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Versions: 00 01 02 03 04 05 06 07 rfc4518                               
Internet-Draft                                      Kurt D. Zeilenga
Intended Category: Standard Track                OpenLDAP Foundation
Expires in six months                                    26 May 2003



                LDAP: Internationalized String Preparation
                   <draft-ietf-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
  document will take place on the IETF LDAP Revision Working Group
  mailing list <ietf-ldapbis@openldap.org>.  Please send editorial
  comments directly to the author <Kurt@OpenLDAP.org>.

  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
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  <http://www.ietf.org/shadow.html>.

  Copyright (C) The Internet Society (2003).  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 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].  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].


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 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 new 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 [X.680][T.61] values are transcoded to Unicode as
  described in Appendix A.

  PrintableString [X.680] value are transcoded directly to Unicode.



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  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).

  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:
      "<SPACE><SPACE>foo<SPACE><SPACE>bar<SPACE><SPACE>"



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  would result in the output string:
      "foo<SPACE>bar"
  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. Contributors

  Appendix A and B of this document were authored by Howard Chu
  <hyc@symas.com> of Symas Corporation (based upon information provided



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  in RFC 1345).


5. Acknowledgments

  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.


6. Author's Address

  Kurt Zeilenga
  E-mail: <kurt@openldap.org>


7. References

7.1. 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.

  [Syntaxes]    Legg, S. (editor), "LDAP: Syntaxes and Matching Rules",
                draft-ietf-ldapbis-syntaxes-xx.txt, a work in progress.

  [ISO10646]    International Organization for Standardization,
                "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/).



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  [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(1997) (also ISO/IEC 8824-1:1998).

  [T.61]        CCITT (now ITU), "Character Repertoire and Coded
                Character Sets for the International Teletex Service",
                T.61, 1988.

7.2. 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.

  [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. Teletex (T.61) to Unicode




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  This appendix defines an algorithm for transcoding [T.61] characters
  to [Unicode] characters for use in string preparation for LDAP
  matching rules.  This appendix is a normative.

  The transcoding algorithm is derived from the T.61-8bit definition
  provided in [RFC1345].  With a few exceptions, the T.61 character
  codes from x00 to x7f are equivalent to the corresponding [Unicode]
  code points, and their values are left unchanged by this algorithm.
  E.g. the T.61 code x20 is identical to (U+0020).  The exceptions are
  for these T.61 codes that are undefined: x23, x24, x5c, x5e, x60, x7b,
  x7d, and x7e.

  The codes from x80 to x9f are also equivalent to the corresponding
  Unicode code points.  This is specified for completeness only, as
  these codes are control characters, and will be mapped to nothing in
  the LDAP String Preparation Mapping step.

  The remaining T.61 codes are mapped below in Table A.1.  Table
  positions marked "??" are undefined.

  Input strings containing undefined T.61 codes SHALL produce an
  Undefined matching result. For diagnostic purposes, this algorithm
  does not fail for undefined input codes.  Instead, undefined codes in
  the input are mapped to the Unicode REPLACEMENT CHARACTER (U+FFFD).
  As the LDAP String Preparation Probhibit step disallows the
  REPLACEMENT CHARACTER from appearing in its output, this transcoding
  yields the desired effect.

  Note: RFC 1345 listed the non-spacing accent codepoints as residing in
        the range starting at (U+E000).  In the current Unicode
        standard, the (U+E000) range is reserved for Private Use, and
        the non-spacing accents are in the range starting at (U+0300).
        The tables here use the (U+0300) range for these accents.

