LDAP DN strings: A String Representation of Distinguished Names

Network Working Group                                           S. Kille
Request for Comments: 1779                              ISODE Consortium
Obsoletes: 1485                                               March 1995
Category: Standards Track


             A 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.

Abstract

   The OSI Directory uses distinguished names as the primary keys to
   entries in the directory.  Distinguished Names are encoded in ASN.1.
   When a distinguished name is communicated between to users not using
   a directory protocol (e.g., in a mail message), there is a need to
   have a user-oriented string representation of distinguished name.
   This specification defines a string format for representing names,
   which is designed to give a clean representation of commonly used
   names, whilst being able to represent any distinguished name.

Table of Contents

   1.   Why a notation is needed ...................................   2
   2.   A notation for Distinguished Name ..........................   2
       2.1    Goals ................................................   2
       2.2    Informal definition ..................................   2
       2.3    Formal definition ....................................   4
   3.   Examples ...................................................   6
   4.   Acknowledgements ...........................................   7
   5.   References .................................................   7
   6.   Security Considerations ....................................   8
   7.   Author's Address ...........................................   8

1.  Why a notation is needed

   Many OSI Applications make use of Distinguished Names (DN) as defined
   in the OSI Directory, commonly known as X.500 [1].  This
   specification assumes familiarity with X.500, and the concept of
   Distinguished Name.  It is important to have a common format to be
   able to unambiguously represent a distinguished name.  This might be
   done to represent a directory name on a business card or in an email
   message.  There is a need for a format to support human to human
   communication, which must be string based (not ASN.1) and user
   oriented.  This notation is targeted towards a general user oriented
   system, and in particular to represent the names of humans.  Other
   syntaxes may be more appropriate for other uses of the directory.
   For example, the OSF Syntax may be more appropriate for some system
   oriented uses.  (The OSF Syntax uses "/" as a separator, and forms
   names in a manner intended to resemble UNIX filenames).

2.  A notation for Distinguished Name

2.1  Goals

   The following goals are laid out:

    o  To provide an unambiguous representation of a distinguished name

    o  To be an intuitive format for the majority of names

    o  To be fully general, and able to represent any distinguished name

    o  To be amenable to a number of different layouts to achieve an
       attractive representation.

    o  To give a clear representation of the contents of the
       distinguished name

2.2  Informal definition

   This notation is designed to be convenient for common forms of name.
   Some examples are given.  The author's directory distinguished name
   would be written:

   CN=Steve Kille,
   O=ISODE Consortium, C=GB
   This may be folded, perhaps to display in multi-column format.  For
   example:

   CN=Steve Kille,
   O=ISODE Consortium,
   C=GB

   Another name might be:

   CN=Christian Huitema, O=INRIA, C=FR

   Semicolon (";") may be used as an alternate separator.  The
   separators may be mixed, but this usage is discouraged.

   CN=Christian Huitema; O=INRIA; C=FR

   In running text, this would be written as <CN=Christian Huitema;
   O=INRIA; C=FR>.  Another example, shows how different attribute types
   are handled:

   CN=James Hacker,
   L=Basingstoke,
   O=Widget Inc,
   C=GB

   Here is an example of a multi-valued Relative Distinguished Name,
   where the namespace is flat within an organisation, and department is
   used to disambiguate certain names:

   OU=Sales + CN=J. Smith, O=Widget Inc., C=US

   The final examples show both methods quoting of a comma in an
   Organisation name:

   CN=L. Eagle, O="Sue, Grabbit and Runn", C=GB

   CN=L. Eagle, O=Sue\, Grabbit and Runn, C=GB

2.3  Formal definition

   A formal definition can now be given.  The structure is specified in
   a BNF grammar in Figure 1.  This BNF uses the grammar defined in RFC
   822, with the terminals enclosed in <> [2].  This definition is in an
   abstract character set, and so may be written in any character set
   supporting the explicitly defined special characters.  The quoting
   mechanism is used for the following cases:

    o  Strings containing ",", "+", "=" or """ , <CR>, "<",
       ">", "#", or ";".

    o  Strings with leading or trailing spaces

    o  Strings containing consecutive spaces

   There is an escape mechanism from the normal user oriented form, so
   that this syntax may be used to print any valid distinguished name.
   This is ugly.  It is expected to be used only in pathological cases.
   There are two parts to this mechanism:

   1.  Attributes types are represented in a (big-endian) dotted
       notation.  (e.g., OID.2.6.53).

   2.  Attribute values are represented in hexadecimal (e.g.  #0A56CF).
       Each pair of hex digits defines an octet, which is the ASN.1 Basic
       Encoding Rules value of the Attribute Value.

   The keyword specification is optional in the BNF, but mandatory for
   this specification.  This is so that the same BNF may be used for the
   related specification on User Friendly Naming [5].  When this
   specification is followed, the attribute type keywords must always be
   present.

