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

The X.500 Directory uses distinguished names as the primary keys to entries in the directory. Distinguished Names are encoded in ASN.1 in Zeilenga LDAPv3 DN [Page 1] INTERNET-DRAFT draft-ietf-ldapbis-dn-03.txt 1 April 2001 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 [X.500], and the concept of Distinguished Name (DN). 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). This document obsoletes RFC 2253. 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 [RFC2119]. 2. Converting DistinguishedName from ASN.1 to a String In X.501 [X.501] 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 [RFC2279] string representation. Zeilenga LDAPv3 DN [Page 2] INTERNET-DRAFT draft-ietf-ldapbis-dn-03.txt 1 April 2001 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 the following table of attribute types associated with LDAP [RFC2252bis], 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 [RFC2251bis]. The type name string is not case sensitive. String X.500 AttributeType ------------------------------------------------CN commonName (2.5.4.3) L localityName (2.5.4.7) ST stateOrProvinceName (2.5.4.8) O organizationName (2.5.4.10) OU organizationalUnitName (2.5.4.11) C countryName (2.5.4.6) Zeilenga LDAPv3 DN [Page 3] INTERNET-DRAFT draft-ietf-ldapbis-dn-03.txt 1 April 2001 STREET streetAddress (2.5.4.9) DC domainComponent (0.9.2342.19200300.100.1.25) UID userId (0.9.2342.19200300.100.1.1) 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 octets 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 [RFC2252bis]). 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. a space (’ ’ ASCII 32) or octothorpe (’#’ ASCII 35) occurring at the beginning of the string a space (’ ’ ASCII 32) character occurring at the end of the string one of the characters ",", "+", """, "\", "<", ">" or ";" (ASCII 44, 43, 34, 92, 60, 62, or 59, respectively) Implementations MAY escape other characters. Each octet of the character to be escaped is replaced by a backslash and two hex digits, which form a single octet in the code of the character. Alternatively, if and only if the character to be escaped is one of ",", "+", """, "\", "<", ">", ";", "#", "=", or " " (ASCII 44, 43, 34, 92, 60, 62, 59, 35, or 32, respectively) it may be prefixed by a backslash (’\’ ASCII 92). Examples of the escaping mechanism are shown in section 4. Zeilenga LDAPv3 DN [Page 4] INTERNET-DRAFT draft-ietf-ldapbis-dn-03.txt 1 April 2001 3. Parsing a String back to a Distinguished Name The structure of the UTF-8 [RFC2279] string is specified using the following Augmented BNF [RFC2234] grammar. distinguishedName = [name] ; may be empty name = name-component *(COMMA name-component) name-component = attributeTypeAndValue *(PLUS attributeTypeAndValue) attributeTypeAndValue = attributeType EQUALS attributeValue attributeType = keyword / oid keyword = ALPHA 1*keychar keychar = ALPHA / DIGIT / MINUS oid = number *(DOT number) number = ( LDIGIT *DIGIT ) / DIGIT attributeValue = string / hexstring string = *( stringchar / pair ) ; the string MUST NOT start with SHARP or SP ; and MUST NOT end with SP stringchar = pair = ESC ( ESC / special / hexpair ) special = escaped / SHARP / EQUALS / SP escaped = COMMA / PLUS / %x22 / %x3C / %x3E / %3B ; "," / "+" / """ / "<" / ">" / ";" hexstring = SHARP 1*hexpair hexpair = HEX HEX HEX = DIGIT / %x41-46 / %x61-66 ; 0-9 / A-F / a-f Zeilenga LDAPv3 DN [Page 5] INTERNET-DRAFT draft-ietf-ldapbis-dn-03.txt 1 April 2001 ALPHA = %x41-5A / %x61-7A ; A-Z / a-z LDIGIT = %x31-39 ; 1-9 DIGIT = %x30 / LDIGIT ; 0-9 SP = %x20 ; space (" ") SHARP = %x23 ; sharp sign ("#") PLUS = %x2B ; plus sign ("+") COMMA = %x2C ; comma (",") MINUS = %x2D ; minus sign ("-") DOT = %x2E ; period (".") EQUALS = %x3D ; equals sign ("=") ESC = %x5C ; backslash ("\") Implementations MUST recognize AttributeType string type names (keywords) listed in the Section 2 table, but MAY recognize other names (keywords). Implementations MAY recognize other DN string representations (such as that described in RFC 1779) but SHALL only generate DN strings in accordance with Section 2 of this document. 4. 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): UID=jsmith,DC=example,DC=net Here is an example name containing three RDNs, in which the first RDN is multi-valued: OU=Sales+CN=J. Smith,DC=example,DC=net This example shows the method of quoting of a comma in a common name: CN=John Smith\, III,DC=example,DC=net An example name in which a value contains a carriage return character: CN=Before\0dAfter,DC=example,DC=net An example name in which an RDN was of an unrecognized type. The Zeilenga LDAPv3 DN [Page 6] INTERNET-DRAFT draft-ietf-ldapbis-dn-03.txt 1 April 2001 value is the BER encoding of an OCTET STRING containing two octets 0x48 and 0x69. 1.3.6.1.4.1.1466.0=#04024869,DC=example,DC=com Finally, an example of an RDN commonName 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): CN=Lu\C4\8Di\C4\87 5. Security Considerations The following security considerations are specific to the handling of distinguished names. LDAP security considerations are discussed in [RFC2251bis] and its normative references. 5.1. 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. 5.2. 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 Zeilenga LDAPv3 DN [Page 7] INTERNET-DRAFT draft-ietf-ldapbis-dn-03.txt 1 April 2001 same BER or DER form. An example of a situation which requires theDER form of a distinguished name is the verification of an X.509certificate.For example, a distinguished name consisting of one RDN with one AVA,in which the type is commonName and the value is of the TeletexStringchoice with the letters ’Sam’ would be represented in LDAP as thestring CN=Sam. Another distinguished name in which the value is still’Sam’ but of the PrintableString choice would have the samerepresentation CN=Sam.Applications which require the reconstruction of the DER form of thevalue SHOULD NOT use the string representation of attribute syntaxeswhen 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. 6. References[X.500] "The Directory -overview of concepts, models andservices," ITU-T Rec. X.500(1993).[X.501] "The Directory -Models," ITU-T Rec. X.501(1993).[RFC2119] Bradner, S., "