     |    0 |    1 |    2 |    3 |    4 |    5 |    6 |    7 |
   --+------+------+------+------+------+------+------+------+
   a0| 00a0 | 00a1 | 00a2 | 00a3 | 0024 | 00a5 | 0023 | 00a7 |
   a8| 00a8 |  ??  |  ??  | 00ab |  ??  |  ??  |  ??  |  ??  |
   b0| 00b0 | 00b1 | 00b2 | 00b3 | 00d7 | 00b5 | 00b6 | 00b7 |
   b8| 00f7 |  ??  |  ??  | 00bb | 00bc | 00bd | 00be | 00bf |
   c0|  ??  | 0300 | 0301 | 0302 | 0303 | 0304 | 0306 | 0307 |
   c8| 0308 |  ??  | 030a | 0327 | 0332 | 030b | 0328 | 030c |
   d0|  ??  |  ??  |  ??  |  ??  |  ??  |  ??  |  ??  |  ??  |
   d8|  ??  |  ??  |  ??  |  ??  |  ??  |  ??  |  ??  |  ??  |
   e0| 2126 | 00c6 | 00d0 | 00aa |  ??  | 0126 | 0132 | 013f |
   e8| 0141 | 00d8 | 0152 | 00ba | 00de | 0166 | 014a | 0149 |
   f0| 0138 | 00e6 | 0111 | 00f0 | 0127 | 0131 | 0133 | 0140 |
   f8| 0142 | 00f8 | 0153 | 00df | 00fe | 0167 | 014b |  ??  |



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   --+------+------+------+------+------+------+------+------+
            Table A.1:  Mapping of 8-bit T.61 codes to Unicode

  T.61 also defines a number of accented characters that are formed by
  combining an accent prefix followed by a base character.  These
  prefixes are in the code range xc1 to xcf. If a prefix character
  appears at the end of a string, the result is undefined.  Otherwise
  these sequences are mapped to Unicode by substituting the
  corresponding non-spacing accent code (as listed in Table A.1) for the
  accent prefix, and exchanging the order so that the base character
  precedes the accent.


Appendix B. Additional Teletex (T.61) to Unicode Tables

  All of the accented characters in T.61 have a corresponding code point
  in Unicode.  For the sake of completeness, the combined character
  codes are presented in the following tables.  This is informational
  only; for matching purposes it is sufficient to map the non-spacing
  accent and exchange the order of the character pair as specified in
  Appendix A.


B.1. Combinations with SPACE

  Accents may be combined with a <SPACE> to generate the accent by
  itself.  For each accent code, the result of combining with <SPACE> is
  listed in Table B.1.

     |    0 |    1 |    2 |    3 |    4 |    5 |    6 |    7 |
   --+------+------+------+------+------+------+------+------+
   c0|  ??  | 0060 | 00b4 | 005e | 007e | 00af | 02d8 | 02d9 |
   c8| 00a8 |  ??  | 02da | 00b8 |  ??  | 02dd | 02db | 02c7 |
   --+------+------+------+------+------+------+------+------+
       Table B.1:  Mapping of T.61 Accents with <SPACE> to Unicode


B.2. Combinations for xc1: (Grave accent)

  T.61 has predefined characters for combinations with A, E, I, O, and
  U.  Unicode also defines combinations for N, W, and Y.  All of these
  combinations are present in Table B.2.

     |    0 |    1 |    2 |    3 |    4 |    5 |    6 |    7 |
   --+------+------+------+------+------+------+------+------+
   40|  ??  | 00c0 |  ??  |  ??  |  ??  | 00c8 |  ??  |  ??  |
   48|  ??  | 00cc |  ??  |  ??  |  ??  |  ??  | 01f8 | 00d2 |
   50|  ??  |  ??  |  ??  |  ??  |  ??  | 00d9 |  ??  | 1e80 |



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   58|  ??  | 1ef2 |  ??  |  ??  |  ??  |  ??  |  ??  |  ??  |
   60|  ??  | 00e0 |  ??  |  ??  |  ??  | 00e8 |  ??  |  ??  |
   68|  ??  | 00ec |  ??  |  ??  |  ??  |  ??  | 01f9 | 00f2 |
   70|  ??  |  ??  |  ??  |  ??  |  ??  | 00f9 |  ??  | 1e81 |
   78|  ??  | 1ef3 |  ??  |  ??  |  ??  |  ??  |  ??  |  ??  |
   --+------+------+------+------+------+------+------+------+
           Table B.2: Mapping of T.61 Grave Accent Combinations


B.3. Combinations for xc2: (Acute accent)

