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RFC 4518 - Lightweight Directory Access Protocol (LDAP): Interna

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Network Working Group                                        K. Zeilenga
Request for Comments: 4518                           OpenLDAP Foundation
Category: Standards Track                                      June 2006

             Lightweight Directory Access Protocol (LDAP):
                  Internationalized String Preparation

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 (2006).


   The previous Lightweight Directory Access Protocol (LDAP) technical
   specifications did not precisely define how character string matching
   is to be performed.  This led to a number of usability and
   interoperability problems.  This document defines string preparation
   algorithms for character-based matching rules defined for use in

1.  Introduction

1.1.  Background

   A Lightweight Directory Access Protocol (LDAP) [RFC4510] matching
   rule [RFC4517] defines an algorithm for determining whether a
   presented value matches an attribute value in accordance with the
   criteria defined for the rule.  The proposition may be evaluated to
   True, False, or Undefined.

      True      - the attribute contains a matching value,

      False     - the attribute contains no matching value,

      Undefined - it cannot be determined whether the attribute contains
                  a matching value.

   For instance, the caseIgnoreMatch matching rule may be used to
   compare whether the commonName attribute contains a particular value
   without regard for case and insignificant spaces.

1.2.  X.500 String Matching Rules

   "X.520: Selected attribute types" [X.520] provides (among other
   things) value syntaxes and matching rules for comparing values
   commonly used in the directory [X.500].  These specifications are
   inadequate for strings composed of Unicode [Unicode] characters.

   The caseIgnoreMatch matching rule [X.520], for example, is simply
   defined as being a case-insensitive comparison where insignificant
   spaces are ignored.  For printableString, there is only one space
   character and case mapping is bijective, hence this definition is
   sufficient.  However, for Unicode string types such as
   universalString, this is not sufficient.  For example, a case-
   insensitive matching implementation that folded lowercase characters
   to uppercase would yield different results than an implementation
   that used uppercase to lowercase folding.  Or one implementation may
   view space as referring to only SPACE (U+0020), a second
   implementation may view any character with the space separator (Zs)
   property as a space, and another implementation may view any
   character with the whitespace (WS) category as a space.

   The lack of precise specification for character string matching has
   led to significant interoperability problems.  When used in
   certificate chain validation, security vulnerabilities can arise.  To
   address these problems, this document defines precise algorithms for
   preparing character strings for matching.

1.3.  Relationship to "stringprep"

   The character string preparation algorithms described in this
   document are based upon the "stringprep" approach [RFC3454].  In
   "stringprep", presented and stored values are first prepared for
   comparison so that a character-by-character comparison yields the
   "correct" result.

   The approach used here is a refinement of the "stringprep" [RFC3454]
   approach.  Each algorithm involves two additional preparation steps.

   a) Prior to applying the Unicode string preparation steps outlined in
      "stringprep", the string is transcoded to Unicode.

   b) After applying the Unicode string preparation steps outlined in
      "stringprep", the string is modified to appropriately handle
      characters insignificant to the matching rule.

   Hence, preparation of character strings for X.500 [X.500] matching
   [X.501] involves the following steps:

      1) Transcode
      2) Map
      3) Normalize
      4) Prohibit
      5) Check Bidi (Bidirectional)
      6) Insignificant Character Handling

   These steps are described in Section 2.

   It is noted that while various tables of Unicode characters included
   or referenced by this specification are derived from Unicode
   [Unicode] data, these tables are to be considered definitive for the
   purpose of implementing this specification.

1.4.  Relationship to the LDAP Technical Specification

   This document is an integral part of the LDAP technical specification
   [RFC4510], which obsoletes the previously defined LDAP technical
   specification [RFC3377] in its entirety.

   This document details new LDAP internationalized character string
   preparation algorithms used by [RFC4517] and possible other technical
   specifications defining LDAP syntaxes and/or matching rules.

1.5.  Relationship to X.500

   LDAP is defined [RFC4510] in X.500 terms as an X.500 access
   mechanism.  As such, there is a strong desire for alignment between
   LDAP and X.500 syntax and semantics.  The character string
   preparation algorithms described in this document are based upon
   "Internationalized String Matching Rules for X.500" [XMATCH] proposal
   to ITU/ISO Joint Study Group 2.

