INTERNET-DRAFT Kurt D. Zeilenga
Intended Category: Standard Track OpenLDAP Foundation
Expires: 4 January 2001 4 July 2000
LDAPv3bis Suggestions:
UTF-8 String Representation of Distinguished Names
<draft-zeilenga-ldapv3bis-rfc2253-00.txt>
Status of Memo
This document is an Internet-Draft and is in full conformance with all
provisions of Section 10 of RFC2026.
This document is intended to be, after appropriate review and
revision, submitted to the RFC Editor as a Standard Track document.
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Copyright 2000, The Internet Society. All Rights Reserved.
Please see the Copyright section near the end of this document for
more information.
Forward
This Internet Draft suggests a number of updates to the "Lightweight
Directory Access Protocol: UTF-8 String Representation of
Distinguished Names" [RFC 2253]. This document is not intended to be
published as an RFC but used to identify LDAPv3bis work items.
The remainer of this documents incorporates the substantive portion of
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RFC 2253 text (less status of memo, appendices, etc). Comments and
suggested updates to this text are inserted as inline notes prefixed
with '//'.
// Start of RFC 2253 text
Abstract
The X.500 Directory uses distinguished names as the primary keys to
entries in the directory. Distinguished Names are encoded in ASN.1 in
the X.500 Directory protocols. In the Lightweight Directory Access
Protocol, a string representation of distinguished names is
transferred. This specification defines the string format for
representing names, which is designed to give a clean representation
of commonly used distinguished names, while being able to represent
any distinguished name.
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 RFC 2119 [6].
1. Background
This specification assumes familiarity with X.500 [1], and the concept
of Distinguished Name. It is important to have a common format to be
able to unambiguously represent a distinguished name. The primary
goal of this specification is ease of encoding and decoding. A
secondary goal is to have names that are human readable. It is not
expected that LDAP clients with a human user interface would display
these strings directly to the user, but would most likely be
performing translations (such as expressing attribute type names in
one of the local national languages).
2. Converting DistinguishedName from ASN.1 to a String
In X.501 [2] the ASN.1 structure of distinguished name is defined as:
DistinguishedName ::= RDNSequence
RDNSequence ::= SEQUENCE OF RelativeDistinguishedName
RelativeDistinguishedName ::= SET SIZE (1..MAX) OF
AttributeTypeAndValue
AttributeTypeAndValue ::= SEQUENCE {
type AttributeType,
value AttributeValue }
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The following sections define the algorithm for converting from an
ASN.1 structured representation to a UTF-8 string representation.
2.1. Converting the RDNSequence
If the RDNSequence is an empty sequence, the result is the empty or
zero length string.
Otherwise, the output consists of the string encodings of each
RelativeDistinguishedName in the RDNSequence (according to 2.2),
starting with the last element of the sequence and moving backwards
toward the first.
The encodings of adjoining RelativeDistinguishedNames are separated by
a comma character (',' ASCII 44).
2.2. Converting RelativeDistinguishedName
When converting from an ASN.1 RelativeDistinguishedName to a string,
the output consists of the string encodings of each
AttributeTypeAndValue (according to 2.3), in any order.
Where there is a multi-valued RDN, the outputs from adjoining
AttributeTypeAndValues are separated by a plus ('+' ASCII 43)
character.
2.3. Converting AttributeTypeAndValue
The AttributeTypeAndValue is encoded as the string representation of
the AttributeType, followed by an equals character ('=' ASCII 61),
followed by the string representation of the AttributeValue. The
encoding of the AttributeValue is given in section 2.4.
