Lightweight Directory Access Protocol (v3): UTF-8 String Representation of Distinguished Names
draft-ietf-asid-ldapv3-dn-03

Network Working Group                                            M. Wahl
INTERNET-DRAFT                                       Critical Angle Inc.
Obsoletes: RFC 1779                                             S. Kille
                                                              Isode Ltd.
                                                                T. Howes
                                           Netscape Communications Corp.
Expires in six months from                                April 29, 1997
Intended Category: Standards Track

             Lightweight Directory Access Protocol (v3):
          UTF-8 String Representation of Distinguished Names
                  <draft-ietf-asid-ldapv3-dn-03.txt>

Status of this Memo

   This document is an Internet-Draft.  Internet-Drafts are working 
   documents of the Internet Engineering Task Force (IETF), its areas, and
   its working groups.  Note that other groups may also distribute working
   documents as Internet-Drafts.
 
   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference material
   or to cite them other than as "work in progress."
 
   To learn the current status of any Internet-Draft, please check the
   "1id-abstracts.txt" listing  contained in the Internet-Drafts Shadow
   Directories on ds.internic.net (US East Coast), nic.nordu.net (Europe),
   ftp.isi.edu (US West Coast), or munnari.oz.au (Pacific Rim).

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.

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).

Wahl,Kille,Howes   String Representations of Distinguished Names   Page 1


INTERNET-DRAFT        draft-ietf-asid-ldapv3-dn-03.txt         April 1997

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.

Wahl,Kille,Howes  String Representations of Distinguished Names    Page 2


INTERNET-DRAFT        draft-ietf-asid-ldapv3-dn-03.txt         April 1997

   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.

Wahl,Kille,Howes  String Representations of Distinguished Names    Page 3


INTERNET-DRAFT        draft-ietf-asid-ldapv3-dn-03.txt         April 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>
   
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.

Wahl,Kille,Howes  String Representations of Distinguished Names    Page 4


INTERNET-DRAFT        draft-ietf-asid-ldapv3-dn-03.txt         April 1997

   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:

Wahl,Kille,Howes  String Representations of Distinguished Names    Page 5


INTERNET-DRAFT        draft-ietf-asid-ldapv3-dn-03.txt         April 1997

   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] M. Wahl, S. Kille, T. Howes, "Lightweight Directory Access 
       Protocol (v3)", INTERNET DRAFT, 
       draft-ietf-asid-ldapv3-protocol-05.txt. April 1997.

   [4] M. Wahl, S. Kille, T. Howes, A. Coulbeck, "Lightweight Directory 
       Access Protocol (v3): Attribute Syntax Definitions", INTERNET 
       DRAFT, draft-ietf-asid-ldapv3-attributes-05.txt.
       April 1997.

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

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.

Wahl,Kille,Howes  String Representations of Distinguished Names    Page 6


INTERNET-DRAFT        draft-ietf-asid-ldapv3-dn-03.txt         April 1997

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.  Author's Address

   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
   Mountain View, CA 94043
   USA
       
   Phone:  +1 415 254-1900
   EMail:   howes@netscape.com

Wahl,Kille,Howes  String Representations of Distinguished Names    Page 7