition.
CBOR encoders do not receive input directly from the network and are
thus not directly attackable in the same way as CBOR decoders.
However, CBOR encoders often have an API that takes input from
another level in the implementation and can be attacked through that
API. The design and implementation of that API should assume the
behavior of its caller may be based on hostile input or on coding
mistakes. It should check inputs for buffer overruns, overflow and
underflow of integer arithmetic, and other such errors that are aimed
to disrupt the encoder.
Protocols should be defined in such a way that potential multiple
interpretations are reliably reduced to a single interpretation. For
example, an attacker could make use of invalid input such as
duplicate keys in maps, or exploit different precision in processing
numbers to make one application base its decisions on a different
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interpretation than the one that will be used by a second
application. To facilitate consistent interpretation, encoder and
decoder implementations should provide a validity checking mode of
operation (Section 5.4). Note, however, that a generic decoder
cannot know about all requirements that an application poses on its
input data; it is therefore not relieving the application from
performing its own input checking. Also, since the set of defined
tag numbers evolves, the application may employ a tag number that is
not yet supported for validity checking by the generic decoder it
uses. Generic decoders therefore need to provide documentation which
tag numbers they support and what validity checking they can provide
for each of them as well as for basic CBOR validity (UTF-8 checking,
duplicate map key checking).
Section 3.4.3 notes that using the non-preferred choice of a bignum
representation instead of a basic integer for encoding a number is
not intended to have application semantics, but it can have such
semantics if an application receiving CBOR data is using a decoder in
the basic generic data model. This disparity causes a security issue
if the two sets of semantics differ. Thus, applications using CBOR
need to specify the data model that they are using for each use of
CBOR data.
It is common to convert CBOR data to other formats. In many cases,
CBOR has more expressive types than other formats; this is
particularly true for the common conversion to JSON. The loss of
type information can cause security issues for the systems that are
processing the less-expressive data.
Section 6.2 describes a possibly-common usage scenario of converting
between CBOR and JSON that could allow an attack if the attcker knows
that the application is performing the conversion.
Security considerations for the use of base16 and base64 from
[RFC4648], and the use of UTF-8 from [RFC3629], are relevant to CBOR
as well.
11. References
11.1. Normative References
[C] International Organization for Standardization,
"Information technology — Programming languages — C", ISO/
IEC 9899:2018, Fourth Edition, June 2018.
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[Cplusplus17]
International Organization for Standardization,
"Programming languages — C++", ISO/IEC 14882:2017, Fifth
Edition, December 2017.
[IEEE754] IEEE, "IEEE Standard for Floating-Point Arithmetic", IEEE
Std 754-2019, DOI 10.1109/IEEESTD.2019.8766229,
<https://ieeexplore.ieee.org/document/8766229>.
[RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message
Bodies", RFC 2045, DOI 10.17487/RFC2045, November 1996,
<https://www.rfc-editor.org/info/rfc2045>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC3339] Klyne, G. and C. Newman, "Date and Time on the Internet:
Timestamps", RFC 3339, DOI 10.17487/RFC3339, July 2002,
<https://www.rfc-editor.org/info/rfc3339>.
[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November
2003, <https://www.rfc-editor.org/info/rfc3629>.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, DOI 10.17487/RFC3986, January 2005,
<https://www.rfc-editor.org/info/rfc3986>.
[RFC4287] Nottingham, M., Ed. and R. Sayre, Ed., "The Atom
Syndication Format", RFC 4287, DOI 10.17487/RFC4287,
December 2005, <https://www.rfc-editor.org/info/rfc4287>.
[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
<https://www.rfc-editor.org/info/rfc4648>.
[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/info/rfc8126>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
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[TIME_T] The Open Group Base Specifications, "Open Group Standard:
Vol. 1: Base Definitions, Issue 7", Section 4.16 'Seconds
Since the Epoch', IEEE Std 1003.1, 2018 Edition, 2018,
<http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/
V1_chap04.html#tag_04_16>.
11.2. Informative References
[ASN.1] International Telecommunication Union, "Information
Technology — ASN.1 encoding rules: Specification of Basic
Encoding Rules (BER), Canonical Encoding Rules (CER) and
Distinguished Encoding Rules (DER)", ITU-T Recommendation
X.690, 1994.
[BSON] Various, "BSON - Binary JSON", 2013,
<http://bsonspec.org/>.
[ECMA262] Ecma International, "ECMAScript 2018 Language
Specification", ECMA Standard ECMA-262, 9th Edition, June
2018, <https://www.ecma-
international.org/publications/files/ECMA-ST/Ecma-
262.pdf>.
[I-D.bormann-cbor-notable-tags]
Bormann, C., "Notable CBOR Tags", Work in Progress,
Internet-Draft, draft-bormann-cbor-notable-tags-02, 25
June 2020, <http://www.ietf.org/internet-drafts/draft-
bormann-cbor-notable-tags-02.txt>.
[IANA.cbor-simple-values]
IANA, "Concise Binary Object Representation (CBOR) Simple
Values",
<http://www.iana.org/assignments/cbor-simple-values>.
[IANA.cbor-tags]
IANA, "Concise Binary Object Representation (CBOR) Tags",
<http://www.iana.org/assignments/cbor-tags>.
[IANA.core-parameters]
IANA, "Constrained RESTful Environments (CoRE)
Parameters",
<http://www.iana.org/assignments/core-parameters>.
[IANA.media-type-structured-suffix]
IANA, "Structured Syntax Suffix Registry",
<http://www.iana.org/assignments/media-type-structured-
suffix>.
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[IANA.media-types]
IANA, "Media Types",
<http://www.iana.org/assignments/media-types>.
[MessagePack]
Furuhashi, S., "MessagePack", 2013, <http://msgpack.org/>.
[PCRE] Ho, A., "PCRE - Perl Compatible Regular Expressions",
2018, <http://www.pcre.org/>.