  T.61 has predefined characters for combinations with A, E, I, O, U, Y,
  C, L, N, R, S, and Z.  Unicode also defines G, K, M, P, and W.  All of
  these combinations are present in Table B.3.

     |    0 |    1 |    2 |    3 |    4 |    5 |    6 |    7 |
   --+------+------+------+------+------+------+------+------+
   40|  ??  | 00c1 |  ??  | 0106 |  ??  | 00c9 |  ??  | 01f4 |
   48|  ??  | 00cd |  ??  | 1e30 | 0139 | 1e3e | 0143 | 00d3 |
   50| 1e54 |  ??  | 0154 | 015a |  ??  | 00da |  ??  | 1e82 |
   58|  ??  | 00dd | 0179 |  ??  |  ??  |  ??  |  ??  |  ??  |
   60|  ??  | 00e1 |  ??  | 0107 |  ??  | 00e9 |  ??  | 01f5 |
   68|  ??  | 00ed |  ??  | 1e31 | 013a | 1e3f | 0144 | 00f3 |
   70| 1e55 |  ??  | 0155 | 015b |  ??  | 00fa |  ??  | 1e83 |
   78|  ??  | 00fd | 017a |  ??  |  ??  |  ??  |  ??  |  ??  |
   --+------+------+------+------+------+------+------+------+
           Table B.3: Mapping of T.61 Acute Accent Combinations


B.4. Combinations for xc3: (Circumflex)

  T.61 has predefined characters for combinations with A, E, I, O, U, Y,
  C, G, H, J, S, and W.  Unicode also defines the combination for Z.
  All of these combinations are present in Table B.4.

     |    0 |    1 |    2 |    3 |    4 |    5 |    6 |    7 |
   --+------+------+------+------+------+------+------+------+
   40|  ??  | 00c2 |  ??  | 0108 |  ??  | 00ca |  ??  | 011c |
   48| 0124 | 00ce | 0134 |  ??  |  ??  |  ??  |  ??  | 00d4 |
   50|  ??  |  ??  |  ??  | 015c |  ??  | 00db |  ??  | 0174 |
   58|  ??  | 0176 | 1e90 |  ??  |  ??  |  ??  |  ??  |  ??  |
   60|  ??  | 00e2 |  ??  | 0109 |  ??  | 00ea |  ??  | 011d |
   68| 0125 | 00ee | 0135 |  ??  |  ??  |  ??  |  ??  | 00f4 |
   70|  ??  |  ??  |  ??  | 015d |  ??  | 00fb |  ??  | 0175 |
   78|  ??  | 0177 | 1e91 |  ??  |  ??  |  ??  |  ??  |  ??  |
   --+------+------+------+------+------+------+------+------+
        Table B.4: Mapping of T.61 Circumflex Accent Combinations




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B.5. Combinations for xc4: (Tilde)

  T.61 has predefined characters for combinations with A, I, O, U, and
  N.  Unicode also defines E, V, and Y.  All of these combinations are
  present in Table B.5.

     |    0 |    1 |    2 |    3 |    4 |    5 |    6 |    7 |
   --+------+------+------+------+------+------+------+------+
   40|  ??  | 00c3 |  ??  |  ??  |  ??  | 1ebc |  ??  |  ??  |
   48|  ??  | 0128 |  ??  |  ??  |  ??  |  ??  | 00d1 | 00d5 |
   50|  ??  |  ??  |  ??  |  ??  |  ??  | 0168 | 1e7c |  ??  |
   58|  ??  | 1ef8 |  ??  |  ??  |  ??  |  ??  |  ??  |  ??  |
   60|  ??  | 00e3 |  ??  |  ??  |  ??  | 1ebd |  ??  |  ??  |
   68|  ??  | 0129 |  ??  |  ??  |  ??  |  ??  | 00f1 | 00f5 |
   70|  ??  |  ??  |  ??  |  ??  |  ??  | 0169 | 1e7d |  ??  |
   78|  ??  | 1ef9 |  ??  |  ??  |  ??  |  ??  |  ??  |  ??  |
   --+------+------+------+------+------+------+------+------+
           Table B.5: Mapping of T.61 Tilde Accent Combinations