1.6.  Conventions and Terms

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in BCP 14 [RFC2119].

   Character names in this document use the notation for code points and
   names from the Unicode Standard [Unicode].  For example, the letter
   "a" may be represented as either <U+0061> or <LATIN SMALL LETTER A>.
   In the lists of mappings and the prohibited characters, the "U+" is

   left off to make the lists easier to read.  The comments for
   character ranges are shown in square brackets (such as "[CONTROL
   CHARACTERS]") and do not come from the standard.

   Note: a glossary of terms used in Unicode can be found in [Glossary].
   Information on the Unicode character encoding model can be found in

   The term "combining mark", as used in this specification, refers to
   any Unicode [Unicode] code point that has a mark property (Mn, Mc,
   Me).  Appendix A provides a definitive list of combining marks.

2.  String Preparation

   The following six-step process SHALL be applied to each presented and
   attribute value in preparation for character string matching rule

      1) Transcode
      2) Map
      3) Normalize
      4) Prohibit
      5) Check bidi
      6) Insignificant Character Handling

   Failure in any step causes the assertion to evaluate to Undefined.

   The character repertoire of this process is Unicode 3.2 [Unicode].

   Note that this six-step process specification is intended to describe
   expected matching behavior.  Implementations are free to use
   alternative processes so long as the matching rule evaluation
   behavior provided is consistent with the behavior described by this

2.1.  Transcode

   Each non-Unicode string value is transcoded to Unicode.

   PrintableString [X.680] values are transcoded directly to Unicode.

   UniversalString, UTF8String, and bmpString [X.680] values need not be
   transcoded as they are Unicode-based strings (in the case of
   bmpString, a subset of Unicode).

   TeletexString [X.680] values are transcoded to Unicode.  As there is
   no standard for mapping TeletexString values to Unicode, the mapping
   is left a local matter.

   For these and other reasons, use of TeletexString is NOT RECOMMENDED.

   The output is the transcoded string.

2.2.  Map

   points are mapped to nothing.  COMBINING GRAPHEME JOINER (U+034F) and
   VARIATION SELECTORs (U+180B-180D, FF00-FE0F) code points are also
   mapped to nothing.  The OBJECT REPLACEMENT CHARACTER (U+FFFC) is
   mapped to nothing.

   (U+000D), and NEXT LINE (NEL) (U+0085) are mapped to SPACE (U+0020).

   All other control code (e.g., Cc) points or code points with a
   control function (e.g., Cf) are mapped to nothing.  The following is
   a complete list of these code points: U+0000-0008, 000E-001F, 007F-
   0084, 0086-009F, 06DD, 070F, 180E, 200C-200F, 202A-202E, 2060-2063,
   206A-206F, FEFF, FFF9-FFFB, 1D173-1D17A, E0001, E0020-E007F.

   ZERO WIDTH SPACE (U+200B) is mapped to nothing.  All other code
   points with Separator (space, line, or paragraph) property (e.g., Zs,
   Zl, or Zp) are mapped to SPACE (U+0020).  The following is a complete
   list of these code points: U+0020, 00A0, 1680, 2000-200A, 2028-2029,
   202F, 205F, 3000.

   For case ignore, numeric, and stored prefix string matching rules,
   characters are case folded per B.2 of [RFC3454].

   The output is the mapped string.

2.3.  Normalize

   The input string is to be normalized to Unicode Form KC
   (compatibility composed) as described in [UAX15].  The output is the
   normalized string.

2.4.  Prohibit

   All Unassigned code points are prohibited.  Unassigned code points
   are listed in Table A.1 of [RFC3454].

   Characters that, per Section 5.8 of [RFC3454], change display
   properties or are deprecated are prohibited.  These characters are
   listed in Table C.8 of [RFC3454].

   Private Use code points are prohibited.  These characters are listed
   in Table C.3 of [RFC3454].

   All non-character code points are prohibited.  These code points are
   listed in Table C.4 of [RFC3454].

   Surrogate codes are prohibited.  These characters are listed in Table
   C.5 of [RFC3454].