[RFC0713] Haverty, J., "MSDTP-Message Services Data Transmission
Protocol", RFC 713, DOI 10.17487/RFC0713, April 1976,
<https://www.rfc-editor.org/info/rfc713>.
[RFC6838] Freed, N., Klensin, J., and T. Hansen, "Media Type
Specifications and Registration Procedures", BCP 13,
RFC 6838, DOI 10.17487/RFC6838, January 2013,
<https://www.rfc-editor.org/info/rfc6838>.
[RFC7049] Bormann, C. and P. Hoffman, "Concise Binary Object
Representation (CBOR)", RFC 7049, DOI 10.17487/RFC7049,
October 2013, <https://www.rfc-editor.org/info/rfc7049>.
[RFC7228] Bormann, C., Ersue, M., and A. Keranen, "Terminology for
Constrained-Node Networks", RFC 7228,
DOI 10.17487/RFC7228, May 2014,
<https://www.rfc-editor.org/info/rfc7228>.
[RFC7493] Bray, T., Ed., "The I-JSON Message Format", RFC 7493,
DOI 10.17487/RFC7493, March 2015,
<https://www.rfc-editor.org/info/rfc7493>.
[RFC7991] Hoffman, P., "The "xml2rfc" Version 3 Vocabulary",
RFC 7991, DOI 10.17487/RFC7991, December 2016,
<https://www.rfc-editor.org/info/rfc7991>.
[RFC8259] Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
Interchange Format", STD 90, RFC 8259,
DOI 10.17487/RFC8259, December 2017,
<https://www.rfc-editor.org/info/rfc8259>.
[RFC8610] Birkholz, H., Vigano, C., and C. Bormann, "Concise Data
Definition Language (CDDL): A Notational Convention to
Express Concise Binary Object Representation (CBOR) and
JSON Data Structures", RFC 8610, DOI 10.17487/RFC8610,
June 2019, <https://www.rfc-editor.org/info/rfc8610>.
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[RFC8618] Dickinson, J., Hague, J., Dickinson, S., Manderson, T.,
and J. Bond, "Compacted-DNS (C-DNS): A Format for DNS
Packet Capture", RFC 8618, DOI 10.17487/RFC8618, September
2019, <https://www.rfc-editor.org/info/rfc8618>.
[RFC8742] Bormann, C., "Concise Binary Object Representation (CBOR)
Sequences", RFC 8742, DOI 10.17487/RFC8742, February 2020,
<https://www.rfc-editor.org/info/rfc8742>.
[RFC8746] Bormann, C., Ed., "Concise Binary Object Representation
(CBOR) Tags for Typed Arrays", RFC 8746,
DOI 10.17487/RFC8746, February 2020,
<https://www.rfc-editor.org/info/rfc8746>.
[SIPHASH_LNCS]
Aumasson, J. and D. Bernstein, "SipHash: A Fast Short-
Input PRF", Lecture Notes in Computer Science pp. 489-508,
DOI 10.1007/978-3-642-34931-7_28, 2012,
<https://doi.org/10.1007/978-3-642-34931-7_28>.
[SIPHASH_OPEN]
Aumasson, J. and D.J. Bernstein, "SipHash: a fast short-
input PRF", <https://131002.net/siphash/siphash.pdf>.
[YAML] Ben-Kiki, O., Evans, C., and I.d. Net, "YAML Ain't Markup
Language (YAML[TM]) Version 1.2", 3rd Edition, October
2009, <http://www.yaml.org/spec/1.2/spec.html>.
Appendix A. Examples of Encoded CBOR Data Items
The following table provides some CBOR-encoded values in hexadecimal
(right column), together with diagnostic notation for these values
(left column). Note that the string "\u00fc" is one form of
diagnostic notation for a UTF-8 string containing the single Unicode
character U+00FC, LATIN SMALL LETTER U WITH DIAERESIS (u umlaut).
Similarly, "\u6c34" is a UTF-8 string in diagnostic notation with a
single character U+6C34 (CJK UNIFIED IDEOGRAPH-6C34, often
representing "water"), and "\ud800\udd51" is a UTF-8 string in
diagnostic notation with a single character U+10151 (GREEK ACROPHONIC
ATTIC FIFTY STATERS). (Note that all these single-character strings
could also be represented in native UTF-8 in diagnostic notation,
just not in an ASCII-only specification.) In the diagnostic notation
provided for bignums, their intended numeric value is shown as a
decimal number (such as 18446744073709551616) instead of showing a
tagged byte string (such as 2(h'010000000000000000')).