   A list of valid keywords for well known attribute types used in
   naming is given in Table 1.  Keywords may contain spaces, but shall
   not have leading or trailing spaces.  This is a list of keywords
   which must be supported.  These are chosen because they appear in
   common forms of name, and can do so in a place which does not
   correspond to the default schema used.  A register of valid keywords
   is maintained by the IANA.

   <name> ::= <name-component> ( <spaced-separator> )
          | <name-component> <spaced-separator> <name>

   <spaced-separator> ::= <optional-space>
                   <separator>
                   <optional-space>

   <separator> ::=  "," | ";"

   <optional-space> ::= ( <CR> ) *( " " )

   <name-component> ::= <attribute>
           | <attribute> <optional-space> "+"
             <optional-space> <name-component>

   <attribute> ::= <string>
           | <key> <optional-space> "=" <optional-space> <string>

   <key> ::= 1*( <keychar> ) | "OID." <oid> | "oid." <oid>
   <keychar> ::= letters, numbers, and space

   <oid> ::= <digitstring> | <digitstring> "." <oid>
   <digitstring> ::= 1*<digit>
   <digit> ::= digits 0-9

   <string> ::= *( <stringchar> | <pair> )
            | '"' *( <stringchar> | <special> | <pair> ) '"'
            | "#" <hex>


   <special> ::= "," | "=" | <CR> | "+" | "<" |  ">"
            | "#" | ";"

   <pair> ::= "\" ( <special> | "\" | '"')
   <stringchar> ::= any character except <special> or "\" or '"'


   <hex> ::= 2*<hexchar>
   <hexchar> ::= 0-9, a-f, A-F



               Figure 1:  BNF Grammar for Distinguished Name

                       Key     Attribute (X.520 keys)
                       ------------------------------
                       CN      CommonName
                       L       LocalityName
                       ST      StateOrProvinceName
                       O       OrganizationName
                       OU      OrganizationalUnitName
                       C       CountryName
                       STREET  StreetAddress


                      Table 1:  Standardised Keywords


   Only string type attributes are considered, but other attribute
   syntaxes could be supported locally (e.g., by use of the syntexes
   defined in [3].)  It is assumed that the interface will translate
   from the supplied string into an appropriate Directory String
   encoding.  The "+" notation is used to specify multi-component RDNs.
   In this case, the types for attributes in the RDN must be explicit.

   The name is presented/input in a little-endian order (most
   significant component last).  When an address is written in a context
   where there is a need to delimit the entire address (e.g., in free
   text), it is recommended that the delimiters <> are used.  The
   terminator > is a special in the notation to facilitate this
   delimitation.

3.  Examples

   This section gives a few examples of distinguished names written
   using this notation:

   CN=Marshall T. Rose, O=Dover Beach Consulting, L=Santa Clara,
   ST=California, C=US

   CN=FTAM Service, CN=Bells, OU=Computer Science,
   O=University College London, C=GB

   CN=Markus Kuhn, O=University of Erlangen, C=DE

   CN=Steve Kille,
   O=ISODE Consortium,
   C=GB
   CN=Steve Kille ,

   O =   ISODE Consortium,
   C=GB

   CN=Steve Kille, O=ISODE Consortium, C=GB

4.  Acknowledgements

   This work was based on research work done at University College
   London [4], and evolved by the IETF OSI-DS WG.

   Input for this version of the document was received from:  Allan
   Cargille (University of Wisconsin); John Dale (COS); Philip Gladstone
   (Onsett); John Hawthorne (US Air Force); Roland Hedberg (University
   of Umea); Kipp Hickman (Mosaic Communications Corp.)  Markus Kuhn
   (University of Erlangen); Elisabeth Roudier (E3X); Mark Wahl (ISODE
   Consortium).

5.  References

   [1] The Directory --- overview of concepts, models and services,
       1993. CCITT X.500 Series Recommendations.

   [2] Crocker, D., "Standard of the Format of ARPA-Internet Text
       Messages", STD 11, RFC 822, University of Delaware, August 1982.

   [3] Yeong, W., Howes, T., and S. Kille, "Lightweight Directory Access
       Protocol", RFC 1777, Performance Systems International,
       University of Michigan, ISODE Consortium, March 1995.

   [4] S.E. Kille. Using the OSI directory to achieve user friendly
       naming. Research Note RN/20/29, Department of Computer Science,
       University College London, February 1990.

   [5] Kille, S., "Using the OSI Directory to Achieve User Friendly
       Naming", RFC 1781, ISODE Consortium, March 1995.

6.  Security Considerations

   Security issues are not discussed in this memo.

7.  Author's Address

   Steve Kille
   ISODE Consortium
   The Dome
   The Square
   Richmond, Surrey
   TW9 1DT
   England

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

   DN: CN=Steve Kille,
   O=ISODE Consortium, C=GB

   UFN: S. Kille,
   ISODE Consortium, GB