Network Working Group D. McGrew
Internet-Draft Cisco Systems, Inc.
Expires: April 26, 2007 October 23, 2006
An Interface and Algorithms for Authenticated Encryption
draft-mcgrew-auth-enc-01.txt
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Copyright (C) The Internet Society (2006).
Abstract
This draft defines algorithms for authenticated encryption with
additional authenticated data (AEAD), and defines a uniform interface
and a registry for such algorithms. The interface and registry can
be used as an application independent set of cryptoalgorithm suites.
This approach provides advantages in efficiency and security, and
promotes the reuse of crypto implementations.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1 Background . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2 Scope . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.3 Benefits . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.4 Conventions Used In This Document . . . . . . . . . . . . 4
2. AEAD Interface . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1 Authenticated Encryption . . . . . . . . . . . . . . . . . 5
2.2 Authenticated Decryption . . . . . . . . . . . . . . . . . 7
2.3 Data Formatting . . . . . . . . . . . . . . . . . . . . . 7
3. Recommended Nonce Formation . . . . . . . . . . . . . . . . . 8
4. Requirements on the use of AEAD algorithms . . . . . . . . . . 10
5. Requirements on AEAD algorithms . . . . . . . . . . . . . . . 11
6. AEAD Algorithms . . . . . . . . . . . . . . . . . . . . . . . 12
6.1 AEAD_AES_128_GCM . . . . . . . . . . . . . . . . . . . . . 12
6.1.1 AEAD_AES_256_GCM . . . . . . . . . . . . . . . . . . . 12
6.2 AEAD_AES_128_CCM . . . . . . . . . . . . . . . . . . . . . 12
6.2.1 AEAD_AES_256_CCM . . . . . . . . . . . . . . . . . . . 13
6.3 AEAD_AES_128_HMAC_SHA1 . . . . . . . . . . . . . . . . . . 13
6.3.1 Test Cases . . . . . . . . . . . . . . . . . . . . . . 15
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16
8. Open Questions . . . . . . . . . . . . . . . . . . . . . . . . 17
9. Security Considerations . . . . . . . . . . . . . . . . . . . 19
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 20
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 21
11.1 Normative References . . . . . . . . . . . . . . . . . . . 21
11.2 Informative References . . . . . . . . . . . . . . . . . . 21
Author's Address . . . . . . . . . . . . . . . . . . . . . . . 22
Intellectual Property and Copyright Statements . . . . . . . . 23
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1. Introduction
Authenticated encryption is a form of encryption that, in addition to
providing confidentiality for the plaintext that is encrypted,
provides a way to check its integrity and authenticity.
Authenticated encryption with Associated Data, or AEAD, adds the
ability to check the integrity and authenticity of some additional
"associated data" that is not encrypted.
1.1 Background
Many cryptographic applications require both confidentiality and
message authentication. Often an encryption method and a message
authentication code (MAC) are used to provide those security
services, with each algorithm using an independent key. More
recently, the idea of providing both security services using a single
cryptoalgorithm has become accepted. In this concept, the cipher and
MAC are replaced by an Authenticated Encryption with Associated Data
(AEAD) algorithm. Several crypto algorithms that implement AEAD
algorithms have been defined, including block cipher modes of
operation and dedicated algorithms. Some of these algorithms have
been adopted and proven useful in practice. Additionally, AEAD is
close to an 'idealized' view of encryption, such as those used in the
automated analysis of cryptographic protocols.
1.2 Scope
In this document we define an AEAD algorithm as an abstraction, by
specifying an interface to an AEAD and defining an IANA registry for
AEAD algorithms. We populate this registry with five AEAD
algorithms: AES in Galois/Counter Mode [GCM] with 128 and 256 bit
keys, AES in Counter and CBC MAC mode [CCM] with 128 and 256 bit
keys, and an algorithm that composes AES-128 CBC and HMAC-SHA1. This
approach enables applications that need cryptographic security
services to more easily adopt those services.