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+==============================+====================================+
|Diagnostic | Encoded |
+==============================+====================================+
|0 | 0x00 |
+------------------------------+------------------------------------+
|1 | 0x01 |
+------------------------------+------------------------------------+
|10 | 0x0a |
+------------------------------+------------------------------------+
|23 | 0x17 |
+------------------------------+------------------------------------+
|24 | 0x1818 |
+------------------------------+------------------------------------+
|25 | 0x1819 |
+------------------------------+------------------------------------+
|100 | 0x1864 |
+------------------------------+------------------------------------+
|1000 | 0x1903e8 |
+------------------------------+------------------------------------+
|1000000 | 0x1a000f4240 |
+------------------------------+------------------------------------+
|1000000000000 | 0x1b000000e8d4a51000 |
+------------------------------+------------------------------------+
|18446744073709551615 | 0x1bffffffffffffffff |
+------------------------------+------------------------------------+
|18446744073709551616 | 0xc249010000000000000000 |
+------------------------------+------------------------------------+
|-18446744073709551616 | 0x3bffffffffffffffff |
+------------------------------+------------------------------------+
|-18446744073709551617 | 0xc349010000000000000000 |
+------------------------------+------------------------------------+
|-1 | 0x20 |
+------------------------------+------------------------------------+
|-10 | 0x29 |
+------------------------------+------------------------------------+
|-100 | 0x3863 |
+------------------------------+------------------------------------+
|-1000 | 0x3903e7 |
+------------------------------+------------------------------------+
|0.0 | 0xf90000 |
+------------------------------+------------------------------------+
|-0.0 | 0xf98000 |
+------------------------------+------------------------------------+
|1.0 | 0xf93c00 |
+------------------------------+------------------------------------+
|1.1 | 0xfb3ff199999999999a |
+------------------------------+------------------------------------+
|1.5 | 0xf93e00 |
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+------------------------------+------------------------------------+
|65504.0 | 0xf97bff |
+------------------------------+------------------------------------+
|100000.0 | 0xfa47c35000 |
+------------------------------+------------------------------------+
|3.4028234663852886e+38 | 0xfa7f7fffff |
+------------------------------+------------------------------------+
|1.0e+300 | 0xfb7e37e43c8800759c |
+------------------------------+------------------------------------+
|5.960464477539063e-8 | 0xf90001 |
+------------------------------+------------------------------------+
|0.00006103515625 | 0xf90400 |
+------------------------------+------------------------------------+
|-4.0 | 0xf9c400 |
+------------------------------+------------------------------------+
|-4.1 | 0xfbc010666666666666 |
+------------------------------+------------------------------------+
|Infinity | 0xf97c00 |
+------------------------------+------------------------------------+
|NaN | 0xf97e00 |
+------------------------------+------------------------------------+
|-Infinity | 0xf9fc00 |
+------------------------------+------------------------------------+
|Infinity | 0xfa7f800000 |
+------------------------------+------------------------------------+
|NaN | 0xfa7fc00000 |
+------------------------------+------------------------------------+
|-Infinity | 0xfaff800000 |
+------------------------------+------------------------------------+
|Infinity | 0xfb7ff0000000000000 |
+------------------------------+------------------------------------+
|NaN | 0xfb7ff8000000000000 |
+------------------------------+------------------------------------+
|-Infinity | 0xfbfff0000000000000 |
+------------------------------+------------------------------------+
|false | 0xf4 |
+------------------------------+------------------------------------+
|true | 0xf5 |
+------------------------------+------------------------------------+
|null | 0xf6 |
+------------------------------+------------------------------------+
|undefined | 0xf7 |
+------------------------------+------------------------------------+
|simple(16) | 0xf0 |
+------------------------------+------------------------------------+
|simple(255) | 0xf8ff |
+------------------------------+------------------------------------+
|0("2013-03-21T20:04:00Z") | 0xc074323031332d30332d32315432303a |
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| | 30343a30305a |
+------------------------------+------------------------------------+
|1(1363896240) | 0xc11a514b67b0 |
+------------------------------+------------------------------------+
|1(1363896240.5) | 0xc1fb41d452d9ec200000 |
+------------------------------+------------------------------------+
|23(h'01020304') | 0xd74401020304 |
+------------------------------+------------------------------------+
|24(h'6449455446') | 0xd818456449455446 |
+------------------------------+------------------------------------+
|32("http://www.example.com") | 0xd82076687474703a2f2f7777772e6578 |
| | 616d706c652e636f6d |
+------------------------------+------------------------------------+
|h'' | 0x40 |
+------------------------------+------------------------------------+
|h'01020304' | 0x4401020304 |
+------------------------------+------------------------------------+
|"" | 0x60 |
+------------------------------+------------------------------------+
|"a" | 0x6161 |
+------------------------------+------------------------------------+
|"IETF" | 0x6449455446 |
+------------------------------+------------------------------------+
|"\"\\" | 0x62225c |
+------------------------------+------------------------------------+
|"\u00fc" | 0x62c3bc |
+------------------------------+------------------------------------+
|"\u6c34" | 0x63e6b0b4 |
+------------------------------+------------------------------------+
|"\ud800\udd51" | 0x64f0908591 |
+------------------------------+------------------------------------+
|[] | 0x80 |
+------------------------------+------------------------------------+
|[1, 2, 3] | 0x83010203 |
+------------------------------+------------------------------------+
|[1, [2, 3], [4, 5]] | 0x8301820203820405 |
+------------------------------+------------------------------------+
|[1, 2, 3, 4, 5, 6, 7, 8, 9, | 0x98190102030405060708090a0b0c0d0e |
|10, 11, 12, 13, 14, 15, 16, | 0f101112131415161718181819 |
|17, 18, 19, 20, 21, 22, 23, | |
|24, 25] | |
+------------------------------+------------------------------------+
|{} | 0xa0 |
+------------------------------+------------------------------------+
|{1: 2, 3: 4} | 0xa201020304 |
+------------------------------+------------------------------------+
|{"a": 1, "b": [2, 3]} | 0xa26161016162820203 |
+------------------------------+------------------------------------+
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|["a", {"b": "c"}] | 0x826161a161626163 |
+------------------------------+------------------------------------+
|{"a": "A", "b": "B", "c": "C",| 0xa5616161416162614261636143616461 |
|"d": "D", "e": "E"} | 4461656145 |
+------------------------------+------------------------------------+
|(_ h'0102', h'030405') | 0x5f42010243030405ff |
+------------------------------+------------------------------------+
|(_ "strea", "ming") | 0x7f657374726561646d696e67ff |
+------------------------------+------------------------------------+
|[_ ] | 0x9fff |
+------------------------------+------------------------------------+
|[_ 1, [2, 3], [_ 4, 5]] | 0x9f018202039f0405ffff |
+------------------------------+------------------------------------+
|[_ 1, [2, 3], [4, 5]] | 0x9f01820203820405ff |
+------------------------------+------------------------------------+
|[1, [2, 3], [_ 4, 5]] | 0x83018202039f0405ff |
+------------------------------+------------------------------------+
|[1, [_ 2, 3], [4, 5]] | 0x83019f0203ff820405 |
+------------------------------+------------------------------------+
|[_ 1, 2, 3, 4, 5, 6, 7, 8, 9, | 0x9f0102030405060708090a0b0c0d0e0f |
|10, 11, 12, 13, 14, 15, 16, | 101112131415161718181819ff |
|17, 18, 19, 20, 21, 22, 23, | |
|24, 25] | |
+------------------------------+------------------------------------+
|{_ "a": 1, "b": [_ 2, 3]} | 0xbf61610161629f0203ffff |
+------------------------------+------------------------------------+
|["a", {_ "b": "c"}] | 0x826161bf61626163ff |
+------------------------------+------------------------------------+
|{_ "Fun": true, "Amt": -2} | 0xbf6346756ef563416d7421ff |
+------------------------------+------------------------------------+
Table 6: Examples of Encoded CBOR Data Items
Appendix B. Jump Table for Initial Byte
For brevity, this jump table does not show initial bytes that are
reserved for future extension. It also only shows a selection of the
initial bytes that can be used for optional features. (All unsigned
integers are in network byte order.)
