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Versions: 00 01 02                                                      
Internet Draft                                         James Seng, BIX
<draft-jseng-utf5-00.txt>                           Martin Duerst, W3C
27th July 1999                                        Tin Wee Tan, NUS
Expires End of January 2000

            UTF-5, a transformation format of Unicode and ISO 10646

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
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as

   Internet-Drafts are draft documents valid for a maximum of six
   months and may be updated, replaced, or obsoleted by other
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   Drafts as reference material or to cite them other than as
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   The list of current Internet-Drafts can be accessed at

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   Distribution of this document is unlimited. Please send comments
   to the authors at jseng@pobox.org.sg, mduerst@w3.org and


   A new transformation format, called UTF-5 for Unicode is proposed.
   The resulting string of this UTF is within a [A-V][0-9] alphanumeric
   range. This enables legacy systems or protocols designed for alpha-
   numerical character set only to be multilingual enabled and inter-
   nationalized immediately. Example of such systems are the domain
   name system and email addresses.

1. Introduction

   The Unicode Standard, version 2.1 [UNICODE], and ISO/IEC 10646-1
   [ISO-10646] jointly define a 16 bit character set, UCS-2, which
   encompasses most of the world's writing systems.  ISO 10646 further
   defines a 31-bit character set, UCS-4, with currently no assignments
   outside of the region corresponding to UCS-2 (the Basic Multilingual
   Plane, BMP).  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 char-
   acters cannot process UCS-4 data. This situation has led to the
   development of so-called UCS transformation formats (UTF), each with
   different characteristics.

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   At this moment, there are 3 standard UTF, namely UTF-7 [UTF7], UTF-8
   [UTF8] and UTF-16 [UTF16], each is a variable length transformation
   which gives 7 bit, 8 bit and 16 bit strings respectively. While
   these are sufficient for most application uses, there are however
   some legacy systems which are, unfortunately, unable to handle even
   7 bit strings either due to technical restriction or common uses.

   The object of this memo is to propose a UTF-5 which gives a trans-
   formed string that is within [A-V][0-9] alphanumerical character set.
   This enables legacy system designed for alphanumerical character set
   only to be multilingual enabled and internationalized immediately.

   UTF-8 is the transformation format for all IETF standards [IETFPC].
   UTF-5 is not here to change this. It is proposed to support legacy
   applications or protocols that cannot be modify in a simple way to
   handle 8 bits using UTF-8 encoding. See Section 4 on the discussion
   on how UTF-5 can be used for Domain Name System [DNS] and Simple Mail
   Transfer Protocol [SMTP] Address.

2. UTF-5 definition

   In UTF-5, each character are encoded using a sequence of 1 to 8
   octets. Two transformations are needed for UTF-5, namely

   1. Determine the quintet ("5-bit") binary sequence.
   2. From a table, translate the quintet to the resulting string.

   Take note that the UTF-5 is not a sequence of quintets but a sequence
   of octets where each octets are in the alphanumeric range. Alpha-
   numeric is defined as A to V (uppercase only) and 0 to 9 in this

   This memo does not specify the binary pattern of the alphanumeric
   characters as the purpose of the transformation is to get a alpha-
   numeric string which represent a multilingual string. However, it
   is presumed that US-ASCII [US-ASCII] is use for most purposes.

   2.1 Determine the quintet binary sequence

   The first quintet of a binary sequence will have the highest-order
   bit set to 1 and the remaining quintet will have the highest-order
   bit set to 0. The remaining 4 bits of every quintet contain bits
   from the value of the character to be encoding.