In the following, we define the AEAD interface (Section 2), and then
outline the requirements that each AEAD algorithm must meet
(Section 5) and provide guidance on the use of AEAD algorithms
(Section 4). Then we define five AEAD algorithms (Section 6), and
establish an IANA registry for AEAD algorithms (Section 7). Lastly,
we discuss some open questions (Section 8).
The AEAD interface specification does not address security protocol
issues such as anti-replay services or access control decisions that
are made on authenticated data. Instead, the specification aims to
abstract the cryptography away from those issues. The interface, and
the guidance about how to use it, are consistent with the
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recommendations from [EEM04].
1.3 Benefits
The approach benefits the application designer by allowing them to
focus on the important issues of security services, canonicalization,
and data marshaling, and relieving them of the need to design crypto
mechanisms that meet their security goals. Importantly, the security
of an AEAD algorithm can be analyzed independent from its use in a
particular application. This property frees the user of the AEAD of
the need to consider security aspects such as the relative order of
authentication and encryption and the security of the particular
combination of cipher and MAC, such as the potential loss of
confidentiality through the MAC. The application designer that uses
the AEAD interface need not select a particular AEAD algorithm during
the design stage. Additionally, the interface to the AEAD is
relatively simple, since it requires only a single key as input and
it requires only a single identifier to indicate the algorithm in use
in a particular case.
The AEAD approach benefits the implementer of the crypto algorithms
by making available optimizations that are otherwise not possible to
reduce the amount of computation, the implementation cost, and/or the
storage requirements. The simpler interface makes testing easier;
this is a considerable benefit for a crypto algorithm implementation.
By providing a uniform interface to access cryptographic services,
the AEAD approach allows a single crypto implementation to easily
support multiple applications. For example, a hardware module that
supports the AEAD interface can easily provide crypto acceleration to
any application using that interface, even to applications that had
not been designed when the module was built.
1.4 Conventions Used In This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
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2. AEAD Interface
An AEAD algorithm has two operations, authenticated encryption and
authenticated decryption. The inputs and outputs of these algorithms
are defined in terms of octet strings.
An implementation MAY accept additional inputs. For example, an
input could be provided to allow the user to select between different
implementation strategies. However, such extensions MUST NOT affect
interoperability.
2.1 Authenticated Encryption
The authenticated encryption operation has four inputs, each of which
is an octet string:
A secret key K, which MUST be generated in a way that is uniformly
random or pseudorandom.
An nonce N. Each nonce provided to distinct invocations of the
Authenticated Encryption operation MUST be distinct, for any
particular value of the key; applications SHOULD use the nonce
formation method defined in Section 3, and MAY use any other
method that meets this requirement.
A plaintext P, which contains the data to be encrypted and
authenticated,
The additional authenticated data A, which contains the data to be
authenticated, but not encrypted.
There is a single output:
A ciphertext C, which is as least as long as the plaintext, or
an indication that the requested encryption operation could not be
performed.
All of the inputs and outputs are variable-length octet strings,
whose lengths obey the following restrictions:
The number of octets in the key K is between one and 256. For
each AEAD algorithm, the length of K MUST be fixed.
The number of octets in the nonce is between one and 2^64 - 1,
inclusive. However, the length SHOULD be twelve (12) octets.
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The number of octets in the plaintext P is between zero and 2^64 -
1, inclusive.
The number of octets in the additional authenticated data AAD is
between zero and 2^64 - 1, inclusive.
The number of octets in the ciphertext C is between zero and 2^64
+ 255.
An AEAD algorithm MAY further restrict the lengths of its inputs and
outputs. A particular AEAD implementation MAY further restrict the
lengths of its inputs and outputs. If a particular implementation of
an AEAD algorithm is requested to process an input that is outside
the range of admissible lengths, or an input that is outside the
range of lengths supported by that implementation, it MUST return an
error code and it MUST NOT output any other information. In
particular, partially encrypted or partially decrypted data MUST NOT
be returned.
Both confidentiality and message authentication is provided on the
plaintext P. When the length of P is zero, the AEAD algorithm acts as
a Message Authentication Code on the input A.
