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Network Working Group                                         M. CrispinRequest for Comments: 5051                      University of WashingtonCategory: Standards Track                                   October 2007i;unicode-casemap - Simple Unicode Collation AlgorithmStatus 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.Abstract   This document describes "i;unicode-casemap", a simple case-   insensitive collation for Unicode strings.  It provides equality,   substring, and ordering operations.1.  Introduction   The "i;ascii-casemap" collation described in [COMPARATOR] is quite   simple to implement and provides case-independent comparisons for the   26 Latin alphabetics.  It is specified as the default and/or baseline   comparator in some application protocols, e.g., [IMAP-SORT].   However, the "i;ascii-casemap" collation does not produce   satisfactory results with non-ASCII characters.  It is possible, with   a modest extension, to provide a more sophisticated collation with   greater multilingual applicability than "i;ascii-casemap".  This   extension provides case-independent comparisons for a much greater   number of characters.  It also collates characters with diacriticals   with the non-diacritical character forms.   This collation, "i;unicode-casemap", is intended to be an alternative   to, and preferred over, "i;ascii-casemap".  It does not replace the   "i;basic" collation described in [BASIC].2.  Unicode Casemap Collation Description   The "i;unicode-casemap" collation is a simple collation which is   case-insensitive in its treatment of characters.  It provides   equality, substring, and ordering operations.  The validity test   operation returns "valid" for any input.Crispin                     Standards Track                     [Page 1]

RFC 5051                   i;unicode-casemap                October 2007   This collation allows strings in arbitrary (and mixed) character   sets, as long as the character set for each string is identified and   it is possible to convert the string to Unicode.  Strings which have   an unidentified character set and/or cannot be converted to Unicode   are not rejected, but are treated as binary.   Each input string is prepared by converting it to a "titlecased   canonicalized UTF-8" string according to the following steps, using   UnicodeData.txt ([UNICODE-DATA]):      (1) A Unicode codepoint is obtained from the input string.          (a) If the input string is in a known charset that can be              converted to Unicode, a sequence in the string's charset              is read and checked for validity according to the rules of              that charset.  If the sequence is valid, it is converted              to a Unicode codepoint.  Note that for input strings in              UTF-8, the UTF-8 sequence must be valid according to the              rules of [UTF-8]; e.g., overlong UTF-8 sequences are              invalid.          (b) If the input string is in an unknown charset, or an              invalid sequence occurs in step (1)(a), conversion ceases.              No further preparation is performed, and any partial              preparation results are discarded.  The original string is              used unchanged with the i;octet comparator.      (2) The following steps, using UnicodeData.txt ([UNICODE-DATA]),          are performed on the resulting codepoint from step (1)(a).          (a) If the codepoint has a titlecase property in              UnicodeData.txt (this is normally the same as the              uppercase property), the codepoint is converted to the              codepoints in the titlecase property.          (b) If the resulting codepoint from (2)(a) has a decomposition              property of any type in UnicodeData.txt, the codepoint is              converted to the codepoints in the decomposition property.              This step is recursively applied to each of the resulting              codepoints until no more decomposition is possible              (effectively Normalization Form KD).          Example: codepoint U+01C4 (LATIN CAPITAL LETTER DZ WITH CARON)          has a titlecase property of U+01C5 (LATIN CAPITAL LETTER D          WITH SMALL LETTER Z WITH CARON).  Codepoint U+01C5 has a          decomposition property of U+0044 (LATIN CAPITAL LETTER D)          U+017E (LATIN SMALL LETTER Z WITH CARON).  U+017E has a          decomposition property of U+007A (LATIN SMALL LETTER Z) U+030cCrispin                     Standards Track                     [Page 2]

RFC 5051                   i;unicode-casemap                October 2007          (COMBINING CARON).  Neither U+0044, U+007A, nor U+030C have          any decomposition properties.  Therefore, U+01C4 is converted          to U+0044 U+007A U+030C by this step.      (3) The resulting codepoint(s) from step (2) is/are appended, in          UTF-8 format, to the "titlecased canonicalized UTF-8" string.      (4) Repeat from step (1) until there is no more data in the input          string.   Following the above preparation process on each string, the equality,   ordering, and substring operations are as for i;octet.   It is permitted to use an alternative implementation of the above   preparation process if it produces the same results.  For example, it   may be more convenient for an implementation to convert all input   strings to a sequence of UTF-16 or UTF-32 values prior to performing   any of the step (2) actions.  Similarly, if all input strings are (or   are convertible to) Unicode, it may be possible to use UTF-32 as an   alternative to UTF-8 in step (3).      Note: UTF-16 is unsuitable as an alternative to UTF-8 in step (3),      because UTF-16 surrogates will cause i;octet to collate codepoints      U+E0000 through U+FFFF after non-BMP codepoints.   This collation is not locale sensitive.  Consequently, care should be   taken when using OS-supplied functions to implement this collation.   Functions such as strcasecmp and toupper are sometimes locale   sensitive and may inconsistently casemap letters.   The i;unicode-casemap collation is well suited to use with many   Internet protocols and computer languages.  Use with natural language   is often inappropriate; even though the collation apparently supports   languages such as Swahili and English, in real-world use it tends to   mis-sort a number of types of string:   o  people and place names containing scripts that are not collated      according to "alphabetical order".   o  words with characters that have diacriticals.  However,      i;unicode-casemap generally does a better job than i;ascii-casemap      for most (but not all) languages.  For example, German umlaut      letters will sort correctly, but some Scandinavian letters will      not.   o  names such as "Lloyd" (which in Welsh sorts after "Lyon", unlike      in English),   o  strings containing other non-letter symbols; e.g., euro and pound      sterling symbols, quotation marks other than '"', dashes/hyphens,      etc.Crispin                     Standards Track                     [Page 3]

