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Versions: 00                                                            
INTERNET-DRAFT                                                Soobok Lee
draft-ietf-idn-hangeulchar-00.txt                         GyeongSeog Gim
Expires 2001-Dec-27                                          2001-Jun-27

           Hangeul NAMEPREP recommendation version 1.0

Status of this Memo

    This document is an Internet-Draft and is in full conformance with
    all provisions of Section 10 of RFC2026.

    Internet-Drafts are working documents of the Internet Engineering
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    Distribution of this document is unlimited.  Please send comments to
    the authors or to the idn working group at idn@ops.ietf.org.


    This document suggests Hangeul-specific NAMEPREP recommendations.
    It defines :
       - mapping tables for half-width jamo and enclosed jamo
       - excluding compatibility Hangeul jamo block
         from compatibility decomposition in normalization step
       - criteria for determining invalid syl-ipf jamo sequence
       - prohibited hangul filler character  in KC norm output.


    Background: UCS Hangeul
    Hangeul Canonical Composition
    Hangeul Compatibility Decomposition
    Summarized Recommendations
    Comments on security implication of inter-lingual similarities
    Security considerations
    A1. Acknowledgements
    A2. Authors
    A3. the mapping table for enclosed jamo
    A4. the mapping table for half-width jamo


    A user can enter a domain name into an application program in a
    myriad of fashions and the characters entered in the domain name
    may or may not be those that are allowed in internationalized host
    names. Thus, there must be a way to normalize the user's input
    before the name is resolved in the DNS, which is the rationale
    for NAMEPREP.

    NAMEPREP design goals are :

     -  to allow users to enter host names in applications and have
        the highest chance of getting the name correct. The user
        should not be limited to only entering exactly the characters
        that might have been used for domain name registration, but
        be able to enter characters that can be unambiguously
        normalized to characters in the registered domain name.

     -  to prohibit as few characters as possible that might be used
        in the future and in the various contexts

     -  to allow the widest possible set of host names as long as
        those host names do not cause other problems, such as
        conflict with other standards.

    The NAMEPREP process to prepare internationalized host names for
    use in the DNS includes the following stages :

     - stage1 : mapping characters to other characters,
                such as to change their case, mapping out some
                meaningless characters

     - stage2 : normalizing characters using normalization form KC.
                KC form consists of two steps detailed in [UTR15]
                - compatibility decomposition
                - canonical composition

     - stage3 : excluding characters that are prohibited from
                appearing in internationalized host names

    This draft defines special Hangeul character mappings and
    exceptions in applying KC normalization. And this draft also
    defines some prohibited Hangeul characters and sequences so that
    Hangeul can be used safely in Internet identifiers such as IDN.

    The content of this draft is subject to change with further
    discussions and studies.

Background : UCS Hangeul

    Korean Hangeul syllables are formed from a set of Hangeul letters,
    called jamo in Korean, in a regular fashion.

    The ISO/IEC 10646 (=Unicode Standard) contains both the complete
    set of precomposed modern Hangeul syllable blocks and the set of
    syl-ipf Hangeul jamo (= conjoining jamo in [UNICODE] ).  This set
    of syl-ipf jamo can be used to encode all modern and old syllable
    blocks.  For a description of syl-ipf Hangeul jamo behavior and
    precomposed Hangeul Syllables, see [UNICODE].

    Hangeul jamo are divided into three classes: choseong (leading
    consonants), jungseong(vowels), and jongseong(trailing consonants).
    In the following paragraphs, these classes are abbreviated as L
    (leading consonant), V(vowel), and T (trailing consonant). And for
    use in composition, two invisible filler characters act as
    placeholders for choseong or jungseong:
      U+115f (Hangeul choseong filler) and
      U+1160 (Hangeul jungseong filler).

    The UCS/Unicode contains a set of Hangeul Compatibility jamo
    (U+3130~U+318F) which consists of a filler, nonsyl-ipf
    Hangeul consonants and vowel elements. These characters are
    provided solely for compatibility with the KS X 1001 (formerly
    KS C 5601) standard.  Unlike the characters found in the Hangeul
    jamo block (U+1100 ..  U+11FF), the compatibility jamo characters
    have no syl-ipf semantics, except for only their filler+L+V+T or
    filler sequence makes a Hangeul syllable according to KS X 1001.

    The UCS/Unicode Standard also contains 52 half-width modern Hangeul
    jamo in the halfwidth and fullwidth forms (U+FFA0 .. U+FFDC) block
    and enclosed Hangeul syllables and jamo in the enclosed CJK letter
    and month block (U+3200 .. U+32FF). Enclosed ones are consisted of
    parenthesized jamo and circled jamo.