The additional authenticated data A is used to protect information
that needs to be authenticated, but which does not need to be kept
confidential. When using an AEAD to secure a network protocol, for
example, this input could include addresses, ports, sequence numbers,
protocol version numbers, and other fields that indicate how the
plaintext or ciphertext should be handled, forwarded, or processed.
In many situations, it is desirable to authenticate these fields,
though they must be left in the clear to allow the network or system
to function properly. When this data is included in the input A,
authentication is provided without copying the data into the
ciphertext.
The nonce is authenticated internally to the algorithm, and it is not
necessary to include it in the AAD input. The nonce MAY be included
in P or A if it convenient to the application.
The nonce MAY be transported along with the plaintext. The entire IV
need not be transmitted; it is sufficient to provide the receiver
with enough information to allow the nonce to be reconstructed.
Because the authenticated decryption process detects incorrect nonce
values, no security failure results when a receiver incorrectly
reconstructs an IV. Any such reconstruction method will need to take
into account the possible loss or reorder of ciphertexts between the
encryption and decryption processes.
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Applications MUST NOT assume any particular structure or formatting
of the ciphertext.
2.2 Authenticated Decryption
The authenticated decryption operation has four inputs: K, IV , A and
C, as defined above. It has only a single output, either a plaintext
value P or a special symbol FAIL that indicates that the inputs are
not authentic. A ciphertext C , nonce N , and additional
authenticated data A are authentic for key K when IV, and C is
generated by the encrypt operation with inputs K, IV, P and A, for
some values of IV, P, and A. The authenticated decrypt operation
will, with high probability, return FAIL whenever its inputs were not
created by the encrypt operation with the identical key (assuming
that the AEAD algorithm is secure).
2.3 Data Formatting
This document does not specify any particular encoding for the AEAD
inputs and outputs, since the encoding does not affect the security
services provided by an AEAD algorithm.
When formatting a ciphertext, an application SHOULD position the
ciphertext C so that it appears after any other data that is needed
to construct the other inputs to the Authenticated Decryption
operation. For example, if the nonce and ciphertext both appear in a
packet, the former value should precede the latter. This rule
facilitates efficient and simple implementations of AEAD algorithms.
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3. Recommended Nonce Formation
It is essential for security that the nonces be constructed in a
manner that respects the requirements that each nonce value be
distinct for each invocation of the Authenticated Encryption
operation, for any fixed value of the key. Special attention must be
paid to the case in which there are multiple encryption devices using
a single key.
The following method to construct nonces is RECOMMENDED. The nonce
is formatted as illustrated in Figure 1, with the initial octets
consisting of a Fixed field, and the final octets consisting of a
Counter field. Both fields have variable lengths.
<----- variable ----> <----------- variable ----------->
+---------------------+----------------------------------+
| Fixed | Counter |
+---------------------+----------------------------------+
Figure 1: Recommended nonce format.
The Counter field is regarded as an unsigned integer in network byte
order. The Counter part is equal to one for the first nonce, and it
increments by one for each successive nonce that is generated. The
integer part is never equal to the all-zero value. Thus at most
2^(8*C) - 1 nonces can be generated when the Counter field is C
octets in length.
The Fixed field MUST remain constant for all nonces that are
generated for a given encryption device. However, if different
devices are performing encryption with a single key, then each
distinct device MUST use a distinct Fixed field, to ensure the
uniqueness of the nonces. Thus at most 2^(8*F) distinct senders can
share a key when the Fixed field is F octets in length. When the
number of encrypters is less than this value, the initial octets of
the Fixed fields MUST be chosen so that they are common across all
encrypters. The final octets of the Fixed field will need to be
distinct across all encrypters. This substructure is shown in
Figure 2.
In some cases it is desirable to not transmit or store an entire
nonce, but instead to reconstruct that value immediately prior to
decryption. In these cases, it is RECOMMENDED that the distinct
final octets of the Fixed field, and the Counter field, be explicitly
transmitted or stored, while the common initial octets of the Fixed
field be stored with the key. The explicit information is shown in
Figure 2. However, applications MAY use any method of reconstructing
nonces that is convenient.