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

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   UCS-4 range (hex.)           UTF-5 quintet sequence (binary)
   0000 0000-0000 000F          1xxxx
   0000 0010-0000 00FF          1xxxx 0xxxx
   0000 0100-0000 0FFF          1xxxx 0xxxx 0xxxx
   0000 1000-0000 FFFF          1xxxx 0xxxx 0xxxx 0xxxx
   1000 0000-7FFF FFFF          1xxxx 0xxxx 0xxxx ..... 0xxxx

   2.2 Translation table for quintet and alphanumeric character

   Translation table for quintet binary pattern and alphanumeric
   character are as follows:

   quintet          quintet         quintet         quintet
   00000   0        01000   8       10000   G       11000   O
   00001   1        01001   9       10001   H       11001   P
   00010   2        01010   A       10010   I       11010   Q
   00011   3        01011   B       10011   J       11011   R
   00100   4        01100   C       10100   K       11100   S
   00101   5        01101   D       10101   L       11101   T
   00110   6        01110   E       10110   M       11110   U
   00111   7        01111   F       10111   N       11111   V

   2.3 Encoding from UCS-4 to UTF-5

   1) Determine the required number of octets from the character value.
      Let U be the UCS-4 value, then the required number of octets is
      log16(U) round up.

   2) Prepare the quintet binary sequence. Put the highest order bit
      of the first quintet as 1 and highest order bit of the rest of
      the quintet as 0.

   3) Fill in the bits marked x from the bits of the character value,
      starting from the lower-order its of the character value and
      putting them first in the last quintet of the sequence, then the
      next to last, etc until all x bits are filled in.

   4) For each quintet, apply the lookup table in Section 2.2 to get
      the corresponding alphanumeric character.

   2.4 Decoding UTF-5 to UCS-4

   1) Determine the length of octet sequence. As according to the UTF-5
      encoding, every character will have the inital octet within 'G'
      to 'V'. Thus, the length of the octet sequence can be determined
      by looking for 'G' to 'V' in the UTF-5 string.

   2) Apply the reverse lookup according to the table in Section 2.2
      to get the quintet binary sequence.

   3) Initialize the 4 octets of the UCS-4 character with all bits set
      to 0.

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   4) Distribute the bits from the sequence to the UCS-4 character,
      first the lower-order bits from the last octet of the sequence
      and proceeding to the left until no x bits are left.

      If the UTF-5 sequence is no more than four octets long, decoding
      can proceed directly to UCS-2 (or equivalently Unicode).

   2.5 Detecting UTF-5 string

   As the UTF-5 string is a alphanumeric string, it is difficult to
   differential between a normal ASCII document or a UTF-5 document.

   Nevertheless, if the string is sufficient long, it is possible to do
   some detection of UTF-5 string base on the fact that
   1. UTF-5 strings only have characters within '0'-'9' and 'A'-'V'.
   2. UTF-5 strings have a well-defined inital octet of 'G' to 'V'.
   3. The 'G' character always occurs as the inital and only octet.

3. Examples of UTF-5

   The Unicode sequence "A<NOT IDENTICAL TO><ALPHA>." (0041, 2262,
   0391, 002E) may be encoded as follows:


   The Unicode sequence "Hi Mom <WHITE SMILING FACE>!" (0048, 0069,
   0020, 004D, 006F, 006D, 0020, 263A, 0021) may be encoded as follows:


   The Unicode sequence representing the Han characters for the
   Japanese word "nihongo" (65E5, 672C, 8A9E) may be encoded as


   Note that from the examples, it is obvious that there is a short-cut
   to the UTF-5 transformation which goes like this:

   1. Write down the hexdecimal of the Unicode character as a string.
   2. For the first character of the hexdecimal string, change 0 to G,
      1 to H, 2 to I, ... F to V.

   This will yield you the UTF-5 string of the Unicode character.

4. Applications

   There are many applications whereby UTF-5 would be useful for
   Internationalization ("i18n"). Here are some of the possible uses.

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   a. Internationalization of Domain Names System

   In the Domain Name System, although the technical standard does not
   prevent 8-bits character to be use as domain names, general use of
   the system restrict it to only A-Z (upper and lower), 0-9 and "-"
   as a valid domain name. This pose some great difficulty when doing
   i18n of domain names as the current UTF-7, UTF-8 and UTF-16 is not
   compatible with the existing software system already in used.

