Internet-Draft Kurt D. Zeilenga Intended Category: Standard Track OpenLDAP Foundation Expires in six months
30 September 200523 January 2006 LDAP: Internationalized String Preparation <draft-ietf-ldapbis-strprep-06.txt><draft-ietf-ldapbis-strprep-07.txt> Status of this Memo This document is intended to be published as a Standard Track RFC. Distribution of this memo is unlimited. Technical discussion of this document will take place on the IETF LDAP Revision Working Group mailing list <firstname.lastname@example.org>. Please send editorial comments directly to the editor <Kurt@OpenLDAP.org>. By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/1id-abstracts.html The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html Copyright (C) The Internet Society (2005).(2006). All Rights Reserved. Please see the Full 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 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. 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 which has a mark property (Mn, Mc, Me). Appendix A provides a completedefinitive list of combining marks. 1. Introduction 1.1. Background A Lightweight Directory Access Protocol (LDAP) [Roadmap] 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 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 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 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 [StringPrep].[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" [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 matching 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 a integral part of the LDAP technical specification [Roadmap] which obsoletes the previously defined LDAP technical specification [RFC3377] in its entirety. This document details new LDAP internationalized character 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 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. 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 described expected matching behavior. Implementations are free 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] value 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 TelexStringTeletexString 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. 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 [StringPrep].[RFC3454]. The output is the mapped string. 2.3. Normalize The input string is 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 [StringPrep].[RFC3454]. Characters which, per Section 5.8 of [Stringprep], change display properties or are deprecated are prohibited. These characters are are listed in Table C.8 of [StringPrep].[RFC3454]. Private Use code points are prohibited. These characters are listed in Table C.3 of [StringPrep].[RFC3454]. All non-character code points are prohibited. These code points are listed in Table C.4 of [StringPrep].[RFC3454]. Surrogate codes are prohibited. These characters are listed in Table C.5 of [StringPrep].[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). If the input string contains at least one non-space character, then the string is modified such that the string starts with exactly one space character, ends with exactly one SPACE character, and that 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>", results in the output "<SPACE>foo<SPACE><SPACE>bar<SPACE>". Otherwise, if the string being prepared is an initial, any, or final substring, then the output string is exactly one SPACE character, else the output string is exactly two SPACEs. 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 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 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 for International Strings ('stringprep')" [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 algorithms described in "Preparation of Internationalized Strings ('stringprep')" [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. Author's Address Kurt D. Zeilenga OpenLDAP Foundation Email: Kurt@OpenLDAP.org 6. References [[Note to the RFC Editor: please replace the citation tags used in referencing Internet-Drafts with tags of the form RFCnnnn where possible.]] 6.1. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14 (also RFC 2119), March 1997. [RFC3454] Hoffman, P. and M. Blanchet, "Preparation of Internationalized Strings ('stringprep')", RFC 3454, December 2002. [Roadmap] Zeilenga, K. (editor), "LDAP: Technical Specification Road Map", draft-ietf-ldapbis-roadmap-xx.txt, a work in progress. [StringPrep] Hoffman P. and M. Blanchet, "Preparation of Internationalized Strings ('stringprep')", draft-hoffman-rfc3454bis-xx.txt, a work in progress.[Syntaxes] Legg, S. (editor), "LDAP: Syntaxes and Matching Rules", draft-ietf-ldapbis-syntaxes-xx.txt, a work in progress. [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). 6.2. 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. [Filters] Smith, M. (editor), LDAPbis WG, "LDAP: String Representation of Search Filters", draft-ietf-ldapbis-filter-xx.txt, a work in progress. [XMATCH] Zeilenga, K., "Internationalized String Matching Rules for X.500", draft-zeilenga-ldapbis-strmatch-xx.txt, a work in progress. [RFC1345] Simonsen, K., "Character Mnemonics & Character Sets", RFC 1345, June 1992.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 absence of substrings matching, the insignificant space handling for case ignore/exact matching could be simplified. Specifically, the handling could be as require all sequences of one or more spaces be replaced with one space and, if string contains non-space characters, removal of all all leading spaces and trailing spaces. In the presence of substrings matching, this simplified space handling this simplified space handlingwould lead to unexpected and undesirable matching behavior. For instance: 1) (CN=foo\20*\20bar) would match the CN value "foobar" but not "foo<SPACE>bar" nor "foo<SPACE><SPACE>bar"; 2) (CN=*\20foobar\20*) would match "foobar", but (CN=*\20*foobar*\20*) would not; 3) (CN=foo\20*\20bar) would match "foo<SPACE>X<SPACE>bar" but not "foo<SPACE><SPACE>bar". Note to readers not familiar with LDAP substrings matching: the LDAP filter [Filters] assertion (CN=A*B*C) says "match any value (of the attribute CN) which 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. The third case illustrates that this simplified space handling causes another partitioning failure. Though both the initial or final strings match different portions of "foo<SPACE>X<SPACE>bar" with neither matching the X portion, they don't match a string consisting of the two matched portions less the unmatched X portion. 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. 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 which differs only by inclusion or 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 which could be considered insignificant. For instance, the assertion (CN=\20*\20*\20) does not match "<SPACE><SPACE><SPACE>" (insignificant spaces present in value) nor " " (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. 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