   The REPLACEMENT CHARACTER (U+FFFD) code point is prohibited.

   The step fails if the input string contains any prohibited code
   point.  Otherwise, the output is the input string.

2.5.  Check bidi

   Bidirectional characters are ignored.

2.6.  Insignificant Character Handling

   In this step, the string is modified to ensure proper handling of
   characters insignificant to the matching rule.  This modification
   differs from matching rule to matching rule.

   Section 2.6.1 applies to case ignore and exact string matching.
   Section 2.6.2 applies to numericString matching.
   Section 2.6.3 applies to telephoneNumber matching.

2.6.1.  Insignificant Space Handling

   For the purposes of this section, a space is defined to be the SPACE
   (U+0020) code point followed by no combining marks.

       NOTE - The previous steps ensure that the string cannot contain
              any code points in the separator class, other than SPACE

   For input strings that are attribute values or non-substring
   assertion values:  If the input string contains no non-space
   character, then the output is exactly two SPACEs.  Otherwise (the
   input string contains at least one non-space character), the string
   is modified such that the string starts with exactly one space
   character, ends with exactly one SPACE character, and any inner
   (non-empty) sequence of space characters is replaced with exactly two
   SPACE characters.  For instance, the input strings
   "foo<SPACE>bar<SPACE><SPACE>", result in the output

   For input strings that are substring assertion values: If the string
   being prepared contains no non-space characters, then the output
   string is exactly one SPACE.  Otherwise, the following steps are

   -  If the input string is an initial substring, it is modified to
      start with exactly one SPACE character;

   -  If the input string is an initial or an any substring that ends in
      one or more space characters, it is modified to end with exactly
      one SPACE character;

   -  If the input string is an any or a final substring that starts in
      one or more space characters, it is modified to start with exactly
      one SPACE character; and

   -  If the input string is a final substring, it is modified to end
      with exactly one SPACE character.

   For instance, for the input string "foo<SPACE>bar<SPACE><SPACE>" as
   an initial substring, the output would be
   "<SPACE>foo<SPACE><SPACE>bar<SPACE>".  As an any or final substring,
   the same input would result in "foo<SPACE>bar<SPACE>".

   Appendix B discusses the rationale for the behavior.

2.6.2.  numericString Insignificant Character Handling

   For the purposes of this section, a space is defined to be the SPACE
   (U+0020) code point followed by no combining marks.

   All spaces are regarded as insignificant and are to be removed.

   For example, removal of spaces from the Form KC string:
   would result in the output string:
   and the Form KC string:
   would result in the output string:
       "" (an empty string).

2.6.3.  telephoneNumber Insignificant Character Handling

   For the purposes of this section, a hyphen is defined to be a
   (U+FE63), or FULLWIDTH HYPHEN-MINUS (U+FF0D) code point followed by

   no combining marks and a space is defined to be the SPACE (U+0020)
   code point followed by no combining marks.

   All hyphens and spaces are considered insignificant and are to be

   For example, removal of hyphens and spaces from the Form KC string:
   would result in the output string:
   and the Form KC string:
   would result in the (empty) output string:

3.  Security Considerations

   "Preparation of Internationalized Strings ("stringprep")" [RFC3454]
   security considerations generally apply to the algorithms described

4.  Acknowledgements

   The approach used in this document is based upon design principles
   and algorithms described in "Preparation of Internationalized Strings
   ('stringprep')" [RFC3454] by Paul Hoffman and Marc Blanchet.  Some
   additional guidance was drawn from Unicode Technical Standards,
   Technical Reports, and Notes.

   This document is a product of the IETF LDAP Revision (LDAPBIS)
   Working Group.

5.  References

5.1.  Normative References

   [RFC2119]     Bradner, S., "Key words for use in RFCs to Indicate
                 Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3454]     Hoffman, P. and M. Blanchet, "Preparation of
                 Internationalized Strings ("stringprep")", RFC 3454,
                 December 2002.

   [RFC4510]     Zeilenga, K., "Lightweight Directory Access Protocol
                 (LDAP): Technical Specification Road Map", RFC 4510,
                 June 2006.