If the AttributeType is in a published table of attribute types
associated with LDAP [4], then the type name string from that table is
used, otherwise it is encoded as the dotted-decimal encoding of the
AttributeType's OBJECT IDENTIFIER. The dotted-decimal notation is
described in [3]. As an example, strings for a few of the attribute
types frequently seen in RDNs include:
String X.500 AttributeType
------------------------------
CN commonName
L localityName
ST stateOrProvinceName
O organizationName
OU organizationalUnitName
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C countryName
STREET streetAddress
DC domainComponent
UID userid
2.4. Converting an AttributeValue from ASN.1 to a String
If the AttributeValue is of a type which does not have a string
representation defined for it, then it is simply encoded as an
octothorpe character ('#' ASCII 35) followed by the hexadecimal
representation of each of the bytes of the BER encoding of the X.500
AttributeValue. This form SHOULD be used if the AttributeType is of
the dotted-decimal form.
Otherwise, if the AttributeValue is of a type which has a string
representation, the value is converted first to a UTF-8 string
according to its syntax specification (see for example section 6 of
[4]).
If the UTF-8 string does not have any of the following characters
which need escaping, then that string can be used as the string
representation of the value.
o a space or "#" character occurring at the beginning of the
string
o a space character occurring at the end of the string
o one of the characters ",", "+", """, "\", "<", ">" or ";"
Implementations MAY escape other characters.
If a character to be escaped is one of the list shown above, then it
is prefixed by a backslash ('\' ASCII 92).
Otherwise the character to be escaped is replaced by a backslash and
two hex digits, which form a single byte in the code of the character.
Examples of the escaping mechanism are shown in section 5.
3. Parsing a String back to a Distinguished Name
The structure of the string is specified in a BNF grammar, based on
the grammar defined in RFC 822 [5]. Server implementations parsing a
DN string generated by an LDAPv2 client MUST also accept (and ignore)
the variants given in section 4 of this document.
distinguishedName = [name] ; may be empty string
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name = name-component *("," name-component)
name-component = attributeTypeAndValue *("+" attributeTypeAndValue)
attributeTypeAndValue = attributeType "=" attributeValue
attributeType = (ALPHA 1*keychar) / oid
keychar = ALPHA / DIGIT / "-"
oid = 1*DIGIT *("." 1*DIGIT)
attributeValue = string
string = *( stringchar / pair )
/ "#" hexstring
/ QUOTATION *( quotechar / pair ) QUOTATION ; only from v2
quotechar = <any character except "\" or QUOTATION >
special = "," / "=" / "+" / "<" / ">" / "#" / ";"
pair = "\" ( special / "\" / QUOTATION / hexpair )
stringchar = <any character except one of special, "\" or QUOTATION >
hexstring = 1*hexpair
hexpair = hexchar hexchar
hexchar = DIGIT / "A" / "B" / "C" / "D" / "E" / "F"
/ "a" / "b" / "c" / "d" / "e" / "f"
ALPHA = <any ASCII alphabetic character>
; (decimal 65-90 and 97-122)
DIGIT = <any ASCII decimal digit> ; (decimal 48-57)
QUOTATION = <the ASCII double quotation mark character '"' decimal
34>
4. Relationship with RFC 1779 and LDAPv2
The syntax given in this document is more restrictive than the syntax
in RFC 1779. Implementations parsing a string generated by an LDAPv2
client MUST accept the syntax of RFC 1779. Implementations MUST NOT,
however, generate any of the RFC 1779 encodings which are not
described above in section 2.
Implementations MUST allow a semicolon character to be used instead of
a comma to separate RDNs in a distinguished name, and MUST also allow
whitespace characters to be present on either side of the comma or
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semicolon. The whitespace characters are ignored, and the semicolon
replaced with a comma.
Implementations MUST allow an oid in the attribute type to be prefixed
by one of the character strings "oid." or "OID.".
Implementations MUST allow for space (' ' ASCII 32) characters to be
present between name-component and ',', between attributeTypeAndValue
and '+', between attributeType and '=', and between '=' and
attributeValue. These space characters are ignored when parsing.
Implementations MUST allow a value to be surrounded by quote ('"'
ASCII 34) characters, which are not part of the value. Inside the
quoted value, the following characters can occur without any escaping:
",", "=", "+", "<", ">", "#" and ";"
5. Examples
This notation is designed to be convenient for common forms of name.
