Internet-Draft                                      Kurt D. Zeilenga
Intended Category: Standard Track                OpenLDAP Foundation
Expires in six months                              30 September 2005                                23 January 2006

                LDAP: Internationalized String Preparation

Status of this Memo

  This document is intended to be published as a Standard Track RFC.
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  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 LDAP.

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 which has a mark property (Mn, Mc,
  Me).  Appendix A provides a complete definitive list of combining marks.

1. Introduction

1.1. Background

  A Lightweight Directory Access Protocol (LDAP) [Roadmap] matching rule
  [Syntaxes] 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 or not.

  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 (amongst other
  things) value syntaxes and matching rules for comparing values
  commonly used in the Directory.  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 which folded lower case characters
  to upper case would yield different different results than an
  implementation which used upper case to lower case 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 [StringPrep]. [RFC3454].  In "stringprep",
  presented and stored values are first prepared for comparison and so
  that a character-by-character comparison yields the "correct" result.

  The approach used here is a refinement of the "stringprep"
  [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 matching 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 a integral part of the LDAP technical specification
  [Roadmap] which obsoletes the previously defined LDAP technical
  specification [RFC3377] in its entirety.

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

1.5. Relationship to X.500

  LDAP is defined [Roadmap] 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.

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 described
  expected matching behavior.   Implementations are free use alternative
  processes so long as the matching rule evaluation behavior provided is
  consistent with the behavior described by this specification.

2.1. Transcode

  Each non-Unicode string value is transcoded to Unicode.

  PrintableString [X.680] value 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 TelexString 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 [StringPrep]. [RFC3454].

  The output is the mapped string.

2.3. Normalize

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

2.4. Prohibit

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

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

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

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

  Surrogate codes are prohibited.  These characters are listed in Table
  C.5 of [StringPrep]. [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 (U+0020).

  If the input string contains at least one non-space character, then
  the string is modified such that the string starts with exactly one
  space character, ends with exactly one SPACE character, and that 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>", results in the output

  Otherwise, if the string being prepared is an initial, any, or final
  substring, then the output string is exactly one SPACE character, else
  the output string is exactly two SPACEs.

  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
  (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 for International Strings ('stringprep')" [StringPrep] [RFC3454]
  security considerations generally apply to the algorithms described

4. Acknowledgments

  The approach used in this document is based upon design principles and
  algorithms described in "Preparation of Internationalized Strings
  ('stringprep')" [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

5. Author's Address

  Kurt D. Zeilenga
  OpenLDAP Foundation


6. References

  [[Note to the RFC Editor: please replace the citation tags used in
  referencing Internet-Drafts with tags of the form RFCnnnn where

6.1. Normative References

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

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

  [Roadmap]     Zeilenga, K. (editor), "LDAP: Technical Specification
                Road Map", draft-ietf-ldapbis-roadmap-xx.txt, a work in

  [StringPrep]  Hoffman P. and M. Blanchet, "Preparation of
                Internationalized Strings ('stringprep')",
                draft-hoffman-rfc3454bis-xx.txt, a work in progress.

  [Syntaxes]    Legg, S. (editor), "LDAP: Syntaxes and Matching Rules",
                draft-ietf-ldapbis-syntaxes-xx.txt, a work in progress.

  [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" ( 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".
                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).

6.2. 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 9594-2:1994).

  [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,
                <>, August

  [Filters]     Smith, M. (editor), LDAPbis WG, "LDAP: String
                Representation of Search Filters",
                draft-ietf-ldapbis-filter-xx.txt, a work in progress.

  [XMATCH]      Zeilenga, K., "Internationalized String Matching Rules
                for X.500", draft-zeilenga-ldapbis-strmatch-xx.txt, a
                work in progress.

  [RFC1345]     Simonsen, K., "Character Mnemonics & Character Sets",
                RFC 1345, June 1992.

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

                In the presence of substrings matching, this simplified
                space handling
  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" but not
    "foo<SPACE>bar" nor "foo<SPACE><SPACE>bar";
  2) (CN=*\20foobar\20*) would match "foobar", but (CN=*\20*foobar*\20*)
    would not;
  3) (CN=foo\20*\20bar) would match "foo<SPACE>X<SPACE>bar" but not

  Note to readers not familiar with LDAP substrings matching: the LDAP
  filter [Filters] assertion (CN=A*B*C) says "match any value (of the
  attribute CN) which 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.

  The third case illustrates that this simplified space handling causes
  another partitioning failure.  Though both the initial or final
  strings match different portions of "foo<SPACE>X<SPACE>bar" with
  neither matching the X portion, they don't match a string consisting
  of the two matched portions less the unmatched X portion.

  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 sequence;
      - 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 value.

  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

  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 which differs only by inclusion
  or 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
  which could be considered insignificant.   For instance, the assertion
  (CN=\20*\20*\20) does not match "<SPACE><SPACE><SPACE>"
  (insignificant spaces present in value) nor " " (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.

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