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<------------- Fixed field ------------>
+-------------------+--------------------+---------------+
| Fixed (common) : Fixed (distinct) | Counter |
+-------------------+--------------------+---------------+
<------------ explicit ------------->
Figure 2: Nonce structure and reconstruction
Rationale. This method of nonce construction incorporates the
best known practice, and facilitates the reconstruction of nonces
from partial explicit information. It is used by both GCM ESP
[RFC4106] and CCM ESP [RFC4309], in which the Fixed field contains
the Salt value and the lowest eight octets of the nonce are
explicitly carried in the ESP packet. In GCM ESP, the Fixed field
must be at least four octets long, so that it can contain the Salt
value. In CCM ESP, the Fixed field must be at least three octets
long for the same reason. This nonce generation method is also
used by several counter mode variants including CTR ESP.
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4. Requirements on the use of AEAD algorithms
This section provides advice that must be followed in order to use an
AEAD algorithm securely.
If the AAD input is constructed out of multiple data elements, then
it is essential that it be unambiguously parseable into its
constituent elements, without the use of any unauthenticated data in
the parsing process. This requirement ensures that an attacker
cannot fool a receiver into accepting a bogus set of data elements as
authentic by altering a set of data elements that were used to
construct an AAD input in an authenticated encryption operation in
such a way that the data elements are different, but the AAD input is
unchanged. This requirement is trivially met if the AAD is composed
of fixed-width elements. If the AAD contains a variable-length
string, for example, this requirement can be met by also including
the length of the string in the AAD.
Similarly, if the plaintext is constructed out of multiple data
elements, then it is essential that it be unambiguously parseable
into its constituent elements, without using any unauthenticated data
in the parsing process. Note that data that included in the AAD may
be used when parsing the plaintext, though of course since the AAD is
not encrypted there is a potential loss of confidentiality when
information about the plaintext is included in the AAD.
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5. Requirements on AEAD algorithms
Each AEAD algorithm MUST only accept keys with a fixed key length
K_LEN, and MUST NOT require any particular data format for the keys
provided as input. An algorithm that requires such structure
internally (e.g. one with subkeys in a particular parity-check
format) will need to provide it internally.
Each AEAD algorithm MUST accept any plaintext with a length between
zero and P_MAX octets, where the value P_MAX is specific to that
algorithm.
Each AEAD algorithm MUST accept any additional authenticated data
with a length between zero and A_MAX octets, where the value A_MAX is
specific to that algorithm.
Each AEAD algorithm MUST accept any IV with a length between N_MIN
and N_MAX octets, where the values of N_MIN and N_MAX are specific to
that algorithm. Each algorithm SHOULD accept an IV with a length of
twelve (12) octets.
An AEAD algorithm MAY structure its ciphertext output in any way; for
example, the ciphertext can incorporate an authentication tag. Each
algorithm SHOULD choose a structure that is amenable to efficient
processing.
An Authenticated Encryption algorithm MAY incorporate a random
source, e.g. for the generation of an internal initialization vector
that is incorporated into the ciphertext output. An algorithm that
uses an internal random initialization vector in this manner MAY have
a value of N_MAX that is equal to zero.
The specification of an AEAD algorithm MUST include the values of
K_LEN, P_MAX, A_MAX, N_MIN, and N_MAX defined above. Additionally,
it MUST specify the number of octets in the largeset possible
ciphertext, which we denote C_MAX.
Each AEAD algorithm MUST provide a description relating the length of
the plaintext to that of the ciphertext.
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6. AEAD Algorithms
This section defines five AEAD algorithms; two are based on AES GCM,
two are based on AES CCM, and one is based on a composition of AES
CBC and HMAC SHA1. Each pair includes an algorithm with a key size
of 128 bits and one with a key size of 256 bits.
6.1 AEAD_AES_128_GCM
The AEAD_AES_128_GCM authenticated encryption algorithm works as
specified in [GCM], by providing the key, nonce, and plaintext, and
additional authenticated data to that mode of operation. An
authentication tag with a length of 16 octets (128 bits) is used.
