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INTERNET DRAFT                                                  J. Abela
Expires: 23 June 1998                                                HSC
<draft-abela-utf9-00.txt>                               23 December 1997

                 UTF-9, a transformation format of UCS

Status of this Memo

   This document is an Internet-Draft.  Internet-Drafts are working
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   Distribution of this document is unlimited.

Abstract

   ISO/IEC 10646 defines a multi-octet character set called the
   Universal Character Set (UCS) which encompasses most of the world's
   writing systems.  Multi-octet characters, however, are not compatible
   with many current applications and protocols, and this has led to the
   development of a few so-called UCS transformation formats (UTF), each
   with different characteristics.  UTF-9, the object of this memo, has
   the characteristic of preserving the full ISO-Latin1 range, providing
   compatibility with file systems, parsers and other software that rely
   on ISO-Latin1 values.

   ISO-Latin1 is almost as widespread as ASCII in many countries,
   especially in most of western Europe, and is the default character
   set for HTML.  A compatible encoding seems desirable, where possible.


1. Introduction

   ISO/IEC 10646-1 [ISO-10646] defines a multi-octet character set
   called the Universal Character Set (UCS), which encompasses most of
   the world's writing systems.  Two multi-octet encodings are defined,
   a four-octet per character encoding called UCS-4 and a two-octet per
   character encoding called UCS-2, able to address only the first 64K
   characters of the UCS (the Basic Multilingual Plane, BMP), outside of
   which there are currently no assignments.

   It is noteworthy that the same set of characters is defined by the
   Unicode standard [UNICODE], which further defines additional
   character properties and other application details of great interest
   to implementors, but does not have the UCS-4 encoding.  Up to the
   present time, changes in Unicode and amendments to ISO/IEC 10646 have
   tracked each other, so that the character repertoires and code point
   assignments have remained in sync.  The relevant standardization
   committees have committed to maintain this very useful synchronism.

   The UCS-2 and UCS-4 encodings, however, are hard to use in many
   current applications and protocols that assume 8 or even 7 bit
   characters.  Even newer systems able to deal with 16 bit characters
   cannot process UCS-4 data. This situation has led to the development
   of so-called UCS transformation formats (UTF), each with different
   characteristics.

   UTF-1 has only historical interest, having been removed from ISO/IEC
   10646.  UTF-7 has the quality of encoding the full BMP repertoire
   using only octets with the high-order bit clear (7 bit US-ASCII
   values, [US-ASCII]), and is thus deemed a mail-safe encoding
   ([RFC2152]).  UTF-8 uses all bits of an octet, but has the quality of
   preserving the full US-ASCII range: US-ASCII characters are encoded
   in one octet having the normal US-ASCII value, and any octet with
   such a value can only stand for an US-ASCII character, and nothing
   else. UTF-9, the object of this memo, has the quality of preserving
   the full ISO-Latin1 range: ISO-Latin1 characters are encoded in one
   octet having the normal ISO-Latin1 value.

   UTF-16 is a scheme for transforming a subset of the UCS-4 repertoire
   into pairs of UCS-2 values from a reserved range.  UTF-16 impacts
   UTF-9 in that UCS-2 values from the reserved range must be treated
   specially in the UTF-9 transformation.

   UTF-9 encodes UCS-2 or UCS-4 characters as a varying number of
   octets, where the number of octets, and the value of each, depend on
   the integer value assigned to the character in ISO/IEC 10646.  This
   transformation format has the following characteristics (all values
   are in hexadecimal):

   -  Character values from 0000 0000 to 0000 007F and 0000 00A0 to 0000
   00FF (Latin1 repertoire) correspond to octets 00 to 7F and A0 to FF
   (8 bit Latin1 values).  A direct consequence is that a plain Latin1
   string is also a valid UTF-9 string.  Note that Latin1 octets in a
   UTF-9 string may be non-Latin1 characters.

   -  US-ASCII values do not appear otherwise in a UTF-9 encoded
   character stream.  This provides compatibility with file systems or
   other software (e.g. the printf() function in C libraries) that parse
   based on US-ASCII values but are transparent to other values.
   However, note that Latin1 octets in a UTF-9 stream may be non-Latin1
   characters when used as part of multi-octet sequences.

   -  Round-trip conversion is easy between UTF-9 and either of UCS-4,
   UCS-2.

   -  The first octet of a multi-octet sequence indicates the number of
   octets in the sequence.