+============+================================================+
| Byte | Structure/Semantics |
+============+================================================+
| 0x00..0x17 | Unsigned integer 0x00..0x17 (0..23) |
+------------+------------------------------------------------+
| 0x18 | Unsigned integer (one-byte uint8_t follows) |
+------------+------------------------------------------------+
| 0x19 | Unsigned integer (two-byte uint16_t follows) |
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+------------+------------------------------------------------+
| 0x1a | Unsigned integer (four-byte uint32_t follows) |
+------------+------------------------------------------------+
| 0x1b | Unsigned integer (eight-byte uint64_t follows) |
+------------+------------------------------------------------+
| 0x20..0x37 | Negative integer -1-0x00..-1-0x17 (-1..-24) |
+------------+------------------------------------------------+
| 0x38 | Negative integer -1-n (one-byte uint8_t for n |
| | follows) |
+------------+------------------------------------------------+
| 0x39 | Negative integer -1-n (two-byte uint16_t for n |
| | follows) |
+------------+------------------------------------------------+
| 0x3a | Negative integer -1-n (four-byte uint32_t for |
| | n follows) |
+------------+------------------------------------------------+
| 0x3b | Negative integer -1-n (eight-byte uint64_t for |
| | n follows) |
+------------+------------------------------------------------+
| 0x40..0x57 | byte string (0x00..0x17 bytes follow) |
+------------+------------------------------------------------+
| 0x58 | byte string (one-byte uint8_t for n, and then |
| | n bytes follow) |
+------------+------------------------------------------------+
| 0x59 | byte string (two-byte uint16_t for n, and then |
| | n bytes follow) |
+------------+------------------------------------------------+
| 0x5a | byte string (four-byte uint32_t for n, and |
| | then n bytes follow) |
+------------+------------------------------------------------+
| 0x5b | byte string (eight-byte uint64_t for n, and |
| | then n bytes follow) |
+------------+------------------------------------------------+
| 0x5f | byte string, byte strings follow, terminated |
| | by "break" |
+------------+------------------------------------------------+
| 0x60..0x77 | UTF-8 string (0x00..0x17 bytes follow) |
+------------+------------------------------------------------+
| 0x78 | UTF-8 string (one-byte uint8_t for n, and then |
| | n bytes follow) |
+------------+------------------------------------------------+
| 0x79 | UTF-8 string (two-byte uint16_t for n, and |
| | then n bytes follow) |
+------------+------------------------------------------------+
| 0x7a | UTF-8 string (four-byte uint32_t for n, and |
| | then n bytes follow) |
+------------+------------------------------------------------+
| 0x7b | UTF-8 string (eight-byte uint64_t for n, and |
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| | then n bytes follow) |
+------------+------------------------------------------------+
| 0x7f | UTF-8 string, UTF-8 strings follow, terminated |
| | by "break" |
+------------+------------------------------------------------+
| 0x80..0x97 | array (0x00..0x17 data items follow) |
+------------+------------------------------------------------+
| 0x98 | array (one-byte uint8_t for n, and then n data |
| | items follow) |
+------------+------------------------------------------------+
| 0x99 | array (two-byte uint16_t for n, and then n |
| | data items follow) |
+------------+------------------------------------------------+
| 0x9a | array (four-byte uint32_t for n, and then n |
| | data items follow) |
+------------+------------------------------------------------+
| 0x9b | array (eight-byte uint64_t for n, and then n |
| | data items follow) |
+------------+------------------------------------------------+
| 0x9f | array, data items follow, terminated by |
| | "break" |
+------------+------------------------------------------------+
| 0xa0..0xb7 | map (0x00..0x17 pairs of data items follow) |
+------------+------------------------------------------------+
| 0xb8 | map (one-byte uint8_t for n, and then n pairs |
| | of data items follow) |
+------------+------------------------------------------------+
| 0xb9 | map (two-byte uint16_t for n, and then n pairs |
| | of data items follow) |
+------------+------------------------------------------------+
| 0xba | map (four-byte uint32_t for n, and then n |
| | pairs of data items follow) |
+------------+------------------------------------------------+
| 0xbb | map (eight-byte uint64_t for n, and then n |
| | pairs of data items follow) |
+------------+------------------------------------------------+
| 0xbf | map, pairs of data items follow, terminated by |
| | "break" |
+------------+------------------------------------------------+
| 0xc0 | Text-based date/time (data item follows; see |
| | Section 3.4.1) |
+------------+------------------------------------------------+
| 0xc1 | Epoch-based date/time (data item follows; see |
| | Section 3.4.2) |
+------------+------------------------------------------------+
| 0xc2 | Positive bignum (data item "byte string" |
| | follows) |
+------------+------------------------------------------------+
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| 0xc3 | Negative bignum (data item "byte string" |
| | follows) |
+------------+------------------------------------------------+
| 0xc4 | Decimal Fraction (data item "array" follows; |
| | see Section 3.4.4) |
+------------+------------------------------------------------+
| 0xc5 | Bigfloat (data item "array" follows; see |
| | Section 3.4.4) |
