Network Working Group                                       F. Yergeau
Request for Comments: 2279                           Alis Technologies
Obsoletes: 2044                                           January 1998
Category: Standards Track


             UTF-8, a transformation format of ISO 10646

Status of this Memo

  This document specifies an Internet standards track protocol for the
  Internet community, and requests discussion and suggestions for
  improvements.  Please refer to the current edition of the "Internet
  Official Protocol Standards" (STD 1) for the standardization state
  and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

  Copyright (C) The Internet Society (1998).  All Rights Reserved.

Abstract

  ISO/IEC 10646-1 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-8, the object of this memo, has
  the characteristic of preserving the full US-ASCII range, providing
  compatibility with file systems, parsers and other software that rely
  on US-ASCII values but are transparent to other values. This memo
  updates and replaces RFC 2044, in particular addressing the question
  of versions of the relevant standards.

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



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  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, the object of this memo, 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-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-8 in that UCS-2 values from the reserved range must be treated
  specially in the UTF-8 transformation.

  UTF-8 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 (US-ASCII repertoire)
     correspond to octets 00 to 7F (7 bit US-ASCII values). A direct
     consequence is that a plain ASCII string is also a valid UTF-8
     string.

  -  US-ASCII values do not appear otherwise in a UTF-8 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.

  -  Round-trip conversion is easy between UTF-8 and either of UCS-4,
     UCS-2.





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

  -  The octet values FE and FF never appear.

  -  Character boundaries are easily found from anywhere in an octet
     stream.

  -  The lexicographic sorting order of UCS-4 strings is preserved.  Of
     course this is of limited interest since the sort order is not
     culturally valid in either case.

  -  The Boyer-Moore fast search algorithm can be used with UTF-8 data.

  -  UTF-8 strings can be fairly reliably recognized as such by a
     simple algorithm, i.e. the probability that a string of characters
     in any other encoding appears as valid UTF-8 is low, diminishing
     with increasing string length.

  UTF-8 was originally a project of the X/Open Joint
  Internationalization Group XOJIG with the objective to specify a File
  System Safe UCS Transformation Format [FSS-UTF] that is compatible
  with UNIX systems, supporting multilingual text in a single encoding.
  The original authors were Gary Miller, Greger Leijonhufvud and John
  Entenmann.  Later, Ken Thompson and Rob Pike did significant work for
  the formal UTF-8.

  A description can also be found in Unicode Technical Report #4 and in
  the Unicode Standard, version 2.0 [UNICODE].  The definitive
  reference, including provisions for UTF-16 data within UTF-8, is
  Annex R of ISO/IEC 10646-1 [ISO-10646].

2.  UTF-8 definition

  In UTF-8, characters are encoded using sequences of 1 to 6 octets.
  The only octet of a "sequence" of one has the higher-order bit set to
  0, the remaining 7 bits being used to encode the character value. In
  a sequence of n octets, n>1, the initial octet has the n higher-order
  bits set to 1, followed by a bit set to 0.  The remaining bit(s) of
  that octet contain bits from the value of the character to be
  encoded.  The following octet(s) all have the higher-order bit set to
  1 and the following bit set to 0, leaving 6 bits in each to contain
  bits from the character to be encoded.

  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.




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  UCS-4 range (hex.)           UTF-8 octet sequence (binary)
  0000 0000-0000 007F   0xxxxxxx
  0000 0080-0000 07FF   110xxxxx 10xxxxxx
  0000 0800-0000 FFFF   1110xxxx 10xxxxxx 10xxxxxx

  0001 0000-001F FFFF   11110xxx 10xxxxxx 10xxxxxx 10xxxxxx
  0020 0000-03FF FFFF   111110xx 10xxxxxx 10xxxxxx 10xxxxxx 10xxxxxx
  0400 0000-7FFF FFFF   1111110x 10xxxxxx ... 10xxxxxx

  Encoding from UCS-4 to UTF-8 proceeds as follows:

  1) Determine the number of octets required from the character value
     and the first column of the table above.  It is important to note
     that the rows of the table are mutually exclusive, i.e. there is
     only one valid way to encode a given UCS-4 character.

  2) Prepare the high-order bits of the octets as per the second column
     of the table.

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

     The algorithm for encoding UCS-2 (or Unicode) to UTF-8 can be
     obtained from the above, in principle, by simply extending each
     UCS-2 character with two zero-valued octets.  However, pairs of
     UCS-2 values between D800 and DFFF (surrogate pairs in Unicode
     parlance), being actually UCS-4 characters transformed through
     UTF-16, need special treatment: the UTF-16 transformation must be
     undone, yielding a UCS-4 character that is then transformed as
     above.