The AEAD_AES_128_GCM ciphertext is formed by appending the
authentication tag provided as an output to the GCM encryption
operation to the ciphertext that is output by that operation. Test
cases are provided in the appendix of [GCM]. The input and output
lengths are as follows:
K_LEN is 16 octets,
P_MAX is 2^36 - 31 octets,
A_MAX is 2^61 - 1 octets,
N_MIN is 1 (one) octet and N_MAX is 2^61 -1 octets; applications
SHOULD use an nonce length of 12 octets, since GCM is optimized
for that length,
C_MAX is 2^36 - 15 octets.
6.1.1 AEAD_AES_256_GCM
This algorithm is identical to AEAD_AES_128_GCM, but with the
following differences:
K_LEN is 32 octets, instead of 16 octets, and
AES-256 GCM is used instead of AES-128 GCM.
6.2 AEAD_AES_128_CCM
The AEAD_AES_128_CCM authenticated encryption algorithm works as
specified in [CCM], by providing the key, nonce, additional
authenticated data, and plaintext to that mode of operation. The
formatting and counter generation function are as specified in
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Appendix A of that reference, and the values of the parameters
identified in that appendix are as follows:
the nonce length n is 12,
the tag length t is 16, and
the value q is 3.
An authentication tag with a length of 16 octets (128 bits) is used.
The AEAD_AES_128_CCM ciphertext is formed by appending the
authentication tag provided as an output to the CCM encryption
operation to the ciphertext that is output by that operation. Test
cases are provided in [CCM]. The input and output lengths are as
follows:
K_LEN is 16 octets,
P_MAX is 2^24 - 1 octets,
A_MAX is 2^64 - 1 octets,
N_MIN and N_MAX are both 12 octets, and
C_MAX is 2^24 + 15 octets.
6.2.1 AEAD_AES_256_CCM
This algorithm is identical to AEAD_AES_128_CCM, but with the
following differences:
K_LEN is 32 octets, instead of 16, and
AES-256 CCM is used instead of AES-128 CCM.
6.3 AEAD_AES_128_HMAC_SHA1
This algorithm random and stateless. It is based on the "generic
composition" of CBC encryption with HMAC authentication, with the the
encrypt-then-MAC method [AE]. It uses the HMAC message
authentication code [RFC2104] with the SHA-1 hash function [SHA1] to
provide message authentication. Test cases for HMAC_SHA1 are
provided in [RFC2202]. For encryption, it uses AES-128 in the cipher
block chaining (CBC) mode of operation as defined in Section 6.2 of
[MODES], with the padding method defined by Appendix A of the same
reference. The input key is 128 bits long, and the CBC IV is
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generated uniformly at random, and is also 128 bits long.
The authenticated encryption algorithm is as follows, or uses an
equivalent set of steps:
1. Generate the secondary keys MAC_KEY and ENC_KEY from the input
key K as follows:
MAC_KEY = HMAC_SHA1(K, "MAC");
ENC_KEY = leftmost(HMAC_SHA1(K, "ENC"), 128);
where the function leftmost(X, m) accepts a bitstring X and a
non-negative integer m and returns the bitstring containing the
leftmost m bits of X. MAC_KEY is 160 bits long, and ENC_KEY is
128 bits long.
2. Generate a 128-bit IV uniformly at random. (A pseudorandom
process MAY be used if its strength is equivalent to AES-128.)
Note that this IV is distinct from the nonce provided as an input
to the authenticated encryption operation.
3. Pad the plaintext by appending a single '1' bit to that data and
then appending to the resulting string as few '0' bits as are
necessary to make the number of bits in the plaintext into a
multiple of 128. Note that padding MUST be added to the data; if
the number of octets in the payload data is a multiple of 16,
then 16 octets of padding will be added.
4. Encrypt the payload using AES-128 in CBC mode, using the ENC_KEY
and the random IV. Form the ciphertext by prepending the IV to
the CBC ciphertext outputs.
5. Compute the authentication tag by applying HMAC_SHA1 to the AAD,
the length of the AAD expressed as a 64-bit unsigned integer in
network byte order, the IV, and the ciphertext, in that order,
using the MAC_KEY.
