Internet-Draft Editor: Kurt D. Zeilenga
Intended Category: Informational OpenLDAP Foundation
Expires in six months 8 September 2002
Internationalized String Matching Rules for X.500
<draft-zeilenga-ldapbis-strmatch-01.txt>
Status of this Memo
This document is an Internet-Draft and is in full conformance with all
provisions of Section 10 of RFC2026.
This document is intended to be submitted to the ITU for publication
as an amendment to X.520 and published as an Informational RFC.
Distribution of this memo is unlimited. Technical discussion of this
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Copyright 2002, The Internet Society. All Rights Reserved.
Please see the Copyright section near the end of this document for
more information.
Abstract
The existing X.500 Directory Service technical specifications do not
precisely define how string matching is to be performed. This has
lead to a number of interoperability problems. This document provides
string preparation profiles for standard syntaxes and matching rules
defined in X.520.
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This document is intended to be submitted to the ITU for publication
as an amendment to X.520 and published as an Informational RFC.
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].
1. Introduction
1.1. Background
An X.500 matching rule [X.501] 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 [X.500]. These specifications are
inadequate for strings composed of characters from the Universal
Character Set (UCS) [ISO10646], a superset of Unicode [UNICODE].
The CaseIgnoreMatch matching rule, 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
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upper case to lower case folding. Or one implementation may view
space as referring to only SPACE (U+00020), 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 updates X.520 [X.520] with a detailed
specification of string syntax and matching rule requirements.
1.3. Relationship to "stringprep"
The matching rule algorithms described in this document are based upon
the "stringprep" approach [STRPREP]. In "stringprep", presented and
stored values are first prepared for comparison and so that a
character-by-character comparison yields the "correct" result.
The algorithm used here is a refinement of the "stringprep" [STRPREP]
approach. The 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
These steps are described in Section 3. Section 2 details design
considerations.
1.4. Relationship to X.500
This document updates X.520 [X.520] with additional normative and
informative information. Sections 3, 4, and 5 are normative parts of
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this update. Other sections are informative.
Section 3 provides a specification for X.500 string preparation. It
is intended to be added as a new section in X.520.
Section 4 replaces section 6.1 of X.520 [X.520]. It updates select
string matching rules.
Section 5 replaces portions of section 6.2 of X.520 [X.520]. It
updates select syntax-based matching rules.
2. Design Considerations
The X.500 string matching rule specification provided in Section 3 is
designed to leverage the "stringprep" framework [STRPREP] for
comparing of strings. As noted above, transcoding and space removal
steps have been added.
This section describes the rationale for these and other design
decisions.
2.1. Transcode
In the past, transcoding only occurred when all of the input strings
were not encoded in the same character set. If all were encoded in
the same character set, no transcoding was to be performed.
Otherwise, all of the strings would be transcoded to one of character
sets used.
As mappings between character sets, such as T.61 and UCS, are not
bijective, this specification requires transliteration of all strings
to a common character encoding set. UCS was the logical choice as all
other character sets (used in X.500) can be transcoded to it without
information loss. None of the other character sets (used in X.500)
offer this property.
2.2. Map
TBD
2.3. Normalize
Normalization is performed to ensure that comparison is always done
between canonical-equivalent strings. As directory strings are often
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used as identifiers, we selected Form KC (compatibility composed) as
it allows a greater number of strings to be treated as equivalent.
Unfortunately, this choice is not best for all applications.
Additional matching rules which use different string preparation
algorithms may be introduced in the future to better support these
applications. In particular, matching rules which use Form C
(composed) normalization instead of Form KC would also be generally
useful.
2.4. Prohibit
TBD
2.5. Check bidi
TBD
2.6. Insignificant Character Removal
TBD
3. 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.
3.1. Transcode
Each non-Unicode string value is transcoded to Unicode.
TeletexString values are transcoded to Unicode as described in
[T61-UCS].
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PrintableString value are transcoded directly to Unicode.
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.
3.2. 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) are 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 [STRPREP].
3.3. Normalize
The input string is be normalized to Unicode Form KC (compatibility
composed) as described in [UAX15].
3.4. 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 is prohibited.
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The first code point of a string is probibited from being a combining
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.
3.5. Check bidi
There are no bidirectional restrictions. The output string is the
input string.
3.6. 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 3.6.1 applies to case ignore and exact string matching.
Section 3.6.2 applies to numericString matching.
Section 3.6.3 applies to telephoneNumber matching
3.6.1. 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.)
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For example, removal of spaces from the Form KC string:
"<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>".
3.6.2. 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.
3.6.3. 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.
4. String Matching Rules
In the matching rules specified in this section, all presented and
stored string values are be prepared for matching as described in
Section 3. String preparation produces strings suitable for
character-by-character matching.