RFC 5051                   i;unicode-casemap                October 20073.  Unicode Casemap Collation Registration   <?xml version='1.0'?>   <!DOCTYPE collation SYSTEM 'collationreg.dtd'>   <collation rfc="5051" scope="global" intendedUse="common">   <identifier>i;unicode-casemap</identifier>   <title>Unicode Casemap</title>   <operations>equality order substring</operations>   <specification>RFC 5051</specification>   <owner>IETF</owner>   <submitter>mrc@cac.washington.edu</submitter>   </collation>4.  Security Considerations   The security considerations for [UTF-8], [STRINGPREP], and [UNICODE-   SECURITY] apply and are normative to this specification.   The results from this comparator will vary depending upon the   implementation for several reasons.  Implementations MUST consider   whether these possibilities are a problem for their use case:   1) New characters added in Unicode may have decomposition or      titlecase properties that will not be known to an implementation      based upon an older revision of Unicode.  This impacts step (2).   2) Step (2)(b) defines a subset of Normalization Form KD (NFKD) that      does not require normalization of out-of-order diacriticals.      However, an implementation MAY use an NFKD library routine that      does such normalization.  This impacts step (2)(b) and possibly      also step (1)(a), and is an issue only with ill-formed UTF-8      input.   3) The set of charsets handled in step (1)(a) is open-ended.  UTF-8      (and, by extension, US-ASCII) are the only mandatory-to-implement      charsets.  This impacts step (1)(a).      Implementations SHOULD, as far as feasible, support all the      charsets they are likely to encounter in the input data, in order      to avoid poor collation caused by the fall through to the (1)(b)      rule.   4) Other charsets may have revisions which add new characters that      are not known to an implementation based upon an older revision.      This impacts step (1)(a) and possibly also step (1)(b).Crispin                     Standards Track                     [Page 4]

RFC 5051                   i;unicode-casemap                October 2007   An attacker may create input that is ill-formed or in an unknown   charset, with the intention of impacting the results of this   comparator or exploiting other parts of the system which process this   input in different ways.  Note, however, that even well-formed data   in a known charset can impact the result of this comparator in   unexpected ways.  For example, an attacker can substitute U+0041   (LATIN CAPITAL LETTER A) with U+0391 (GREEK CAPITAL LETTER ALPHA) or   U+0410 (CYRILLIC CAPITAL LETTER A) in the intention of causing a   non-match of strings which visually appear the same and/or causing   the string to appear elsewhere in a sort.5.  IANA Considerations   The i;unicode-casemap collation defined insection 2 has been added   to the registry of collations defined in [COMPARATOR].6.  Normative References   [COMPARATOR]          Newman, C., Duerst, M., and A. Gulbrandsen,                         "Internet Application Protocol Collation                         Registry",RFC 4790, February 2007.   [STRINGPREP]          Hoffman, P. and M. Blanchet, "Preparation of                         Internationalized Strings ("stringprep")",RFC3454, December 2002.   [UTF-8]               Yergeau, F., "UTF-8, a transformation format of                         ISO 10646", STD 63,RFC 3629, November 2003.   [UNICODE-DATA]        <http://www.unicode.org/Public/UNIDATA/UnicodeData.txt>                         Although the UnicodeData.txt file referenced                         here is part of the Unicode standard, it is                         subject to change as new characters are added                         to Unicode and errors are corrected in Unicode                         revisions.  As a result, it may be less stable                         than might otherwise be implied by the                         standards status of this specification.   [UNICODE-SECURITY]    Davis, M. and M. Suignard, "Unicode Security                         Considerations", February 2006,                         <http://www.unicode.org/reports/tr36/>.Crispin                     Standards Track                     [Page 5]

RFC 5051                   i;unicode-casemap                October 20077.  Informative References   [BASIC]               Newman, C., Duerst, M., and A. Gulbrandsen,                         "i;basic - the Unicode Collation Algorithm",                         Work in Progress, March 2007.   [IMAP-SORT]           Crispin, M. and K. Murchison, "Internet Message                         Access Protocol - SORT and THREAD Extensions",                         Work in Progress, September 2007.Author's Address   Mark R. Crispin   Networks and Distributed Computing   University of Washington   4545 15th Avenue NE   Seattle, WA  98105-4527   Phone: +1 (206) 543-5762   EMail: MRC@CAC.Washington.EDUCrispin                     Standards Track                     [Page 6]

RFC 5051                   i;unicode-casemap                October 2007Full Copyright Statement   Copyright (C) The IETF Trust (2007).   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, THE IETF TRUST 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.Crispin                     Standards Track                     [Page 7]