B.6. Combinations for xc5: (Macron)

  T.61 has predefined characters for combinations with A, E, I, O, and
  U.  Unicode also defines Y, G, and AE.  All of these combinations are
  present in Table B.6.

     |    0 |    1 |    2 |    3 |    4 |    5 |    6 |    7 |
   --+------+------+------+------+------+------+------+------+
   40|  ??  | 0100 |  ??  |  ??  |  ??  | 0112 |  ??  | 1e20 |
   48|  ??  | 012a |  ??  |  ??  |  ??  |  ??  |  ??  | 014c |
   50|  ??  |  ??  |  ??  |  ??  |  ??  | 016a |  ??  |  ??  |
   58|  ??  | 0232 |  ??  |  ??  |  ??  |  ??  |  ??  |  ??  |
   60|  ??  | 0101 |  ??  |  ??  |  ??  | 0113 |  ??  | 1e21 |
   68|  ??  | 012b |  ??  |  ??  |  ??  |  ??  |  ??  | 014d |
   70|  ??  |  ??  |  ??  |  ??  |  ??  | 016b |  ??  |  ??  |
   78|  ??  | 0233 |  ??  |  ??  |  ??  |  ??  |  ??  |  ??  |
   e0|  ??  | 01e2 |  ??  |  ??  |  ??  |  ??  |  ??  |  ??  |
   f0|  ??  | 01e3 |  ??  |  ??  |  ??  |  ??  |  ??  |  ??  |
   --+------+------+------+------+------+------+------+------+
          Table B.6: Mapping of T.61 Macron Accent Combinations


B.7. Combinations for xc6: (Breve)

  T.61 has predefined characters for combinations with A, U, and G.
  Unicode also defines E, I, and O.  All of these combinations are
  present in Table B.7.




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     |    0 |    1 |    2 |    3 |    4 |    5 |    6 |    7 |
   --+------+------+------+------+------+------+------+------+
   40|  ??  | 0102 |  ??  |  ??  |  ??  | 0114 |  ??  | 011e |
   48|  ??  | 012c |  ??  |  ??  |  ??  |  ??  |  ??  | 014e |
   50|  ??  |  ??  |  ??  |  ??  |  ??  | 016c |  ??  |  ??  |
   58|  ??  |  ??  |  ??  |  ??  |  ??  |  ??  |  ??  |  ??  |
   60|  ??  | 0103 |  ??  |  ??  |  ??  | 0115 |  ??  | 011f |
   68|  ??  | 012d |  ??  |  ??  |  ??  |  ??  | 00f1 | 014f |
   70|  ??  |  ??  |  ??  |  ??  |  ??  | 016d |  ??  |  ??  |
   78|  ??  |  ??  |  ??  |  ??  |  ??  |  ??  |  ??  |  ??  |
   --+------+------+------+------+------+------+------+------+
           Table B.7: Mapping of T.61 Breve Accent Combinations


B.8. Combinations for xc7: (Dot Above)

  T.61 has predefined characters for C, E, G, I, and Z.  Unicode also
  defines A, O, B, D, F, H, M, N, P, R, S, T, W, X, and Y.  All of these
  combinations are present in Table B.8.

     |    0 |    1 |    2 |    3 |    4 |    5 |    6 |    7 |
   --+------+------+------+------+------+------+------+------+
   40|  ??  | 0226 | 1e02 | 010a | 1e0a | 0116 | 1e1e | 0120 |
   48| 1e22 | 0130 |  ??  |  ??  |  ??  | 1e40 | 1e44 | 022e |
   50| 1e56 |  ??  | 1e58 | 1e60 | 1e6a |  ??  |  ??  | 1e86 |
   58| 1e8a | 1e8e | 017b |  ??  |  ??  |  ??  |  ??  |  ??  |
   60|  ??  | 0227 | 1e03 | 010b | 1e0b | 0117 | 1e1f | 0121 |
   68| 1e23 |  ??  |  ??  |  ??  |  ??  | 1e41 | 1e45 | 022f |
   70| 1e57 |  ??  | 1e59 | 1e61 | 1e6b |  ??  |  ??  | 1e87 |
   78| 1e8b | 1e8f | 017c |  ??  |  ??  |  ??  |  ??  |  ??  |
   --+------+------+------+------+------+------+------+------+
         Table B.8: Mapping of T.61 Dot Above Accent Combinations