   [RFC4517]     Legg, S., Ed., "Lightweight Directory Access Protocol
                 (LDAP): Syntaxes and Matching Rules", RFC 4517, June

   [Unicode]     The Unicode Consortium, "The Unicode Standard, Version
                 3.2.0" is defined by "The Unicode Standard, Version
                 3.0" (Reading, MA, Addison-Wesley, 2000.  ISBN 0-201-
                 61633-5), as amended by the "Unicode Standard Annex
                 #27: Unicode 3.1"
                 (http://www.unicode.org/reports/tr27/) and by the
                 "Unicode Standard Annex #28: Unicode 3.2"

   [UAX15]       Davis, M. and M. Duerst, "Unicode Standard Annex #15:
                 Unicode Normalization Forms, Version 3.2.0".
                 22.html>, March 2002.

   [X.680]       International Telecommunication Union -
                 Telecommunication Standardization Sector, "Abstract
                 Syntax Notation One (ASN.1) - Specification of Basic
                 Notation", X.680(2002) (also ISO/IEC 8824-1:2002).

5.2.  Informative References

   [X.500]       International Telecommunication Union -
                 Telecommunication Standardization Sector, "The
                 Directory -- Overview of concepts, models and
                 services," X.500(1993) (also ISO/IEC 9594-1:1994).

   [X.501]       International Telecommunication Union -
                 Telecommunication Standardization Sector, "The
                 Directory -- Models," X.501(1993) (also ISO/IEC 9594-

   [X.520]       International Telecommunication Union -
                 Telecommunication Standardization Sector, "The
                 Directory: Selected Attribute Types", X.520(1993) (also
                 ISO/IEC 9594-6:1994).

   [Glossary]    The Unicode Consortium, "Unicode Glossary",

   [CharModel]   Whistler, K. and M. Davis, "Unicode Technical Report
                 #17, Character Encoding Model", UTR17,
                 <http://www.unicode.org/unicode/reports/tr17/>, August

   [RFC3377]     Hodges, J. and R. Morgan, "Lightweight Directory Access
                 Protocol (v3): Technical Specification", RFC 3377,
                 September 2002.

   [RFC4515]     Smith, M., Ed. and T. Howes, "Lightweight Directory
                 Access Protocol (LDAP): String Representation of Search
                 Filters", RFC 4515, June 2006.

   [XMATCH]      Zeilenga, K., "Internationalized String Matching Rules
                 for X.500", Work in Progress.

Appendix A.  Combining Marks

   This appendix is normative.

   This table was derived from Unicode [Unicode] data files; it lists
   all code points with the Mn, Mc, or Me properties.  This table is to
   be considered definitive for the purposes of implementation of this

         0300-034F 0360-036F 0483-0486 0488-0489 0591-05A1
         05A3-05B9 05BB-05BC 05BF 05C1-05C2 05C4 064B-0655 0670
         06D6-06DC 06DE-06E4 06E7-06E8 06EA-06ED 0711 0730-074A
         07A6-07B0 0901-0903 093C 093E-094F 0951-0954 0962-0963
         0981-0983 09BC 09BE-09C4 09C7-09C8 09CB-09CD 09D7
         09E2-09E3 0A02 0A3C 0A3E-0A42 0A47-0A48 0A4B-0A4D
         0A70-0A71 0A81-0A83 0ABC 0ABE-0AC5 0AC7-0AC9 0ACB-0ACD
         0B01-0B03 0B3C 0B3E-0B43 0B47-0B48 0B4B-0B4D 0B56-0B57
         0B82 0BBE-0BC2 0BC6-0BC8 0BCA-0BCD 0BD7 0C01-0C03
         0C3E-0C44 0C46-0C48 0C4A-0C4D 0C55-0C56 0C82-0C83
         0CBE-0CC4 0CC6-0CC8 0CCA-0CCD 0CD5-0CD6 0D02-0D03
         0D3E-0D43 0D46-0D48 0D4A-0D4D 0D57 0D82-0D83 0DCA
         0DCF-0DD4 0DD6 0DD8-0DDF 0DF2-0DF3 0E31 0E34-0E3A
         0E47-0E4E 0EB1 0EB4-0EB9 0EBB-0EBC 0EC8-0ECD 0F18-0F19
         0F35 0F37 0F39 0F3E-0F3F 0F71-0F84 0F86-0F87 0F90-0F97
         0F99-0FBC 0FC6 102C-1032 1036-1039 1056-1059 1712-1714
         1732-1734 1752-1753 1772-1773 17B4-17D3 180B-180D 18A9
         20D0-20EA 302A-302F 3099-309A FB1E FE00-FE0F FE20-FE23
         1D165-1D169 1D16D-1D172 1D17B-1D182 1D185-1D18B

Appendix B.  Substrings Matching

   This appendix is non-normative.

