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 © 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

9. Full Copyright Statement

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

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