     Decoding from UTF-8 to UCS-4 proceeds as follows:

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

  2) Determine which bits encode the character value from the number of
     octets in the sequence and the second column of the table above
     (the bits marked x).

  3) 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-8 sequence is no more than three octets long, decoding
     can proceed directly to UCS-2.



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       NOTE -- actual implementations of the decoding algorithm above
       should protect against decoding invalid sequences.  For
       instance, a naive implementation may (wrongly) decode the
       invalid UTF-8 sequence C0 80 into the character U+0000, which
       may have security consequences and/or cause other problems.  See
       the Security Considerations section below.

  A more detailed algorithm and formulae can be found in [FSS_UTF],
  [UNICODE] or Annex R to [ISO-10646].

3.  Versions of the standards

  ISO/IEC 10646 is updated from time to time by published amendments;
  similarly, different versions of the Unicode standard exist: 1.0, 1.1
  and 2.0 as of this writing.  Each new version obsoletes and replaces
  the previous one, but implementations, and more significantly data,
  are not updated instantly.

  In general, the changes amount to adding new characters, which does
  not pose particular problems with old data.  Amendment 5 to ISO/IEC
  10646, however, has moved and expanded the Korean Hangul block,
  thereby making any previous data containing Hangul characters invalid
  under the new version.  Unicode 2.0 has the same difference from
  Unicode 1.1. The official justification for allowing such an
  incompatible change was that no implementations and no data
  containing Hangul existed, a statement that is likely to be true but
  remains unprovable.  The incident has been dubbed the "Korean mess",
  and the relevant committees have pledged to never, ever again make
  such an incompatible change.

  New versions, and in particular any incompatible changes, have q
  conseuences regarding MIME character encoding labels, to be discussed
  in section 5.

4.  Examples

  The UCS-2 sequence "A<NOT IDENTICAL TO><ALPHA>." (0041, 2262, 0391,
  002E) may be encoded in UTF-8 as follows:

  41 E2 89 A2 CE 91 2E

  The UCS-2 sequence representing the Hangul characters for the Korean
  word "hangugo" (D55C, AD6D, C5B4) may be encoded as follows:

  ED 95 9C EA B5 AD EC 96 B4






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RFC 2279                         UTF-8                      January 1998


  The UCS-2 sequence representing the Han characters for the Japanese
  word "nihongo" (65E5, 672C, 8A9E) may be encoded as follows:

  E6 97 A5 E6 9C AC E8 AA 9E

5.  MIME registration

  This memo is meant to serve as the basis for registration of a MIME
  character set parameter (charset) [CHARSET-REG].  The proposed
  charset parameter value is "UTF-8".  This string labels media types
  containing text consisting of characters from the repertoire of
  ISO/IEC 10646 including all amendments at least up to amendment 5
  (Korean block), encoded to a sequence of octets using the encoding
  scheme outlined above.  UTF-8 is suitable for use in MIME content
  types under the "text" top-level type.

  It is noteworthy that the label "UTF-8" does not contain a version
  identification, referring generically to ISO/IEC 10646.  This is
  intentional, the rationale being as follows:

  A MIME charset label is designed to give just the information needed
  to interpret a sequence of bytes received on the wire into a sequence
  of characters, nothing more (see RFC 2045, section 2.2, in [MIME]).
  As long as a character set standard does not change incompatibly,
  version numbers serve no purpose, because one gains nothing by
  learning from the tag that newly assigned characters may be received
  that one doesn't know about.  The tag itself doesn't teach anything
  about the new characters, which are going to be received anyway.

  Hence, as long as the standards evolve compatibly, the apparent
  advantage of having labels that identify the versions is only that,
  apparent.  But there is a disadvantage to such version-dependent
  labels: when an older application receives data accompanied by a
  newer, unknown label, it may fail to recognize the label and be
  completely unable to deal with the data, whereas a generic, known
  label would have triggered mostly correct processing of the data,
  which may well not contain any new characters.

  Now the "Korean mess" (ISO/IEC 10646 amendment 5) is an incompatible
  change, in principle contradicting the appropriateness of a version
  independent MIME charset label as described above.  But the
  compatibility problem can only appear with data containing Korean
  Hangul characters encoded according to Unicode 1.1 (or equivalently
  ISO/IEC 10646 before amendment 5), and there is arguably no such data
  to worry about, this being the very reason the incompatible change
  was deemed acceptable.