4.1. Case Exact / Ignore Match
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The Case Exact Match rule compares for equality a presented string
with an attribute value of type DirectoryString or one of the data
types appearing in the choice type DirectoryString, e.g. UTF8String,
without regards to insignificant spaces (3.4.1).
caseExactMatch MATCHING-RULE ::= {
SYNTAX DirectoryString {ub-match}
ID id-mr-caseExactMatch }
The Case Ignore Match rule compares for equality a presented string
with an attribute value of type DirectoryString or one of the data
types appearing in the choice type DirectoryString, e.g. UTF8String,
without regard to the case (upper or lower) of the strings (e.g.
"Dundee" and "DUNDEE" match) and insignificant spaces (3.4.1). The
rule is identical to the caseExactMatch rule except upper case
characters are folded to lower case during string preparation as
discussed in 3.2.
caseIgnoreMatch MATCHING-RULE ::= {
SYNTAX DirectoryString {ub-match}
ID id-mr-caseIgnoreMatch }
Both rules return TRUE if the prepared strings are the same length and
corresponding characters are identical.
4.2. Case Exact / Ignore Ordering Match
The Case Exact Ordering Match rule compares the collation order of a
presented string with an attribute value of type DirectoryString or
one of the data types appearing in the choice type DirectoryString,
e.g. UTF8String, without regard to insignificant spaces (3.4.1).
caseExactOrderingMatch MATCHING-RULE ::= {
SYNTAX DirectoryString {ub-match}
ID id-mr-caseExactOrderingMatch }
The Case Ignore Ordering Match rule compares the collation order of a
presented string an attribute value of type DirectoryString or one of
the data types appearing in the choice type DirectoryString, e.g.
UTF8String, without regard to the case (upper or lower) of the strings
and insignificant spaces (3.4.1). The rule is identical to the
caseExactOrderingMatch rule except upper case characters are folded to
lower case during string preparation as discussed in 3.2.
caseIgnoreOrderingMatch MATCHING-RULE ::= {
SYNTAX DirectoryString {ub-match}
ID id-mr-caseIgnoreOrderingMatch }
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Both rules return TRUE if the attribute value is "less" or appears
earlier than the presented value, when the prepared strings are
compared using Unicode code point collation order.
4.3. Case Exact / Ignore Substrings Match
The Case Exact Substrings Match rule determines whether a presented
value is a substring of an attribute value of type DirectoryString or
one of the data types appearing in the choice type DirectoryString,
e.g. UTF8String, without regard to insignficant spaces (3.4.1).
caseExactSubstringsMatch MATCHING-RULE ::= {
SYNTAX SubstringAssertion
ID id-mr-caseExactSubstringsMatch }
SubstringAssertion ::= SEQUENCE OF CHOICE {
initial [0] DirectoryString {ub-match},
any [1] DirectoryString {ub-match},
final [2] DirectoryString {ub-match},
control Attribute }
-- Used to specify interpretation of the following items
-- at most one initial and one final component
The Case Ignore Substrings Match rule determines whether a presented
value is a substring of an attribute value of type DirectoryString or
one of the data types appearing in the choice type DirectoryString,
e.g. UTF8String, without regard to the case (upper or lower) of the
strings and insignificant spaces (3.4.1). The rule is identical to
the caseExactSubstringsMatch rule except upper case characters are
folded to lower case during string preparation as discussed in 3.2.
caseIgnoreSubstringsMatch MATCHING-RULE ::= {
SYNTAX SubstringAssertion
ID id-mr-caseIgnoreSubstringsMatch }
Both rules return TRUE if there is a partitioning of the prepared
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;
- any, if present, matches some arbitrary portion of the value;
- final, if present, matches the last portion of the value.
- control is not used for the caseExactSubstringsMatch,
caseIgnoreSubstringsMatch, telephoneNumberSubstringsMatch, or any
other form of substring match for which only initial, any, or
final elements are used in the matching algorithm; if a control
element is encountered, it is ignored. The control element is
only used for matching rules that explicitly specify its use in
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the matching algorithm. Such a matching rule may also redefine the
semantics of the initial, any and final substrings.
NOTE - The generalWordMatch matching rule is an example of such
a matching rule.
There shall be at most one initial, and at most one final in the
SubstringAssertion. If initial is present, it shall be the first
element. If final is present, it shall be the last element. There
shall be zero or more any elements.
For a component of substrings to match a portion of the attribute
value, corresponding characters must be identical (including all
combining characters in the combining character sequences).
4.4. Numeric String Match
The Numeric String Match rule compares for equality a presented
numeric string with an attribute value of type NumericString.
numericStringMatch MATCHING-RULE ::= {
SYNTAX NumericString
ID id-mr-numericStringMatch }
The rule is identical to the caseIgnoreMatch rule (case is irrelevant
as characters are numeric) except that all space characters are
removed during string preparation as detailed in Section 3.6.2.
