Internet-Draft Editor: Kurt D. Zeilenga
Intended Category: Standard Track OpenLDAP Foundation
Expires in six months 4 May 2003
LDAP: Internationalized String Preparation
<draft-zeilenga-ldapbis-strprep-00.txt>
Status of this Memo
This document is an Internet-Draft and is in full conformance with all
provisions of Section 10 of RFC2026.
Distribution of this memo is unlimited. Technical discussion of this
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Copyright 2003, The Internet Society. All Rights Reserved.
Please see the 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 string matching is to be
performed. This lead to a number of usability and interoperability
problems. This document defines string preparation algorithms for
matching rules defined for use in LDAP.
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Conventions
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] and ISO/IEC 10646-1
[ISO10646]. 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
standards.
Note: a glossary of terms used in Unicode and ISO/IEC 10646 can be
found in [GLOSSARY]. Information on the ISO/IEC 10646/Unicode
character encoding model can be found in [UTR17].
1. Introduction
1.1. Background
An LDAP 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 characters from the Universal Character Set
(UCS) [ISO10646], a superset of Unicode [UNICODE].
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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 UCS-based 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 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
strings for matching.
1.3. Relationship to "stringprep"
The 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 and 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", characters insignificant to the matching rules are
removed.
Hence, preparation of strings for X.500 matching involves the
following steps:
1) Transcode
2) Map
3) Normalize
4) Prohibit
5) Check Bidi (Bidirectional)
6) Insignificant Character Removal
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These steps are described in Section 2.
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 LDAP internationalized 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 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 string match rule evaluation.
1) Transcode
2) Map
3) Normalize
4) Prohibit
5) Check bidi
6) Insignificant Character Removal
Failure in any step is be cause the assertion to be Undefined.
The character repertoire of this process is Unicode 3.2 [UNICODE].
2.1. Transcode
Each non-Unicode string value is transcoded to Unicode.
TeletexString values are transcoded to Unicode as described in
Appendix A.
PrintableString value are transcoded directly to Unicode.
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UniversalString, UTF8String, and bmpString values need not be
transcoded as they are Unicode-based strings (in the case of
bmpString, restricted to a subset of Unicode).
If the implementation is unable or unwilling to perform the
transcoding as described above, or the transcoding fails, this step
fails and the assertion is evaluated to Undefined.
The transcoded string is the output 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 points (e.g., Cc) or code points with a control
function (e.g., Cf) are mapped to nothing.
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).
For case ignore, numeric, and stored prefix string matching rules,
characters are case folded per B.2 of [RFC3454].
2.3. Normalize
The input string is be normalized to Unicode Form KC (compatibility
composed) as described in [UAX15].
2.4. Prohibit
All Unassigned, Private Use, and non-character code points are
prohibited. Surrogate codes (U+D800-DFFFF) are prohibited.
The REPLACEMENT CHARACTER (U+FFFD) code point is prohibited.
The first code point of a string is prohibited from being a combining
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character.
Empty strings are prohibited.
The step fails and the assertion is evaluated to Undefined if the
input string contains any prohibited code point. The output string is
the input string.
2.5. Check bidi
There are no bidirectional restrictions. The output string is the
input string.
2.5. Insignificant Character Removal
In this step, characters insignificant to the matching rule are to be
removed. The characters to be removed differ 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 Removal
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).
The following spaces are regarded as not significant and are to be
removed:
- 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).
(A string consisting entirely of spaces is equivalent to a string
containing exactly one space.)
For example, removal of spaces from the Form KC string:
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"<SPACE><SPACE>foo<SPACE><SPACE>bar<SPACE><SPACE>" would result in
the output string:
"<SPACE>foo<SPACE>bar<SPACE>".
and the Form KC string:
"<SPACE><SPACE><SPACE>" would result in the output string:
"<SPACE>".
2.6.2. NumericString Insignificant Character Removal
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 not significant 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 an empty output string.
2.6.3. TelephoneNumber Insignificant Character Removal
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 regarded as not significant and are to be
removed.
3. Security Considerations
"Preparation for International Strings ('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
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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.
5. Editor's Address
Kurt Zeilenga
E-mail: <kurt@openldap.org>
6. References
6.1. Normative References
[RFC2119] S. Bradner, "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14 (also RFC 2119), March 1997.
[RFC3454] P. Hoffman, M. Blanchet, "Preparation of Internationalized
Strings ('stringprep')", RFC 3454, December 2002.
[Roadmap] K. Zeilenga, "LDAP: Technical Specification Road Map",
draft-ietf-ldapbis-roadmap-xx.txt, a work in progress.
[Syntaxes] S. Legg (editor), "LDAP: Syntaxes and Matching Rules",
draft-ietf-ldapbis-syntaxes-xx.txt, a work in progress.
[ISO10646] Universal Multiple-Octet Coded Character Set (UCS) -
Architecture and Basic Multilingual Plane, ISO/IEC 10646-1
: 1993.
[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] M. Davis, 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.
6.2. Informative References
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[X.500] International Telephone Union, "The Directory: Overview of
Concepts, Models and Service", X.500, 2000.
[X.501] International Telephone Union, "The Directory: The Models",
X.501, 2000.
[X.520] International Telephone Union, "The Directory: Selected
Attribute Types", X.520, 2000.
[XMATCH] K. Zeilenga, "Internationalized String Matching
Rules for X.500", draft-zeilenga-ldapbis-strmatch-xx.txt a
work in progress.
[GLOSSARY] The Unicode Consortium, "Unicode Glossary",
<http://www.unicode.org/glossary/>.
[UTR17] K. Whistler, M. Davis, "Unicode Technical Report
#17, Character Encoding Model", UTR17,
<http://www.unicode.org/unicode/reports/tr17/>, August
2000.
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Appendix A. Teletex (T.61) to Unicode
TBD.
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