Hangeul canonical composition

    Modern Hangeul syllables can be expressed with either two or
    three jamo, either in the form consonant + vowel or in the form
    consonant + vowel + consonant. There are 19 possible leading
    (initial) consonants (choseong), 21 vowels (jungseong), and 27
    trailing (final) consonants (jongseong).  Thus there are 399
    possible two-jamo syllables and 10,773 possible three-jamo
    syllables, for a total of 11,172 modern Hangeul syllables.

    Each of the Hangeul syllables may be encoded by an equivalent
    sequence of syl-ipf jamo; however, the converse is not true
    because thousands of archaic Hangeul syllables may be encoded
    only as a sequence of syl-ipf jamo. Implementaions that
    use a syl-ipf jamo encoding are able to represent these archaic
    Hangeul syllables.

    The Hangeul syllables can be derived from syl-ipf jamo by a
    regular process of composition. The algorithm that maps a sequence
    of syl-ipf jamo to the encoding point for a Hangeul syllable
    is detailed in [UNICODE].

    In canonical composition, the syl-ipf jamo sequence for modern
    Hangeul syllable is transformed into the modern Hangeul syllable,
    but the sequence for archaic Hangeul syllable and the invalid jamo
    sequence (defective combining character sequence) are preserved
    in this process.

    In normalization form KC, all input sequence of code points go
    through this canonical composition [UTR15]. If any invalid jamo
    sequence is detected after KC normaliation stage, as it is not
    displayable correctly and distinguishably, the sequence should be
    prohibited from being an identifier. Whether a syl-ipf jamo
    sequence is valid or not can be determined according to
    the criteria detailed in [UNICODE].

Hangeul compatibility decomposition

    In normalization form KC, all input code sequence go through this
    compatibility decomposition and then canonical composition.

    Every Hangeul compatibility jamo and half-width jamo have its
    corresponding compatibility equivalent Hangeul syl-ipf jamo
    defined in [UNICODE_CHART].

    But this equivalence does violate the semantics and combining rules
    for compatibility jamo sequence in [KSC5601] from which UCS
    compatibility jamo came.

    In [KSC5601], a valid compatibility jamo sequence should start with
    a filler followed by choseong,jungseong and jongseong (or filler)
    to denote a Hangeul syllable. If the sequence does not fulfill this
    criterion, its jamo should remain unchaged as compatibility jamo.
    The same for half-width Hangeul jamo.

    Current compatibility decomposition blindly transforms compatibility
    jamo sequence even without a leading filler on a jamo by jamo basis.
    For example, a valid jamo sequence "filler gi-eog a gi-eug" (U+3164
    U+3131 U+314F U+3131) denoting a Hangeul syllable "gag"(U+AC01)
    is errornously transformed into "jungsong_filler chosung_gi-eog
    jungseong_a chosung_gi-eog" (U+1160 U+1100 U+1161 U+1100) that are
    canonically composed into "syllable_ga choseong_gi-eog"
    (U+AC00 U+1100) which are false.

    If false composistion could be avoided, NAMEPREP should exclude
    compatibility jamo and half-width jamo from its compatibility
    decomposition step. And, only valid compatibility jamo sequence
    should be recognized and transformed into a syl-ipf jamo sequence
    at the mapping step before KC normalization step in NAMEPREP.

    Hangeul consonant sequence can be used as abbreviated form of long
    Hangeul syllables sequence that represent Hangeul business name.
    And, there may be future need to represent Hangeul syllables in
    compatibility jamo sequences for an alternative syllable writing/
    displaying scheme.

    In NAMEPREP KC normalization and its internal compatibility
    decomposition step, each parethesized Hangeul jamo is transformed
    into its compatibility equivalent character sequence consisted of
    one pair of parentheses with inner Hangeul jamo and then that
    sequence is treated as an invalid domain since the paranthesis is
    prohibited in the domain names.  For example, parenthesized gi-eog
    (U+3200) is decomposed into U+0028 + U+1100 + U+0029 which includes
    prohibited left and right parentheses (U+0028,U+0029 respectively).

    Each parethesized Hangeul syllable is transformed into its
    compatibility equivalent character sequence consisted of one
    pair of parentheses with inner Hangeul syllable and then that
    sequence is treated as an invalid domain since the paranthesis is
    prohibited in the domain names.