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  In practice, then, a version-independent label is warranted, provided
  the label is understood to refer to all versions after Amendment 5,
  and provided no incompatible change actually occurs.  Should
  incompatible changes occur in a later version of ISO/IEC 10646, the
  MIME charset label defined here will stay aligned with the previous
  version until and unless the IETF specifically decides otherwise.

  It is also proposed to register the charset parameter value
  "UNICODE-1-1-UTF-8", for the exclusive purpose of labelling text data
  containing Hangul syllables encoded to UTF-8 without taking into
  account Amendment 5 of ISO/IEC 10646 (i.e. using the pre-amendment 5
  code point assignments).  Any other UTF-8 data SHOULD NOT use this
  label, in particular data not containing any Hangul syllables, and it
  is felt important to strongly recommend against creating any new
  Hangul-containing data without taking Amendment 5 of ISO/IEC 10646
  into account.

6.  Security Considerations

  Implementors of UTF-8 need to consider the security aspects of how
  they handle illegal UTF-8 sequences.  It is conceivable that in some
  circumstances an attacker would be able to exploit an incautious
  UTF-8 parser by sending it an octet sequence that is not permitted by
  the UTF-8 syntax.

  A particularly subtle form of this attack could be carried out
  against a parser which performs security-critical validity checks
  against the UTF-8 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 C0 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 C0 AE 2E 2F.

Acknowledgments

  The following have participated in the drafting and discussion of
  this memo:

  James E. Agenbroad    Andries Brouwer
  Martin J. D|rst       Ned Freed
  David Goldsmith       Edwin F. Hart
  Kent Karlsson         Markus Kuhn
  Michael Kung          Alain LaBonte
  John Gardiner Myers   Murray Sargent
  Keld Simonsen         Arnold Winkler




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Bibliography

  [CHARSET-REG]  Freed, N., and J. Postel, "IANA Charset Registration
                 Procedures", BCP 19, RFC 2278, January 1998.

  [FSS_UTF]      X/Open CAE Specification C501 ISBN 1-85912-082-2 28cm.
                 22p. pbk. 172g.  4/95, X/Open Company Ltd., "File
                 System Safe UCS Transformation Format (FSS_UTF)",
                 X/Open Preleminary Specification, Document Number
                 P316.  Also published in Unicode Technical Report #4.

  [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.
                 UTF-8 is described in Annex R, published as Amendment
                 2.  UTF-16 is described in Annex Q, published as
                 Amendment 1. 17 other amendments are currently at
                 various stages of standardization.

  [MIME]         Freed, N., and N. Borenstein, "Multipurpose Internet
                 Mail Extensions (MIME) Part One:  Format of Internet
                 Message Bodies", RFC 2045.  N. Freed, N. Borenstein,
                 "Multipurpose Internet Mail Extensions (MIME) Part
                 Two:  Media Types", RFC 2046.  K. Moore, "MIME
                 (Multipurpose Internet Mail Extensions) Part Three:
                 Message Header Extensions for Non-ASCII Text", RFC
                 2047.  N.  Freed, J. Klensin, J. Postel, "Multipurpose
                 Internet Mail Extensions (MIME) Part Four:
                 Registration Procedures", RFC 2048.  N. Freed, N.
                 Borenstein, " Multipurpose Internet Mail Extensions
                 (MIME) Part Five: Conformance Criteria and Examples",
                 RFC 2049.  All November 1996.

  [RFC2152]      Goldsmith, D., and 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.







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RFC 2279                         UTF-8                      January 1998


Author's Address

  Francois Yergeau
  Alis Technologies
  100, boul. Alexis-Nihon
  Suite 600
  Montreal  QC  H4M 2P2
  Canada

  Phone: +1 (514) 747-2547
  Fax:   +1 (514) 747-2561
  EMail: [email protected]







































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Full Copyright Statement

  Copyright (C) The Internet Society (1998).  All Rights Reserved.

  This document and translations of it may be copied and furnished to
  others, and derivative works that comment on or otherwise explain it
  or assist in its implementation may be prepared, copied, published
  and distributed, in whole or in part, without restriction of any
  kind, provided that the above copyright notice and this paragraph are
  included on all such copies and derivative works.  However, this
  document itself may not be modified in any way, such as by removing
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  The limited permissions granted above are perpetual and will not be
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  This document and the information contained herein is provided on an
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  TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
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