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

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.

   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
       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 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>

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
   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.

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

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