   In the absence of substrings matching, the insignificant space
   handling for case ignore/exact matching could be simplified.
   Specifically, the handling could be to require that all sequences of
   one or more spaces be replaced with one space and, if the string
   contains non-space characters, removal of all leading spaces and
   trailing spaces.

   In the presence of substrings matching, this simplified space
   handling would lead to unexpected and undesirable matching behavior.
   For instance:

   1) (CN=foo\20*\20bar) would match the CN value "foobar";

   2) (CN=*\20foobar\20*) would match "foobar", but
      (CN=*\20*foobar*\20*) would not.

   Note to readers not familiar with LDAP substrings matching: the LDAP
   filter [RFC4515] assertion (CN=A*B*C) says to "match any value (of
   the attribute CN) that begins with A, contains B after A, ends with C
   where C is also after B."

   The first case illustrates that this simplified space handling would
   cause leading and trailing spaces in substrings of the string to be
   regarded as insignificant.  However, only leading and trailing (as
   well as multiple consecutive spaces) of the string (as a whole) are

   The second case illustrates that this simplified space handling would
   cause sub-partitioning failures.  That is, if a prepared any
   substring matches a partition of the attribute value, then an
   assertion constructed by subdividing that substring into multiple
   substrings should also match.

   In designing an appropriate approach for space handling for
   substrings matching, one must study key aspects of X.500 case
   exact/ignore matching.  X.520 [X.520] says:

      The [substrings] rule returns TRUE if there is a partitioning of
      the attribute value (into portions) such that:

         -  the specified substrings (initial, any, final) match
            different portions of the value in the order of the strings

         -  initial, if present, matches the first portion of the value;

         -  final, if present, matches the last portion of the value;

         -  any, if present, matches some arbitrary portion of the

   That is, the substrings assertion (CN=foo\20*\20bar) matches the
   attribute value "foo<SPACE><SPACE>bar" as the value can be
   partitioned into the portions "foo<SPACE>" and "<SPACE>bar" meeting
   the above requirements.

   X.520 also says:

      [T]he following spaces are regarded as not significant:

         -  leading spaces (i.e., those preceding the first character
            that is not a space);

         -  trailing spaces (i.e., those following the last character
            that is not a space);

         -  multiple consecutive spaces (these are taken as equivalent
            to a single space character).

   This statement applies to the assertion values and attribute values
   as whole strings, and not individually to substrings of an assertion
   value.  In particular, the statements should be taken to mean that if
   an assertion value and attribute value match without any
   consideration to insignificant characters, then that assertion value
   should also match any attribute value that differs only by inclusion
   nor removal of insignificant characters.

   Hence the assertion (CN=foo\20*\20bar) matches
   "foo<SPACE><SPACE><SPACE>bar" and "foo<SPACE>bar" as these values
   only differ from "foo<SPACE><SPACE>bar" by the inclusion or removal
   of insignificant spaces.

   Astute readers of this text will also note that there are special
   cases where the specified space handling does not ignore spaces that
   could be considered insignificant.  For instance, the assertion
   (CN=\20*\20*\20) does not match "<SPACE><SPACE><SPACE>"
   (insignificant spaces present in value) or " " (insignificant spaces
   not present in value).  However, as these cases have no practical
   application that cannot be met by simple assertions, e.g., (cn=\20),
   and this minor anomaly can only be fully addressed by a preparation
   algorithm to be used in conjunction with character-by-character
   partitioning and matching, the anomaly is considered acceptable.

Author's Address

   Kurt D. Zeilenga
   OpenLDAP Foundation

   EMail: Kurt@OpenLDAP.org

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