   -  UTF-9 encoding length is never bigger than UTF-8.

   -  unlike UTF-8, there is no reliable way to find character
   boundaries in a UTF-9 octet stream.

   UTF-9 is heavily based on UTF-8 definition. More information about
   UTF, Unicode, and their various versions can be found in RFC-2044.

UTF-9 definition

   In UTF-9, characters are encoded using sequences of 1 to 5 octets.
   The only octet of a "sequence" of one is in the ranges 00 to 7F or
   A0-FF. In a sequence of n octets, n>1, the initial octet is in the
   range 80 to 9F. This octet specifies the length of the sequence and
   contains value bits if in the range 80 to 8F. All the bits of the
   remaining octets are used to encode the character.

   The table below summarizes the format of these different octet types.
   The letter x indicates bits available for encoding bits of the UCS-4
   character value.

    UCS-4 range (hex)     UTF-9 octet sequence (binary)
    0000 0000-0000 007F   0xxxxxxx
    0000 00A0-0000 00BF   101xxxxx
    0000 00C0-0000 00FF   11xxxxxx
    0000 0100-0000 07FF   1000xxxx 1xxxxxxx
    0000 0800-0000 FFFF   100100xx 1xxxxxxx 1xxxxxxx
    0001 0000-007F FFFF   100101xx 1xxxxxxx 1xxxxxxx 1xxxxxxx
    0080 0000-7FFF FFFF   10011xxx 1xxxxxxx 1xxxxxxx 1xxxxxxx 1xxxxxxx

Examples

   The Latin1 sequence "No<e diaeresis>l" should be encoded as follows:

    UCS-2: 004E 006F 00EB 006C
    UTF-9: 4E   6F   EB   6C
    UTF-8: 4E   6F   C3AB 6C

   The UCS-2 sequence "A<NOT IDENTICAL TO><ALPHA>." should be encoded as
   follows:

    UCS-2: 0041  2262      0391   002E
    UTF-9: 41    90 C4 E2  87 91  2E
    UTF-8: 41    E2 89 A2  CE 91  2E

   The UCS-2 sequence representing the Hangul characters for the Korean
   word "hangugo" should be encoded as follows:

    UCS-2: D55C      AD6D      C5B4
    UTF-9: 93 AA DC  92 DA ED  93 8B B4
    UTF-8: ED 95 9C  EA B5 AD  EC 96 B4

Security Considerations

   Implementors of UTF-9 need to consider the security aspects of how
   they handle illegal UTF-9 sequences.  It is conceivable that in some
   circumstances an attacker would be able to exploit an incautious
   UTF-9 parser by sending it an octet sequence that is not permitted by
   the UTF-9 syntax.

   A particularly subtle form of this attack could be carried out
   against a parser which performs security-critical validity checks
   against the UTF-9 encoded form of its input, but interprets certain
   illegal octet sequences as characters.  For example, a parser might
   prohibit the NUL character when encoded as the single-octet sequence
   00, but allow the illegal two-octet sequence 80 80 and interpret it
   as a NUL character.  Another example might be a parser which
   prohibits the octet sequence 2F 2E 2E 2F ("/../"), yet permits the
   illegal octet sequence 2F 2E 80 AE 2F.

Acknowledgments

   Most of the text of this memo comes from the UTF-8 memo from Francois
   Yergeau.  The following have participated in the drafting of this
   memo: Antoine Leca and Francois Yergeau

Bibliography

      [ISO-10646]    ISO/IEC 10646-1:1993. International Standard --
                     Information technology -- Universal Multiple-Octet
                     Coded Character Set (UCS) -- Part 1: Architecture
                     and Basic Multilingual Plane.  Five amendments and
                     a technical corrigendum have been published up to
                     now.

      [RFC2152]      D. Goldsmith, M. Davis, "UTF-7: A Mail-safe
                     Transformation Format of Unicode", RFC 1642,
                     Taligent inc., May 1997. (Obsoletes RFC1642)

      [UNICODE]      The Unicode Consortium, "The Unicode Standard --
                     Version 2.0", Addison-Wesley, 1996.

      [US-ASCII]     Coded Character Set--7-bit American Standard Code
                     for Information Interchange, ANSI X3.4-1986.

Author's Address

   Jerome Abela
   Herve Schauer Consultants
   142, rue de Rivoli
   75001 Paris
   France

   Phone: +33 141 409 700
   Fax:   +33 141 409 709

   EMail: Jerome.Abela@hsc.fr


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