+------------+------------------------------------------------+
| 0xc6..0xd4 | (tag) |
+------------+------------------------------------------------+
| 0xd5..0xd7 | Expected Conversion (data item follows; see |
| | Section 3.4.5.2) |
+------------+------------------------------------------------+
| 0xd8..0xdb | (more tags; 1/2/4/8 bytes of tag number and |
| | then a data item follow) |
+------------+------------------------------------------------+
| 0xe0..0xf3 | (simple value) |
+------------+------------------------------------------------+
| 0xf4 | False |
+------------+------------------------------------------------+
| 0xf5 | True |
+------------+------------------------------------------------+
| 0xf6 | Null |
+------------+------------------------------------------------+
| 0xf7 | Undefined |
+------------+------------------------------------------------+
| 0xf8 | (simple value, one byte follows) |
+------------+------------------------------------------------+
| 0xf9 | Half-Precision Float (two-byte IEEE 754) |
+------------+------------------------------------------------+
| 0xfa | Single-Precision Float (four-byte IEEE 754) |
+------------+------------------------------------------------+
| 0xfb | Double-Precision Float (eight-byte IEEE 754) |
+------------+------------------------------------------------+
| 0xff | "break" stop code |
+------------+------------------------------------------------+
Table 7: Jump Table for Initial Byte
Appendix C. Pseudocode
The well-formedness of a CBOR item can be checked by the pseudocode
in Figure 1. The data is well-formed if and only if:
* the pseudocode does not "fail";
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* after execution of the pseudocode, no bytes are left in the input
(except in streaming applications)
The pseudocode has the following prerequisites:
* take(n) reads n bytes from the input data and returns them as a
byte string. If n bytes are no longer available, take(n) fails.
* uint() converts a byte string into an unsigned integer by
interpreting the byte string in network byte order.
* Arithmetic works as in C.
* All variables are unsigned integers of sufficient range.
Note that "well_formed" returns the major type for well-formed
definite length items, but 99 for an indefinite length item (or -1
for a "break" stop code, only if "breakable" is set). This is used
in "well_formed_indefinite" to ascertain that indefinite length
strings only contain definite length strings as chunks.
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well_formed(breakable = false) {
// process initial bytes
ib = uint(take(1));
mt = ib >> 5;
val = ai = ib & 0x1f;
switch (ai) {
case 24: val = uint(take(1)); break;
case 25: val = uint(take(2)); break;
case 26: val = uint(take(4)); break;
case 27: val = uint(take(8)); break;
case 28: case 29: case 30: fail();
case 31:
return well_formed_indefinite(mt, breakable);
}
// process content
switch (mt) {
// case 0, 1, 7 do not have content; just use val
case 2: case 3: take(val); break; // bytes/UTF-8
case 4: for (i = 0; i < val; i++) well_formed(); break;
case 5: for (i = 0; i < val*2; i++) well_formed(); break;
case 6: well_formed(); break; // 1 embedded data item
case 7: if (ai == 24 && val < 32) fail(); // bad simple
}
return mt; // definite-length data item
}
well_formed_indefinite(mt, breakable) {
switch (mt) {
case 2: case 3:
while ((it = well_formed(true)) != -1)
if (it != mt) // need definite-length chunk
fail(); // of same type
break;
case 4: while (well_formed(true) != -1); break;
case 5: while (well_formed(true) != -1) well_formed(); break;
case 7:
if (breakable)
return -1; // signal break out
else fail(); // no enclosing indefinite
default: fail(); // wrong mt
}
return 99; // indefinite-length data item
}
Figure 1: Pseudocode for Well-Formedness Check
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Note that the remaining complexity of a complete CBOR decoder is
about presenting data that has been decoded to the application in an
appropriate form.
Major types 0 and 1 are designed in such a way that they can be
encoded in C from a signed integer without actually doing an if-then-
else for positive/negative (Figure 2). This uses the fact that
(-1-n), the transformation for major type 1, is the same as ~n
(bitwise complement) in C unsigned arithmetic; ~n can then be
expressed as (-1)^n for the negative case, while 0^n leaves n
unchanged for non-negative. The sign of a number can be converted to
-1 for negative and 0 for non-negative (0 or positive) by arithmetic-
shifting the number by one bit less than the bit length of the number
(for example, by 63 for 64-bit numbers).
void encode_sint(int64_t n) {
uint64t ui = n >> 63; // extend sign to whole length
unsigned mt = ui & 0x20; // extract (shifted) major type
ui ^= n; // complement negatives
if (ui < 24)
*p++ = mt + ui;
else if (ui < 256) {
*p++ = mt + 24;
*p++ = ui;
} else
...
Figure 2: Pseudocode for Encoding a Signed Integer
See Section 1.2 for some specific assumptions about the profile of
the C language used in these pieces of code.
Appendix D. Half-Precision
As half-precision floating-point numbers were only added to IEEE 754
in 2008 [IEEE754], today's programming platforms often still only
have limited support for them. It is very easy to include at least
decoding support for them even without such support. An example of a
small decoder for half-precision floating-point numbers in the C
language is shown in Figure 3. A similar program for Python is in
Figure 4; this code assumes that the 2-byte value has already been
decoded as an (unsigned short) integer in network byte order (as
would be done by the pseudocode in Appendix C).