This section gives a few examples of distinguished names written using
this notation. First is a name containing three relative
distinguished names (RDNs):
CN=Steve Kille,O=Isode Limited,C=GB
Here is an example name containing three RDNs, in which the first RDN
is multi-valued:
OU=Sales+CN=J. Smith,O=Widget Inc.,C=US
This example shows the method of quoting of a comma in an organization
name:
CN=L. Eagle,O=Sue\, Grabbit and Runn,C=GB
An example name in which a value contains a carriage return character:
CN=Before\0DAfter,O=Test,C=GB
An example name in which an RDN was of an unrecognized type. The
value is the BER encoding of an OCTET STRING containing two bytes 0x48
and 0x69.
1.3.6.1.4.1.1466.0=#04024869,O=Test,C=GB
Finally, an example of an RDN surname value consisting of 5 letters:
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Unicode Letter Description 10646 code UTF-8 Quoted
=============================== ========== ====== =======
LATIN CAPITAL LETTER L U0000004C 0x4C L
LATIN SMALL LETTER U U00000075 0x75 u
LATIN SMALL LETTER C WITH CARON U0000010D 0xC48D \C4\8D
LATIN SMALL LETTER I U00000069 0x69 i
LATIN SMALL LETTER C WITH ACUTE U00000107 0xC487 \C4\87
Could be written in printable ASCII (useful for debugging purposes):
SN=Lu\C4\8Di\C4\87
6. References
[1] The Directory -- overview of concepts, models and services.
ITU-T Rec. X.500(1993).
[2] The Directory -- Models. ITU-T Rec. X.501(1993).
[3] Wahl, M., Howes, T., and S. Kille, "Lightweight Directory
Access Protocol (v3)", RFC 2251, December 1997.
[4] Wahl, M., Coulbeck, A., Howes, T. and S. Kille, "Lightweight
Directory Access Protocol (v3): Attribute Syntax Definitions",
RFC 2252, December 1997.
[5] Crocker, D., "Standard of the Format of ARPA-Internet Text
Messages", STD 11, RFC 822, August 1982.
[6] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", RFC 2119.
7. Security Considerations
// Add consideration requiring the use of strong authentication
// to update the directory.
7.1. Disclosure
Distinguished Names typically consist of descriptive information about
the entries they name, which can be people, organizations, devices or
other real-world objects. This frequently includes some of the
following kinds of information:
- the common name of the object (i.e. a person's full name) - an email
or TCP/IP address - its physical location (country, locality, city,
street address) - organizational attributes (such as department name
or affiliation)
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Most countries have privacy laws regarding the publication of
information about people.
7.2. Use of Distinguished Names in Security Applications
The transformations of an AttributeValue value from its X.501 form to
an LDAP string representation are not always reversible back to the
same BER or DER form. An example of a situation which requires the
DER form of a distinguished name is the verification of an X.509
certificate.
For example, a distinguished name consisting of one RDN with one AVA,
in which the type is commonName and the value is of the TeletexString
choice with the letters 'Sam' would be represented in LDAP as the
string CN=Sam. Another distinguished name in which the value is still
'Sam' but of the PrintableString choice would have the same
representation CN=Sam.
Applications which require the reconstruction of the DER form of the
value SHOULD NOT use the string representation of attribute syntaxes
when converting a distinguished name to the LDAP format. Instead,
they SHOULD use the hexadecimal form prefixed by the octothorpe ('#')
as described in the first paragraph of section 2.4.
// remainder trimmed
// End of RFC 2253 text
Additional Information
Discussions regarding these suggestions may directed to the author:
Kurt D. Zeilenga
OpenLDAP Foundation
<Kurt@OpenLDAP.org>
or the LDAPext Working Group mailing list:
<ietf-ldapext@netscape.com>
Copyright 2000, The Internet Society. All Rights Reserved.
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