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Versions: 00                                                            
Internet-Draft                              Editor: Kurt D. Zeilenga
Intended Category: Standard Track                OpenLDAP Foundation
Expires in six months                                     4 May 2003



                LDAP: Internationalized String Preparation
                 <draft-zeilenga-ldapbis-strprep-00.txt>


Status of this Memo

  This document is an Internet-Draft and is in full conformance with all
  provisions of Section 10 of RFC2026.

  Distribution of this memo is unlimited.  Technical discussion of this
  document will take place on the IETF LDAP Revision Working Group
  mailing list <ietf-ldapbis@openldap.org>.  Please send editorial
  comments directly to the author <Kurt@OpenLDAP.org>.

  Internet-Drafts are working documents of the Internet Engineering Task
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  Copyright 2003, The Internet Society.  All Rights Reserved.

  Please see the Copyright section near the end of this document for
  more information.


Abstract

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





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Conventions

  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] and ISO/IEC 10646-1
  [ISO10646].  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
  standards.

  Note: a glossary of terms used in Unicode and ISO/IEC 10646 can be
  found in [GLOSSARY].  Information on the ISO/IEC 10646/Unicode
  character encoding model can be found in [UTR17].


1. Introduction

1.1. Background

  An LDAP 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 characters from the Universal Character Set
  (UCS) [ISO10646], a superset of Unicode [UNICODE].



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  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 UCS-based 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 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
  strings for matching.


1.3. Relationship to "stringprep"

  The 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 and 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", characters insignificant to the matching rules are
     removed.

  Hence, preparation of strings for X.500 matching involves the
  following steps:

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




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  These steps are described in Section 2.


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 LDAP internationalized 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 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 string match rule evaluation.

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

  Failure in any step is be cause the assertion to be Undefined.

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


2.1. Transcode

  Each non-Unicode string value is transcoded to Unicode.

  TeletexString values are transcoded to Unicode as described in
  Appendix A.

  PrintableString value are transcoded directly to Unicode.



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

  If the implementation is unable or unwilling to perform the
  transcoding as described above, or the transcoding fails, this step
  fails and the assertion is evaluated to Undefined.

  The transcoded string is the output 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.

  CHARACTER TABULATION (U+0009), LINE FEED (LF) (U+000A), LINE
  TABULATION (U+000B), FORM FEED (FF) (U+000C), CARRIAGE RETURN (CR)
  (U+000D), and NEXT LINE (NEL) (U+0085) are mapped to SPACE (U+0020).

  All other control code points (e.g., Cc) or code points with a control
  function (e.g., Cf) are mapped to nothing.

  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).

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


2.3. Normalize

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


2.4. Prohibit

  All Unassigned, Private Use, and non-character code points are
  prohibited.  Surrogate codes (U+D800-DFFFF) are prohibited.

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

  The first code point of a string is prohibited from being a combining



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  character.

  Empty strings are prohibited.

  The step fails and the assertion is evaluated to Undefined if the
  input string contains any prohibited code point.  The output string is
  the input string.


2.5. Check bidi

  There are no bidirectional restrictions.  The output string is the
  input string.


2.5. Insignificant Character Removal

  In this step, characters insignificant to the matching rule are to be
  removed.  The characters to be removed differ 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 Removal

  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).

  The following spaces are regarded as not significant and are to be
  removed:
    - 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).

  (A string consisting entirely of spaces is equivalent to a string
  containing exactly one space.)

  For example, removal of spaces from the Form KC string:



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      "<SPACE><SPACE>foo<SPACE><SPACE>bar<SPACE><SPACE>" would result in
  the output string:
      "<SPACE>foo<SPACE>bar<SPACE>".

  and the Form KC string:
      "<SPACE><SPACE><SPACE>" would result in the output string:
      "<SPACE>".


2.6.2. NumericString Insignificant Character Removal

  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 not significant and are to be removed.

  For example, removal of spaces from the Form KC string:
      "<SPACE><SPACE>123<SPACE><SPACE>456<SPACE><SPACE>" would result in
  the output string:
      "123456".

  and the Form KC string:
      "<SPACE><SPACE><SPACE>" would result in an empty output string.


2.6.3. TelephoneNumber Insignificant Character Removal

  For the purposes of this section, a hyphen is defined to be
  HYPHEN-MINUS (U+002D), ARMENIAN HYPHEN (U+058A), HYPHEN (U+2010),
  NON-BREAKING HYPHEN (U+2011), MINUS SIGN (U+2212), SMALL HYPHEN-MINUS
  (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 regarded as not significant and are to be
  removed.


3. Security Considerations

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


4. Acknowledgments

  The approach used in this document is based upon design principles and



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  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.


5. Editor's Address

  Kurt Zeilenga
  E-mail: <kurt@openldap.org>


6. References

6.1. Normative References

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

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

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

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

  [ISO10646]   Universal Multiple-Octet Coded Character Set (UCS) -
             Architecture and Basic Multilingual Plane, ISO/IEC 10646-1
             : 1993.

  [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"
             (http://www.unicode.org/reports/tr28/).

  [UAX15]    M. Davis, M. Duerst, "Unicode Standard Annex #15: Unicode
             Normalization Forms, Version 3.2.0".
             <http://www.unicode.org/unicode/reports/tr15/tr15-22.html>,
             March 2002.


6.2. Informative References




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  [X.500]    International Telephone Union, "The Directory: Overview of
             Concepts, Models and Service", X.500, 2000.

  [X.501]    International Telephone Union, "The Directory: The Models",
             X.501, 2000.

  [X.520]    International Telephone Union, "The Directory: Selected
             Attribute Types", X.520, 2000.

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

             [GLOSSARY] The Unicode Consortium, "Unicode Glossary",
             <http://www.unicode.org/glossary/>.

             [UTR17]    K. Whistler, M. Davis, "Unicode Technical Report
             #17, Character Encoding Model", UTR17,
             <http://www.unicode.org/unicode/reports/tr17/>, August
             2000.



Copyright 2003, The Internet Society.  All Rights Reserved.

  This document and translations of it may be copied and furnished to
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  INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
  WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.




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  Appendix A. Teletex (T.61) to Unicode

  TBD.
















































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