6. Return the ciphertext and the authentication tag.
The authenticated decryption algorithm is as follows, or uses an
equivalent set of steps:
1. Generate the secondary keys MAC_KEY and ENC_KEY from the input
key K as follows:
MAC_KEY = HMAC_SHA1(K, "MAC");
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ENC_KEY = leftmost(HMAC_SHA1(K, "ENC"), 128);
2. Compute the MAC value by applying HMAC_SHA1 to the AAD, he length
of the AAD expressed as a 64-bit unsigned integer in network byte
order, the IV, and the ciphertext, in that order, using the
MAC_KEY. Compare this value to the authentication tag. If they
match, then continue with the processing. Otherwise, discard the
data and return FAIL.
3. Decrypt the payload using AES-128 in CBC mode, with the ENC_KEY,
using the first 128 bits of the ciphertext as the random IV.
4. Remove padding by deleting the final '1' bit and all of the
following '0' bits. The remaining data forms the payload data.
5. Return the plaintext.
The length of the ciphertext can be inferred from that of the
plaintext. The number L of octets in the ciphertext is given by
L = 16 * ( floor(M / 16) + 2)
where M denotes the number of octets in the payload, and the function
floor() rounds its argument down to the nearest integer. This fact
is needed by the encoding function, since the length of the
ciphertext, rather than the length of the payload, must be
authenticated.
The lengths of the inputs are restricted as follows:
K_LEN is 16 octets,
P_MAX is 2^32 - 1 octets,
A_MAX is 2^64 - 1 octets,
N_MIN and N_MAX are both zero octets, and
C_MAX is 2^32 + 19 octets.
6.3.1 Test Cases
A future version of this document will include test cases for
AEAD_AES_128_HMAC_SHA1.
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7. IANA Considerations
IANA will define the "AEAD Registry" described below. Additions and
changes to the AEAD Registry are by expert review. Each entry in the
registry contains the following elements:
a short name, such as "AEAD_AES_128_GCM", that starts with the
string "AEAD",
a positive number, and
a reference to a specification that completely defines an AEAD
algorithm and provides test cases that can be used to verify the
correctness of an implementation.
Requests to add an entry to the registry MUST include the name and
the reference. The number is assigned by IANA. These number
assignments SHOULD use the smallest available positive number.
IANA will add the following five entries to the AEAD Registry:
+----------------------------+---------------+--------------------+
| Name | Reference | Numeric Identifier |
+----------------------------+---------------+--------------------+
| AEAD_AES_128_GCM | Section 6.1 | 1 |
| | | |
| AEAD_AES_256_GCM | Section 6.1.1 | 2 |
| | | |
| AEAD_AES_128_CCM | Section 6.2 | 3 |
| | | |
| AEAD_AES_256_CCM | Section 6.2.1 | 4 |
| | | |
| AEAD_AES_128_CBC_HMAC_SHA1 | Section 6.3 | 5 |
+----------------------------+---------------+--------------------+
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8. Open Questions
The additional authenticated data input is well suited to
authenticating headers. Some cryptographic protocols have trailers
that should be authenticated. For example, in the Secure RTP
protocol the authenticated data consists of the RTP header, the
ciphertext containing the encrypted payload, and some additional
data, in that order. It is impossible for an AEAD to accommodate
both the authenticated header and authenticated trailer without
adding an additional input for the trailer. Because none of the
specified AEAD algorithms can handle both a trailer and a footer,
this specification does not include a trailer in its interface. We
expect that protocols like SRTP will need to define different
processing rules that include all of the authenticated-only data into
a single AAD input in order to make use of this specification.
However, new rules would need to be defined in order to use either
GCM or CCM or any other AEAD transforms, so this need is not
especially onerous.
The TLS protocol as currently defined assumes that authentication
will precede encryption. Thus, in order to accommodate this
specification, new processing rules would need to be written that
make no assumptions about the relative ordering of the cryptographic
services. However, as above, these new rules would need to be
defined anyway in order to use any AEAD algorithm.
