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rfc:rfc2253

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 RFC 2026, 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.

Wahl, et. al. Proposed Standard [Page 1] RFC 2253 LADPv3 Distinguished Names December 1997

 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 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

Wahl, et. al. Proposed Standard [Page 2] RFC 2253 LADPv3 Distinguished Names December 1997

     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:

Wahl, et. al. Proposed Standard [Page 3] RFC 2253 LADPv3 Distinguished Names December 1997

                  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.

Wahl, et. al. Proposed Standard [Page 4] RFC 2253 LADPv3 Distinguished Names December 1997

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

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>

Wahl, et. al. Proposed Standard [Page 5] RFC 2253 LADPv3 Distinguished Names December 1997

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 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

Wahl, et. al. Proposed Standard [Page 6] RFC 2253 LADPv3 Distinguished Names December 1997

 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)", 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.

Wahl, et. al. Proposed Standard [Page 7] RFC 2253 LADPv3 Distinguished Names December 1997

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:
  1. the common name of the object (i.e. a person's full name)
  2. an email or TCP/IP address
  3. its physical location (country, locality, city, street address)
  4. 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

Wahl, et. al. Proposed Standard [Page 8] RFC 2253 LADPv3 Distinguished Names December 1997

 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

Wahl, et. al. Proposed Standard [Page 9] RFC 2253 LADPv3 Distinguished Names December 1997

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
 HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
 MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Wahl, et. al. Proposed Standard [Page 10]

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