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#include <math.h>
double decode_half(unsigned char *halfp) {
unsigned half = (halfp[0] << 8) + halfp[1];
unsigned exp = (half >> 10) & 0x1f;
unsigned mant = half & 0x3ff;
double val;
if (exp == 0) val = ldexp(mant, -24);
else if (exp != 31) val = ldexp(mant + 1024, exp - 25);
else val = mant == 0 ? INFINITY : NAN;
return half & 0x8000 ? -val : val;
}
Figure 3: C Code for a Half-Precision Decoder
import struct
from math import ldexp
def decode_single(single):
return struct.unpack("!f", struct.pack("!I", single))[0]
def decode_half(half):
valu = (half & 0x7fff) << 13 | (half & 0x8000) << 16
if ((half & 0x7c00) != 0x7c00):
return ldexp(decode_single(valu), 112)
return decode_single(valu | 0x7f800000)
Figure 4: Python Code for a Half-Precision Decoder
Appendix E. Comparison of Other Binary Formats to CBOR's Design
Objectives
The proposal for CBOR follows a history of binary formats that is as
long as the history of computers themselves. Different formats have
had different objectives. In most cases, the objectives of the
format were never stated, although they can sometimes be implied by
the context where the format was first used. Some formats were meant
to be universally usable, although history has proven that no binary
format meets the needs of all protocols and applications.
CBOR differs from many of these formats due to it starting with a set
of objectives and attempting to meet just those. This section
compares a few of the dozens of formats with CBOR's objectives in
order to help the reader decide if they want to use CBOR or a
different format for a particular protocol or application.
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Note that the discussion here is not meant to be a criticism of any
format: to the best of our knowledge, no format before CBOR was meant
to cover CBOR's objectives in the priority we have assigned them. A
brief recap of the objectives from Section 1.1 is:
1. unambiguous encoding of most common data formats from Internet
standards
2. code compactness for encoder or decoder
3. no schema description needed
4. reasonably compact serialization
5. applicability to constrained and unconstrained applications
6. good JSON conversion
7. extensibility
A discussion of CBOR and other formats with respect to a different
set of design objectives is provided in Section 5 and Appendix C of
[RFC8618].
E.1. ASN.1 DER, BER, and PER
[ASN.1] has many serializations. In the IETF, DER and BER are the
most common. The serialized output is not particularly compact for
many items, and the code needed to decode numeric items can be
complex on a constrained device.
Few (if any) IETF protocols have adopted one of the several variants
of Packed Encoding Rules (PER). There could be many reasons for
this, but one that is commonly stated is that PER makes use of the
schema even for parsing the surface structure of the data item,
requiring significant tool support. There are different versions of
the ASN.1 schema language in use, which has also hampered adoption.
E.2. MessagePack
[MessagePack] is a concise, widely implemented counted binary
serialization format, similar in many properties to CBOR, although
somewhat less regular. While the data model can be used to represent
JSON data, MessagePack has also been used in many remote procedure
call (RPC) applications and for long-term storage of data.
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MessagePack has been essentially stable since it was first published
around 2011; it has not yet had a transition. The evolution of
MessagePack is impeded by an imperative to maintain complete
backwards compatibility with existing stored data, while only few
bytecodes are still available for extension. Repeated requests over
the years from the MessagePack user community to separate out binary
and text strings in the encoding recently have led to an extension
proposal that would leave MessagePack's "raw" data ambiguous between
its usages for binary and text data. The extension mechanism for
MessagePack remains unclear.
E.3. BSON
[BSON] is a data format that was developed for the storage of JSON-
like maps (JSON objects) in the MongoDB database. Its major
distinguishing feature is the capability for in-place update, which
prevents a compact representation. BSON uses a counted
representation except for map keys, which are null-byte terminated.
While BSON can be used for the representation of JSON-like objects on
the wire, its specification is dominated by the requirements of the
database application and has become somewhat baroque. The status of
how BSON extensions will be implemented remains unclear.
E.4. MSDTP: RFC 713
Message Services Data Transmission (MSDTP) is a very early example of
a compact message format; it is described in [RFC0713], written in
1976. It is included here for its historical value, not because it
was ever widely used.
E.5. Conciseness on the Wire
While CBOR's design objective of code compactness for encoders and
decoders is a higher priority than its objective of conciseness on
the wire, many people focus on the wire size. Table 8 shows some
encoding examples for the simple nested array [1, [2, 3]]; where some
form of indefinite-length encoding is supported by the encoding,
[_ 1, [2, 3]] (indefinite length on the outer array) is also shown.
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+=============+============================+================+
| Format | [1, [2, 3]] | [_ 1, [2, 3]] |
+=============+============================+================+
| RFC 713 | c2 05 81 c2 02 82 83 | |
+-------------+----------------------------+----------------+
| ASN.1 BER | 30 0b 02 01 01 30 06 02 01 | 30 80 02 01 01 |
| | 02 02 01 03 | 30 06 02 01 02 |
| | | 02 01 03 00 00 |
+-------------+----------------------------+----------------+
| MessagePack | 92 01 92 02 03 | |
+-------------+----------------------------+----------------+
| BSON | 22 00 00 00 10 30 00 01 00 | |
| | 00 00 04 31 00 13 00 00 00 | |
| | 10 30 00 02 00 00 00 10 31 | |
| | 00 03 00 00 00 00 00 | |
+-------------+----------------------------+----------------+
| CBOR | 82 01 82 02 03 | 9f 01 82 02 03 |
| | | ff |
+-------------+----------------------------+----------------+
Table 8: Examples for Different Levels of Conciseness
Appendix F. Well-formedness errors and examples
There are three basic kinds of well-formedness errors that can occur
in decoding a CBOR data item:
* Too much data: There are input bytes left that were not consumed.
This is only an error if the application assumed that the input
bytes would span exactly one data item. Where the application
uses the self-delimiting nature of CBOR encoding to permit
additional data after the data item, as is for example done in
CBOR sequences [RFC8742], the CBOR decoder can simply indicate
what part of the input has not been consumed.
* Too little data: The input data available would need additional
bytes added at their end for a complete CBOR data item. This may
indicate the input is truncated; it is also a common error when
trying to decode random data as CBOR. For some applications,
however, this may not actually be an error, as the application may
not be certain it has all the data yet and can obtain or wait for
additional input bytes. Some of these applications may have an
upper limit for how much additional data can show up; here the
decoder may be able to indicate that the encoded CBOR data item
cannot be completed within this limit.