The AEAD algorithms selected reflect those that have been already
adopted by standards. It is an open question as to what other AEAD
algorithms should be added. Many variations on basic algorithms are
possible, each with its own advantages. While it is desirable to
admit any algorithms that are found to be useful in practice, it is
also desirable to limit the total number of registered algorithms.
The current specification requires that a registered algorithm
provide a complete specification and a set of validation data; it is
hoped that these prerequisites set the admission criteria
appropriately.
Some users may view an IANA assignment as a recommendation or an
endorsement of a particular AEAD algorithm. Other users may desire
to register an AEAD algorithm in order to allow for experimental or
specialized use. Because of these conflicting perspectives, it may
be desirable to allocate a second IANA registry for experimental use.
It may be desirable to replace HMAC-SHA1 with AES CMAC [CMAC] in the
generic composition algorithm, or to introduce an additional
algorithm that does so.
Directly testing a randomized AEAD encryption algorithm using a test
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cases with fixed inputs and outputs is not possible, since the
encryption process is non-deterministic. However, it is easy to test
a randomized AEAD algorithm against fixed test cases. The
authenticated decryption algorithm is deterministic, and it can be
directly tested. The authenticated encryption algorithm can be
tested by encrypting a plaintext, decrypting the resulting
ciphertext, and comparing the original plaintext to the post-
decryption plaintext.
Some of the terminology in this specification is unwieldy, and could
perhaps be improved. For example, "AEAD algorithm" could be replaced
with "crypto transform", which would be meaningful to a broader
community, but is less precise.
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9. Security Considerations
A future version of this document will define the security services
that must be provided by an AEAD algorithm.
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10. Acknowledgments
Many reviewers provided valuable comments on earlier drafts of this
document.
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11. References
11.1 Normative References
[CCM] "NIST Special Publication 800-38C: The CCM Mode for
Authentication and Confidentiality",
http://csrc.nist.gov/publications/nistpubs/SP800-38C.pdf.
[GCM] McGrew, D. and J. Viega, "The Galois/Counter Mode of
Operation (GCM)", Submission to NIST. http://
csrc.nist.gov/CryptoToolkit/modes/proposedmodes/gcm/
gcm-spec.pdf, January 2004.
[MODES] "NIST Special Publication 800-38", Reccomendation for
Block Cipher Modes of Operation http://csrc.nist.gov/
publications/nistpubs/800-38a/sp800-38a.pdf.
[RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
Hashing for Message Authentication", RFC 2104,
February 1997.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2202] Cheng, P. and R. Glenn, "Test Cases for HMAC-MD5 and HMAC-
SHA-1", RFC 2202, September 1997.
[RFC4106] Viega, J. and D. McGrew, "The Use of Galois/Counter Mode
(GCM) in IPsec Encapsulating Security Payload (ESP)",
RFC 4106, June 2005.
[RFC4309] Housley, R., "Using Advanced Encryption Standard (AES) CCM
Mode with IPsec Encapsulating Security Payload (ESP)",
RFC 4309, December 2005.
[SHA1] "FIPS 180-1: Secure Hash Standard,", Federal Information
Processing Standard
(FIPS) http://www.itl.nist.gov/fipspubs/fip180-1.htm.
11.2 Informative References
[AE] "Authenticated encryption: Relations among notions and
analysis of the generic composition paradigm", Proceedings
of ASIACRYPT 2000, Springer-Verlag, LNCS 1976, pp.
531-545 http://www.
[CMAC] "NIST Special Publication 800-38B", http://csrc.nist.gov/
CryptoToolkit/modes/800-38_Series_Publications/
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SP800-38B.pdf.
[EEM04] "Breaking and provably repairing the SSH authenticated
encryption scheme: A case study of the Encode-then-Encrypt-
and-MAC paradigm", ACM Transactions on Information and
System Security, http://www-cse.ucsd.edu/users/tkohno/
papers/TISSEC04/.
Author's Address
David A. McGrew
Cisco Systems, Inc.
510 McCarthy Blvd.
Milpitas, CA 95035
US
Phone: (408) 525 8651
Email: mcgrew@cisco.com
URI: http://www.mindspring.com/~dmcgrew/dam.htm
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