   Please see draft-xxx-xxx-xxx.txt for detail discussion on
   Internationalization of Domain Names System ("iDNS").

   b. Internationalization of Simple Mail Transfer Protocol Address

   While it is possible for a person to send SMTP Mail in different
   language on different character set to each another using Multi-
   purpose Internet Mail Extensions [MIME], the SMTP Mail Address
   remains a challenge to be Internationalized. Internationalization of
   SMTP Address has two barrier, 1. the Internationalization of Domain
   Name System and 2. the Internationalization of the mailbox or
   username. SMTP mailbox have a very strict check [RFC822] dues to
   many potential security risks when using symbols or special char-
   acters in mailbox. UTF-5 will allow Unicode to be used immediately
   as mailbox with minimual change in system and without additional
   security risks.

   Please see draft-xxx-xxx-xxx.txt for detail discussion on Inter-
   nationalization of Simple Mail Transfer Protocol Address

   Internationalization of URIs is not discussed in this memo. Please
   refer to http://www.w3.org/International/0-URL-and-ident.html.

   However, uses for UTF-5 goes beyond Internet back to old legacy
   system such as Telegram system or even Morse code allowing
   Multilingual characters to be transmitted.

5. Security Considerations

   This memo does not address any security consideration at the moment.

6. Acknowledgments

   UTF-5 was first defined by Martin Duerst at the University of Zurich
   in draft-duerst-dns-i18n-00.txt.

   Contributors (not in any order):
   Marc Blanchet <Marc.Blanchet@viagenic.qc.ca>

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7. Bibliography

   [ISO-10646]    ISO/IEC 10646-1:1993. International Standard --
   [UTF16]        Information technology -- Universal Multiple-Octet
                  Coded Character Set (UCS) -- Part 1: Architecture
                  and Basic Multilingual Plane. UTF-8 is described in
                  Annex R, adopted but not yet published. UTF-16 is
                  described in Annex Q, adopted but not yet published.

   [UTF7]         Goldsmith, D., and M. Davis, "UTF-7: A Mail-safe
                  Transformation Format of Unicode", RFC 1642,
                  Taligent, Inc., July 1994.

   [UTF8]         F. Yergeau "UTF-8: a transformation format of Unicode
                  and ISO 10646", RFC2044, Alis Technologies, October

   [UNICODE]      The Unicode Consortium, "The Unicode Standard --
                  Worldwide Character Encoding -- Version 1.0",
                  Addison-Wesley, Volume 1, 1991, Volume 2, 1992.

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

   [DNS]          P. Mockapetris "Domain Names - Concepts and
                  Facilities", RFC1034, ISI, November 1987, "Domain
                  Names - Implementation and Specification", RFC1035,
                  ISI, November 1987.

   [SMTP]         Jonathan B. Postel "Simple Mail Transfer Protocol",
   [RFC822]       RFC821, ISI, August 1982. David H. Crocker "Standard
                  for ARPA Internet Text Messages", RFC822, Dept of
                  Electrical Engineering, Univeristy of Delaware,
                  August 1982.

   [MIME]         "Multipurpose Internet Mail Extensions", RFC1341,
                  N. Borensten, Bellcore, N. Freed, Innosoft, June

   [IETFPC]       "IETF Policy on Character Sets and Languages",
                  RFC2277 BCP18, H. Alvestrand, Jan 1998.

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8. Author Address

   James C.H Seng
   BioInformatrix Pte Ltd
   102 Elm Street
   Menlo Park CA 94025

   Tel: (650) 322-6505
   E-mail: jseng@pobox.org.sg

   Martin J. Duerst
   World Wide Web Consortium
   Keio Research Institute at SFC
   Keio University
   252-8520 Japan

   Tel: +81 446 49 11 70
   E-mail: mduerst@w3.org

   NOTE -- Please write the author's name with u-Umlaut wherever
   possible, e.g. in HTML as D&uuml;rst.

   Tin Wee Tan, Dr
   National University of Singapore
   c/o BioInformatic Center
   National University Hospital
   Lower Kent Ridge Road
   Singapore 119074

   Tel: +65 774 7149
   E-mail: tinwee@post1.com

   This memo is also archived at http://www.idns.org/technical.html

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