4.5. Numeric String Ordering Match
The Numeric String Ordering Match rule compares the collation order of
a presented string with an attribute value of type NumericString.
numericStringOrderingMatch MATCHING-RULE ::= {
SYNTAX NumericString
ID id-mr-numericStringOrderingMatch }
The rule is identical to the caseIgnoreOrderingMatch rule (case is
irrelevant as characters are numeric) except that all space characters
are removed during string preparation as detailed in Section 3.6.
4.6. Numeric String Substrings Match
The Numeric String Substrings Match rule determines whether a
presented value is a substring of an attribute value of type
NumericString.
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numericStringSubstringsMatch MATCHING-RULE ::= {
SYNTAX SubstringAssertion
ID id-mr-numericStringSubstringsMatch }
The rule is identical to the caseIgnoreSubstringsMatch rule (case is
irrelevant as characters are numeric) except that all space characters
are removed during string preparation as detailed in Section 3.6.
4.7. Case Ignore List Match
The Case Ignore List Match rule compares for equality a presented
sequence of strings with an attribute value which is a sequence of
DirectoryStrings, without regard to the case (upper or lower) of the
strings and insignificant spaces (3.6.1).
caseIgnoreListMatch MATCHING-RULE ::= {
SYNTAX CaseIgnoreList
ID id-mr-caseIgnoreListMatch }
CaseIgnoreList ::= SEQUENCE OF DirectoryString {ub-match}
The rule returns TRUE if and only if the number of strings in each is
the same, and corresponding strings match. The latter matching is as
for the caseIgnoreMatch matching rule.
4.8. Case Ignore List Substrings Match
The Case Ignore List Substring rule compares a presented substring
with an attribute value which is a sequence of DirectoryStrings, but
without regard for the case (upper or lower) of the strings and
insignificant spaces (3.6.1).
caseIgnoreListSubstringsMatch MATCHING-RULE ::= {
SYNTAX SubstringAssertion
ID id-mr-caseIgnoreListSubstringsMatch }
A presented value matches a stored value if and only if the presented
value matches the string formed by concatenating the strings of the
stored value. This matching is done according to the
caseIgnoreSubstringsMatch rule; however, none of the initial, any, or
final values of the presented value are considered to match a
substring of the concatenated string which spans more than one of the
strings of the stored value.
4.9. Stored Prefix Match
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The Stored Prefix Match rule determines whether an attribute value,
whose syntax is DirectoryString, is a prefix (i.e. initial substring)
of the presented value, without regard to the case (upper or lower) of
the strings and insignficant spaces (3.6.1).
NOTE - It can be used, for example, to compare values in
the Directory which are telephone area codes with a value
which is a purported telephone number.
storedPrefixMatch MATCHING-RULE ::= {
SYNTAX DirectoryString {ub-match}
ID id-mr-storedPrefixMatch }
The rule returns TRUE if the attribute value is an initial substring
of the presented value with corresponding characters identical except
with regard to case.
5. Other changes to X.520
This document makes the following changes to X.520:
The section 6.2.8 (Telephone Number Match) sentence:
The rules for matching are identical to those for caseIgnoreMatch,
except that all space and "-" characters are skipped during the
comparison.
is replaced with:
The rules for matching are identical to those for caseIgnoreMatch,
except that all hyphens and spaces are insignficant (3.6.3) and
removed during the insignificant character removal step.
The section 6.2.9 (Telephone Number Substrings Match) sentence:
The rules for matching are identical to those for
caseExactSubstringsMatch, except that all space and "-" characters
are skipped during the comparison.
is replaced with:
The rules for matching are identical to those for
caseExactSubstringsMatch, except that all hyphens and spaces are
insignficant (3.6.3) and removed during the insignificant
character removal step.
6. Security Considerations
TBD
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7. Acknowledgments
The approach used in this document is based upon design principles and
algorithms described in "Preparation of Internationalized Strings
('stringprep')" [STRPREP] by Paul Hoffman and Marc Blanchet.
Additional guidance was drawn from Unicode Technical Standards,
Technical Reports, and Notes.
Sections 3.3 and 4 of this document are derived from Section 6.1 of
[X.520].
This document is the product of IETF and ITU-T collaboration [IETF-
ITU].
8. Editor's Address
Kurt Zeilenga
E-mail: <kurt@openldap.org>
9. References
9.1. Normative References
[RFC2119] S. Bradner, "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14 (also RFC 2119), March 1997.
[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.
[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/).
[STRPREP] P. Hoffman, M. Blanchet, "Preparation of Internationalized
Strings ('stringprep')", draft-hoffman-stringprep-xx.txt (a
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work in progress).
[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>.
[T61-UCS] TBD
9.2. Informative References
[X.500] International Telephone Union, "The Directory: Overview of
Concepts, Models and Service", X.500, 2000.
[IETF-ITU] G. Fishman, S. Bradner, "Internet Engineering Task Force
and International Telecommunication Union -
Telecommunications Standardization Sector Collaboration
Guidelines", TSAG A-Series Supplement 3, November 2001
(also RFC 3356, published August 2002).
Copyright 2002, The Internet Society. All Rights Reserved.
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