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rfc:rfc4518

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.

Zeilenga Standards Track [Page 1] RFC 4518 LDAP: Internationalized String Preparation June 2006

 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.

Zeilenga Standards Track [Page 2] RFC 4518 LDAP: Internationalized String Preparation June 2006

 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

Zeilenga Standards Track [Page 3] RFC 4518 LDAP: Internationalized String Preparation June 2006

 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.

Zeilenga Standards Track [Page 4] RFC 4518 LDAP: Internationalized String Preparation June 2006

 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].

Zeilenga Standards Track [Page 5] RFC 4518 LDAP: Internationalized String Preparation June 2006

 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>".

Zeilenga Standards Track [Page 6] RFC 4518 LDAP: Internationalized String Preparation June 2006

 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:
  1. If the input string is an initial substring, it is modified to

start with exactly one SPACE character;

  1. 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;
  1. 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
  1. 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

Zeilenga Standards Track [Page 7] RFC 4518 LDAP: Internationalized String Preparation June 2006

 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:
     "<SPACE><HYPHEN>123<SPACE><SPACE>456<SPACE><HYPHEN>"
 would result in the output string:
     "123456"
 and the Form KC string:
     "<HYPHEN><HYPHEN><HYPHEN>"
 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.

Zeilenga Standards Track [Page 8] RFC 4518 LDAP: Internationalized String Preparation June 2006

 [RFC4517]     Legg, S., Ed., "Lightweight Directory Access Protocol
               (LDAP): Syntaxes and Matching Rules", RFC 4517, June
               2006.
 [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]       Davis, M. and 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.
 [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-
               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",
               <http://www.unicode.org/glossary/>.
 [CharModel]   Whistler, K. and M. Davis, "Unicode Technical Report
               #17, Character Encoding Model", UTR17,
               <http://www.unicode.org/unicode/reports/tr17/>, August
               2000.

Zeilenga Standards Track [Page 9] RFC 4518 LDAP: Internationalized String Preparation June 2006

 [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.

Zeilenga Standards Track [Page 10] RFC 4518 LDAP: Internationalized String Preparation June 2006

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
       1D1AA-1D1AD

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";

Zeilenga Standards Track [Page 11] RFC 4518 LDAP: Internationalized String Preparation June 2006

 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:
  1. the specified substrings (initial, any, final) match

different portions of the value in the order of the strings

          sequence;
  1. initial, if present, matches the first portion of the value;
  1. final, if present, matches the last portion of the value;
  1. 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.

Zeilenga Standards Track [Page 12] RFC 4518 LDAP: Internationalized String Preparation June 2006

 X.520 also says:
    [T]he following spaces are regarded as not significant:
  1. leading spaces (i.e., those preceding the first character

that is not a space);

  1. trailing spaces (i.e., those following the last character

that is not a space);

  1. 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

Zeilenga Standards Track [Page 13] RFC 4518 LDAP: Internationalized String Preparation June 2006

Full Copyright Statement

 Copyright (C) The Internet Society (2006).
 This document is subject to the rights, licenses and restrictions
 contained in BCP 78, and except as set forth therein, the authors
 retain all their rights.
 This document and the information contained herein are provided on an
 "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
 ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
 INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
 INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
 WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

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 Intellectual Property Rights or other rights that might be claimed to
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Acknowledgement

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 Administrative Support Activity (IASA).

Zeilenga Standards Track [Page 14]

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