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

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", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 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 [CharModel].

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 evaluation.

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 specification.

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

SOFT HYPHEN (U+00AD) and MONGOLIAN TODO SOFT HYPHEN (U+1806) code 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 (U+0020).

   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 taken:

  • 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 removed.

   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 here.

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"
                 ( 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,
                 <>, 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 specification.

         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 insignificant.

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 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 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


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