B.9. Combinations for xc8: (Diaeresis)

  T.61 has predefined characters for A, E, I, O, U, and Y.  Unicode also
  defines H, W, X, and t.  All of these combinations are present in
  Table B.9.

     |    0 |    1 |    2 |    3 |    4 |    5 |    6 |    7 |
   --+------+------+------+------+------+------+------+------+
   40|  ??  | 00c4 |  ??  |  ??  |  ??  | 00cb |  ??  |  ??  |
   48| 1e26 | 00cf |  ??  |  ??  |  ??  |  ??  |  ??  | 00d6 |
   50|  ??  |  ??  |  ??  |  ??  |  ??  | 00dc |  ??  | 1e84 |
   58| 1e8c | 0178 |  ??  |  ??  |  ??  |  ??  |  ??  |  ??  |
   60|  ??  | 00e4 |  ??  |  ??  |  ??  | 00eb |  ??  |  ??  |
   68| 1e27 | 00ef |  ??  |  ??  |  ??  |  ??  |  ??  | 00f6 |



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   70|  ??  |  ??  |  ??  |  ??  | 1e97 | 00fc |  ??  | 1e85 |
   78| 1e8d | 00ff |  ??  |  ??  |  ??  |  ??  |  ??  |  ??  |
   --+------+------+------+------+------+------+------+------+
         Table B.8: Mapping of T.61 Diaeresis Accent Combinations


B.10. Combinations for xca: (Ring Above)

  T.61 has predefined characters for A, and U.  Unicode also defines w
  and y.  All of these combinations are present in Table B.10.

     |    0 |    1 |    2 |    3 |    4 |    5 |    6 |    7 |
   --+------+------+------+------+------+------+------+------+
   40|  ??  | 00c5 |  ??  |  ??  |  ??  |  ??  |  ??  |  ??  |
   48|  ??  |  ??  |  ??  |  ??  |  ??  |  ??  |  ??  |  ??  |
   50|  ??  |  ??  |  ??  |  ??  |  ??  | 016e |  ??  |  ??  |
   58|  ??  |  ??  |  ??  |  ??  |  ??  |  ??  |  ??  |  ??  |
   60|  ??  | 00e5 |  ??  |  ??  |  ??  |  ??  |  ??  |  ??  |
   68|  ??  |  ??  |  ??  |  ??  |  ??  |  ??  |  ??  |  ??  |
   70|  ??  |  ??  |  ??  |  ??  |  ??  | 016f |  ??  | 1e98 |
   78|  ??  | 1e99 |  ??  |  ??  |  ??  |  ??  |  ??  |  ??  |
   --+------+------+------+------+------+------+------+------+
        Table B.10: Mapping of T.61 Ring Above Accent Combinations


B.11. Combinations for xcb: (Cedilla)

  T.61 has predefined characters for C, G, K, L, N, R, S, and T.
  Unicode also defines E, D, and H.  All of these combinations are
  present in Table B.11.

     |    0 |    1 |    2 |    3 |    4 |    5 |    6 |    7 |
   --+------+------+------+------+------+------+------+------+
   40|  ??  |  ??  |  ??  | 00c7 | 1e10 | 0228 |  ??  | 0122 |
   48| 1e28 |  ??  |  ??  | 0136 | 013b |  ??  | 0145 |  ??  |
   50|  ??  |  ??  | 0156 | 015e | 0162 |  ??  |  ??  |  ??  |
   58|  ??  |  ??  |  ??  |  ??  |  ??  |  ??  |  ??  |  ??  |
   60|  ??  |  ??  |  ??  | 00e7 | 1e11 | 0229 |  ??  | 0123 |
   68| 1e29 |  ??  |  ??  | 0137 | 013c |  ??  | 0146 |  ??  |
   70|  ??  |  ??  | 0157 | 015f | 0163 |  ??  |  ??  |  ??  |
   78|  ??  |  ??  |  ??  |  ??  |  ??  |  ??  |  ??  |  ??  |
   --+------+------+------+------+------+------+------+------+
         Table B.11: Mapping of T.61 Cedilla Accent Combinations