    In NAMEPREP KC normalization and its internal compatibility
    decomposition step, Circled Hangeul jamo is transformed into its
    compatibility equivalent Hangeul jamo which is not appropriate
    in IDN context, and preferrably, this NAMEPREP process should map
    this circled one into the corresponding compatibility Hangeul jamo
    before KC normalization to bypass this inappropriate
    compatibility decomposition.

Summarized Recommendations

    KC normalization employed in NAMEPREP process does not preserve
    some Hangeul code semantics and so we recommend the following
    additional NAMEPREP actions for Hangeul codes:

    * Stage 1: mapping

      -  circled Hangeul jamo
         = map into the corresponding Hangeul compatibility jamo
           code range: U+3160 ~ U+326D
           mapping table detailed in appendix 3.

      -  half-width Hangeul jamo
         = map into the corresponding Hangeul compatibility jamo
           code range: U+FFA0 ~ U+FFDC
           mapping table detailed in appendix 4.

      -  transform compatibility jamo sequence with leading filler
         (U+3164) into syl-ipf jamo sequence
         = if and only if
           the sequence is of filler+ L+ V+ T (or filler) form.
         = preserve unchanged if the sequence is not of this form
         = so that each resulting jamo is given intended choseong
           or jongseong semantics implied in the input sequence

    * Stage 2: KC normalization

      -  compatibility decomposition
         = exclude compatibility Hangeul jamo; preserve them
           code range: U+3130 ~ U+318F

    * Stage 3: prohibitions

      -  prohibit invalid syl-ipf Hangeul jamo sequences
         = return error if not meaningful LV or LVT sequence

      -  compatibility Hangeul filler (U+3164) not combined
         = return error

Comments on security implication of inter-lingual similarities

    We have found  many similarities between hangeul jamo and
    other language scripts like japanese katakana and latin.

    To list some of them:

     - hangeul jamo gi-eog  and katakana hu
     - hangeul jamo mi-eum  and katakana ro
     - hangeul jamo i-eung  and latin 'o'
     - hangeul jamo ji-euth and katakana su
     - hangeul jamo ki-eog  and katakana wo
     - hangeul jamo a       and katakana to

     - hangeul syllable ma  and katakana ro-to
     - hangeul syllable ja  and katakana su-to
     - hangeul syllable ga  and katakana hu-to
     - hangeul syllable i   and digits '01'

    Some hangeul domains similiar to katakana domains
    can mislead some japanese to believe hangeul hostnames or
    hangeul email addresses are the japanese ones they trust.

    To mitigate these inherent security problems, there should be
    well-prepared registration/dispute resolution policy that
    can be enforced to every zone masters (including root zone
    and its lower-level zones) and every email account masters.
    Of course, whether this is feasible or not is beyond NAMEPREP scope.

Security considerations

    This suggestion improves IDN security by prohibiting/correcting
    non-displayable or invalid hangeul syllables/sequences in IDN.


    [IDNREQ] Requirements of Internationalized Domain Names

    [UNICODE] The Unicode Consortium, "The Unicode Standard",

    [UNICODE_CHART] THe Unicode Code Charts

    [IDNA]  Patrik Falstrom, Paul Hoffman,
    "Internationalizing Host Names In Applications (IDNA)",

    [NAMEPREP]  Paul Hoffman, Marc Blanchet,
    "Preparation of Internationalized Host Names",  Feb 2001,

    [UTR15] Mark Davis and Martin Duerst.
    Unicode Normalization Forms.  Unicode Technical Report;15.

    [VERSION] M Blanchet
    "Handling versions of internationalized domain names protocols",

    [ISO10646] ISO/IEC, Information Technology - Universal
    Multiple-Octet Coded Character Set (UCS) - Part 1: Architecture
    and Basic Multilingual Plane, Oct. 2000, with amendments.

    [KSC5601]  Korean Standard KS C 5601- 1987

A1. Acknowledgements

    Dongman Lee <dlee@icu.ac.kr> and Yangwoo Ko <newcat@peacenet.or.kr>
    made valuable contributions to narrowing down the issues of the
    prohibition and preservation of some hangeul characters.

    Thank Mark Davis for his advice on useful UNICODE reference

A2. Authors

    Soobok Lee <lsb@postel.co.kr>
    Postel Services, Inc.
    Tel: +82-11-9774-2737

    GyeongSeog Gim <gimgs@asadal.pusan.ac.kr>
    Department of Computer Engineering
    Pusan National University
    Republic of Korea
    Tel: +82-51-510-2292

A3. the mapping table for enclosed jamo in the format of [VERSION]



A4. the mapping table for half-width jamo in the format of [VERSION]