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* Syntax error: The input data are not consistent with the
requirements of the CBOR encoding, and this cannot be remedied by
adding (or removing) data at the end.
In Appendix C, errors of the first kind are addressed in the first
paragraph/bullet list (requiring "no bytes are left"), and errors of
the second kind are addressed in the second paragraph/bullet list
(failing "if n bytes are no longer available"). Errors of the third
kind are identified in the pseudocode by specific instances of
calling fail(), in order:
* a reserved value is used for additional information (28, 29, 30)
* major type 7, additional information 24, value < 32 (incorrect)
* incorrect substructure of indefinite length byte/text string (may
only contain definite length strings of the same major type)
* "break" stop code (mt=7, ai=31) occurs in a value position of a
map or except at a position directly in an indefinite length item
where also another enclosed data item could occur
* additional information 31 used with major type 0, 1, or 6
F.1. Examples for CBOR data items that are not well-formed
This subsection shows a few examples for CBOR data items that are not
well-formed. Each example is a sequence of bytes each shown in
hexadecimal; multiple examples in a list are separated by commas.
Examples for well-formedness error kind 1 (too much data) can easily
be formed by adding data to a well-formed encoded CBOR data item.
Similarly, examples for well-formedness error kind 2 (too little
data) can be formed by truncating a well-formed encoded CBOR data
item. In test suites, it may be beneficial to specifically test with
incomplete data items that would require large amounts of addition to
be completed (for instance by starting the encoding of a string of a
very large size).
A premature end of the input can occur in a head or within the
enclosed data, which may be bare strings or enclosed data items that
are either counted or should have been ended by a "break" stop code.
* End of input in a head: 18, 19, 1a, 1b, 19 01, 1a 01 02, 1b 01 02
03 04 05 06 07, 38, 58, 78, 98, 9a 01 ff 00, b8, d8, f8, f9 00, fa
00 00, fb 00 00 00
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* Definite length strings with short data: 41, 61, 5a ff ff ff ff
00, 5b ff ff ff ff ff ff ff ff 01 02 03, 7a ff ff ff ff 00, 7b 7f
ff ff ff ff ff ff ff 01 02 03
* Definite length maps and arrays not closed with enough items: 81,
81 81 81 81 81 81 81 81 81, 82 00, a1, a2 01 02, a1 00, a2 00 00
00
* Tag number not followed by tag content: c0
* Indefinite length strings not closed by a "break" stop code: 5f 41
00, 7f 61 00
* Indefinite length maps and arrays not closed by a "break" stop
code: 9f, 9f 01 02, bf, bf 01 02 01 02, 81 9f, 9f 80 00, 9f 9f 9f
9f 9f ff ff ff ff, 9f 81 9f 81 9f 9f ff ff ff
A few examples for the five subkinds of well-formedness error kind 3
(syntax error) are shown below.
Subkind 1:
* Reserved additional information values: 1c, 1d, 1e, 3c, 3d, 3e,
5c, 5d, 5e, 7c, 7d, 7e, 9c, 9d, 9e, bc, bd, be, dc, dd, de, fc,
fd, fe,
Subkind 2:
* Reserved two-byte encodings of simple values: f8 00, f8 01, f8 18,
f8 1f
Subkind 3:
* Indefinite length string chunks not of the correct type: 5f 00 ff,
5f 21 ff, 5f 61 00 ff, 5f 80 ff, 5f a0 ff, 5f c0 00 ff, 5f e0 ff,
7f 41 00 ff
* Indefinite length string chunks not definite length: 5f 5f 41 00
ff ff, 7f 7f 61 00 ff ff
Subkind 4:
* Break occurring on its own outside of an indefinite length item:
ff
* Break occurring in a definite length array or map or a tag: 81 ff,
82 00 ff, a1 ff, a1 ff 00, a1 00 ff, a2 00 00 ff, 9f 81 ff, 9f 82
9f 81 9f 9f ff ff ff ff
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* Break in indefinite length map would lead to odd number of items
(break in a value position): bf 00 ff, bf 00 00 00 ff
Subkind 5:
* Major type 0, 1, 6 with additional information 31: 1f, 3f, df
Appendix G. Changes from RFC 7049
As discussed in the introduction, this document is a revised edition
of RFC 7049, with editorial improvements, added detail, and fixed
errata. This document formally obsoletes RFC 7049, while keeping
full compatibility of the interchange format from RFC 7049. This
document does not create a new version of the format.
G.1. Errata processing, clerical changes
The two verified errata on RFC 7049, EID 3764 and EID 3770, concerned
two encoding examples in the text that have been corrected
(Section 3.4.3: "29" -> "49", Section 5.5: "0b000_11101" ->
"0b000_11001"). Also, RFC 7049 contained an example using the
numeric value 24 for a simple value (EID 5917), which is not well-
formed; this example has been removed. Errata report 5763 pointed to
an accident in the wording of the definition of tags; this was
resolved during a re-write of Section 3.4. Errata report 5434
pointed out that the UBJSON example in Appendix E no longer complied
with the version of UBJSON current at the time of submitting the
report. It turned out that the UBJSON specification had completely
changed since 2013; this example therefore also was removed. Further
errata reports (4409, 4963, 4964) complained that the map key sorting
rules for canonical encoding were onerous; these led to a
reconsideration of the canonical encoding suggestions and replacement
by the deterministic encoding suggestions (described below). An
editorial suggestion in errata report 4294 was also implemented
(improved symmetry by adding "Second value" to a comment to the last
example in Section 3.2.2).
