rfc4518.txt

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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).

Abstract

   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.






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   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.



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   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





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   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.



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   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.

   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 (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].




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   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
   "<SPACE>foo<SPACE><SPACE>bar<SPACE>".




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   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:
       "<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 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
   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



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