B.12. Combinations for xcd: (Double Acute Accent)

  T.61 has predefined characters for O, and U.  These combinations are



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  present in Table B.12.

     |    0 |    1 |    2 |    3 |    4 |    5 |    6 |    7 |
   --+------+------+------+------+------+------+------+------+
   48|  ??  |  ??  |  ??  |  ??  |  ??  |  ??  |  ??  | 0150 |
   50|  ??  |  ??  |  ??  |  ??  |  ??  | 0170 |  ??  |  ??  |
   68|  ??  |  ??  |  ??  |  ??  |  ??  |  ??  |  ??  | 0151 |
   70|  ??  |  ??  |  ??  |  ??  |  ??  | 0171 |  ??  |  ??  |
   --+------+------+------+------+------+------+------+------+
       Table B.12: Mapping of T.61 Double Acute Accent Combinations


B.13. Combinations for xce: (Ogonek)

  T.61 has predefined characters for A, E, I, and U.  Unicode also
  defines the combination for O.  All of these combinations are present
  in Table B.13.

     |    0 |    1 |    2 |    3 |    4 |    5 |    6 |    7 |
   --+------+------+------+------+------+------+------+------+
   40|  ??  | 0104 |  ??  |  ??  |  ??  | 0118 |  ??  |  ??  |
   48|  ??  | 012e |  ??  |  ??  |  ??  |  ??  |  ??  | 01ea |
   50|  ??  |  ??  |  ??  |  ??  |  ??  | 0172 |  ??  |  ??  |
   58|  ??  |  ??  |  ??  |  ??  |  ??  |  ??  |  ??  |  ??  |
   60|  ??  | 0105 |  ??  |  ??  |  ??  | 0119 |  ??  |  ??  |
   68|  ??  | 012f |  ??  |  ??  |  ??  |  ??  |  ??  | 01eb |
   70|  ??  |  ??  |  ??  |  ??  |  ??  | 0173 |  ??  |  ??  |
   78|  ??  |  ??  |  ??  |  ??  |  ??  |  ??  |  ??  |  ??  |
   --+------+------+------+------+------+------+------+------+
          Table B.13: Mapping of T.61 Ogonek Accent Combinations


B.14. Combinations for xcf: (Caron)

  T.61 has predefined characters for C, D, E, L, N, R, S, T, and Z.
  Unicode also defines A, I, O, U, G, H, j,and K.  All of these
  combinations are present in Table B.14.

     |    0 |    1 |    2 |    3 |    4 |    5 |    6 |    7 |
   --+------+------+------+------+------+------+------+------+
   40|  ??  | 01cd |  ??  | 010c | 010e | 011a |  ??  | 01e6 |
   48| 021e | 01cf |  ??  | 01e8 | 013d |  ??  | 0147 | 01d1 |
   50|  ??  |  ??  | 0158 | 0160 | 0164 | 01d3 |  ??  |  ??  |
   58|  ??  |  ??  | 017d |  ??  |  ??  |  ??  |  ??  |  ??  |
   60|  ??  | 01ce |  ??  | 010d | 010f | 011b |  ??  | 01e7 |
   68| 021f | 01d0 | 01f0 | 01e9 | 013e |  ??  | 0148 | 01d2 |
   70|  ??  |  ??  | 0159 | 0161 | 0165 | 01d4 |  ??  |  ??  |
   78|  ??  |  ??  | 017e |  ??  |  ??  |  ??  |  ??  |  ??  |



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   --+------+------+------+------+------+------+------+------+
          Table B.14: Mapping of T.61 Caron Accent Combinations




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