Other more clerical changes include:
* use of new RFCXML functionality [RFC7991];
* explain some more of the notation used;
* updated references, e.g. for RFC4627 to [RFC8259] in many places,
for CNN-TERMS to [RFC7228]; added missing reference to [IEEE754]
(importing required definitions) and updated to [ECMA262]; added a
reference to [RFC8618] that further illustrates the discussion in
Appendix E;
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* the discussion of diagnostic notation mentions the "Extended
Diagnostic Notation" (EDN) defined in [RFC8610] as well as the gap
diagnostic notation has in representing NaN payloads; an
explanation was added on how to represent indefinite length
strings with no chunks;
* the addition of this appendix.
G.2. Changes in IANA considerations
The IANA considerations were generally updated (clerical changes,
e.g., now pointing to the CBOR working group as the author of the
specification). References to the respective IANA registries have
been added to the informative references.
Tags in the space from 256 to 32767 (lower half of "1+2") are no
longer assigned by First Come First Served; this range is now
Specification Required.
G.3. Changes in suggestions and other informational components
In revising the document, beyond processing errata reports, the WG
could use nearly seven years of experience with the use of CBOR in a
diverse set of applications. This led to a number of editorial
changes, including adding tables for illustration, but also to
emphasizing some aspects and de-emphasizing others.
A significant addition in this revision is Section 2, which discusses
the CBOR data model and its small variations involved in the
processing of CBOR. Introducing terms for those (basic generic,
extended generic, specific) enables more concise language in other
places of the document, but also helps in clarifying expectations on
implementations and on the extensibility features of the format.
RFC 7049, as a format derived from the JSON ecosystem, was influenced
by the JSON number system that was in turn inherited from JavaScript
at the time. JSON does not provide distinct integers and floating-
point values (and the latter are decimal in the format). CBOR
provides binary representations of numbers, which do differ between
integers and floating-point values. Experience from implementation
and use now suggested that the separation between these two number
domains should be more clearly drawn in the document; language that
suggested an integer could seamlessly stand in for a floating-point
value was removed. Also, a suggestion (based on I-JSON [RFC7493])
was added for handling these types when converting JSON to CBOR, and
the use of a specific rounding mechanism has been recommended.
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For a single value in the data model, CBOR often provides multiple
encoding options. The revision adds a new section Section 4, which
first introduces the term "preferred serialization" (Section 4.1) and
defines it for various kinds of data items. On the basis of this
terminology, the section goes on to discuss how a CBOR-based protocol
can define "deterministic encoding" (Section 4.2), which now avoids
the RFC 7049 terms "canonical" and "canonicalization". The
suggestion of "Core Deterministic Encoding Requirements"
Section 4.2.1 enables generic support for such protocol-defined
encoding requirements. The present revision further eases the
implementation of deterministic encoding by simplifying the map
ordering suggested in RFC 7049 to simple lexicographic ordering of
encoded keys. A description of the older suggestion is kept as an
alternative, now termed "length-first map key ordering"
(Section 4.2.3).
The terminology for well-formed and valid data was sharpened and more
stringently used, avoiding less well-defined alternative terms such
as "syntax error", "decoding error" and "strict mode" outside
examples. Also, a third level of requirements beyond CBOR-level
validity that an application has on its input data is now explicitly
called out. Well-formed (processable at all), valid (checked by a
validity-checking generic decoder), and expected input (as checked by
the application) are treated as a hierarchy of layers of
acceptability.
The handling of non-well-formed simple values was clarified in text
and pseudocode. Appendix F was added to discuss well-formedness
errors and provide examples for them. The pseudocode was updated to
be more portable and some portability considerations were added.
The discussion of validity has been sharpened in two areas. Map
validity (handling of duplicate keys) was clarified and the domain of
applicability of certain implementation choices explained. Also,
while streamlining the terminology for tags, tag numbers, and tag
content, discussion was added on tag validity, and the restrictions
were clarified on tag content, in general and specifically for tag 1.
An implementation note (and note for future tag definitions) was
added to Section 3.4 about defining tags with semantics that depend
on serialization order.
Tag 35 is no longer defined in this updated document; the
registration based on the definition in RFC 7049 remains in place.
Terminology was introduced in Section 3 for "argument" and "head",
simplifying further discussion.
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The security considerations were mostly rewritten and significantly
expanded; in multiple other places, the document is now more explicit
that a decoder cannot simply condone well-formedness errors.
Acknowledgements
CBOR was inspired by MessagePack. MessagePack was developed and
promoted by Sadayuki Furuhashi ("frsyuki"). This reference to
MessagePack is solely for attribution; CBOR is not intended as a
version of or replacement for MessagePack, as it has different design
goals and requirements.
The need for functionality beyond the original MessagePack
Specification became obvious to many people at about the same time
around the year 2012. BinaryPack is a minor derivation of
MessagePack that was developed by Eric Zhang for the binaryjs
project. A similar, but different, extension was made by Tim Caswell
for his msgpack-js and msgpack-js-browser projects. Many people have
contributed to the discussion about extending MessagePack to separate
text string representation from byte string representation.
The encoding of the additional information in CBOR was inspired by
the encoding of length information designed by Klaus Hartke for CoAP.
This document also incorporates suggestions made by many people,
notably Dan Frost, James Manger, Jeffrey Yasskin, Joe Hildebrand,
Keith Moore, Laurence Lundblade, Matthew Lepinski, Michael
Richardson, Nico Williams, Peter Occil, Phillip Hallam-Baker, Ray
Polk, Stuart Cheshire, Tim Bray, Tony Finch, Tony Hansen, and Yaron
Sheffer. Benjamin Kaduk provided an extensive review during IESG
processing. Éric Vyncke, Erik Kline, Robert Wilton, and Roman Danyliw
provided further IESG comments, which included an IoT directorate
review by Eve Schooler.
Authors' Addresses
Carsten Bormann
Universitaet Bremen TZI
Postfach 330440
D-28359 Bremen
Germany
Phone: +49-421-218-63921
Email: cabo@tzi.org
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Paul Hoffman
ICANN
Email: paul.hoffman@icann.org
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