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Network Working Group                                        K. ZeilengaRequest for Comments: 4518                           OpenLDAP FoundationCategory: Standards Track                                      June 2006Lightweight Directory Access Protocol (LDAP):Internationalized String PreparationStatus 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.  Introduction1.1.  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 inSection 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 inBCP 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+" isZeilenga                    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, perSection 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:   -  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 byZeilenga                    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.  References5.1.  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 2006Appendix 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-1D1ADAppendix 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:         -  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 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:         -  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   EMail: Kurt@OpenLDAP.orgZeilenga                    Standards Track                    [Page 13]

RFC 4518       LDAP: Internationalized String Preparation      June 2006Full Copyright Statement   Copyright (C) The Internet Society (2006).   This document is subject to the rights, licenses and restrictions   contained inBCP 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.Intellectual Property   The IETF takes no position regarding the validity or scope of any   Intellectual Property Rights or other rights that might be claimed to   pertain to the implementation or use of the technology described in   this document or the extent to which any license under such rights   might or might not be available; nor does it represent that it has   made any independent effort to identify any such rights.  Information   on the procedures with respect to rights in RFC documents can be   found inBCP 78 andBCP 79.   Copies of IPR disclosures made to the IETF Secretariat and any   assurances of licenses to be made available, or the result of an   attempt made to obtain a general license or permission for the use of   such proprietary rights by implementers or users of this   specification can be obtained from the IETF on-line IPR repository athttp://www.ietf.org/ipr.   The IETF invites any interested party to bring to its attention any   copyrights, patents or patent applications, or other proprietary   rights that may cover technology that may be required to implement   this standard.  Please address the information to the IETF at   ietf-ipr@ietf.org.Acknowledgement   Funding for the RFC Editor function is provided by the IETF   Administrative Support Activity (IASA).Zeilenga                    Standards Track                    [Page 14]