RFC2253 - Lightweight Directory Access Protocol (v3): UTF-8 String Representatio

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　　Network Working Group M. Wahl
Request for Comments: 2253 Critical Angle Inc.
Obsoletes: 1779 S. Kille
Category: Standards Track Isode Ltd.
T. Howes
Netscape Communications Corp.
December 1997

Lightweight Directory Access Protocol (v3):
UTF-8 String Representation of Distinguished Names

Status of this Memo

This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.

Copyright Notice

Copyright (C) The Internet Society (1997). All Rights Reserved.

IESG Note

This document describes a directory access protocol that provides
both read and update access. Update access requires secure
authentication, but this document does not mandate implementation of
any satisfactory authentication mechanisms.

In accordance with RFC2026, section 4.4.1, this specification is
being approved by IESG as a Proposed Standard despite this
limitation, for the following reasons:

a. to encourage implementation and interoperability testing of
these protocols (with or without update access) before they
are deployed, and

b. to encourage deployment and use of these protocols in read-only
applications. (e.g. applications where LDAPv3 is used as
a query language for directories which are updated by some
secure mechanism other than LDAP), and

c. to avoid delaying the advancement and deployment of other Internet
standards-track protocols which require the ability to query, but
not update, LDAPv3 directory servers.

Readers are hereby warned that until mandatory authentication
mechanisms are standardized, clients and servers written according to
this specification which make use of update functionality are
UNLIKELY TO INTEROPERATE, or MAY INTEROPERATE ONLY IF AUTHENTICATION
IS REDUCED TO AN UNACCEPTABLY WEAK LEVEL.

Implementors are hereby discouraged from deploying LDAPv3 clients or
servers which implement the update functionality, until a Proposed
Standard for mandatory authentication in LDAPv3 has been approved and
published as an RFC.

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 RFC2119 [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 }

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
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 RFC822 [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

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 RFC1779 and LDAPv2

The syntax given in this document is more restrictive than the syntax
in RFC1779. Implementations parsing a string generated by an LDAPv2
client MUST accept the syntax of RFC1779. Implementations MUST NOT,
however, generate any of the RFC1779 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 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:

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)", RFC2251, December 1997.

[4] Wahl, M., Coulbeck, A., Howes, T. and S. Kille, "Lightweight
Directory Access Protocol (v3): Attribute Syntax Definitions",
RFC2252, December 1997.

[5] Crocker, D., "Standard of the Format of ARPA-Internet Text
Messages", STD 11, RFC822, August 1982.

[6] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", RFC2119.

7. Security Considerations

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)

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.

8. Authors' Addresses

Mark Wahl
Critical Angle Inc.
4815 W. Braker Lane #502-385
Austin, TX 78759
USA

EMail: M.Wahl@critical-angle.com

Steve Kille
Isode Ltd.
The Dome
The Square
Richmond, Surrey
TW9 1DT
England

Phone: +44-181-332-9091
EMail: S.Kille@ISODE.COM

Tim Howes
Netscape Communications Corp.
501 E. Middlefield Rd, MS MV068
Mountain View, CA 94043
USA

Phone: +1 650 937-3419
EMail: howes@netscape.com

9. Full Copyright Statement

Copyright (C) The Internet Society (1997). All Rights Reserved.

This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.

The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.

This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
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