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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.


 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

Yergeau Standards Track [Page 1] RFC 2279 UTF-8 January 1998

 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
 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):
  1. 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
  1. 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
  1. Round-trip conversion is easy between UTF-8 and either of UCS-4,


Yergeau Standards Track [Page 2] RFC 2279 UTF-8 January 1998

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

octets in the sequence.

  1. The octet values FE and FF never appear.
  1. Character boundaries are easily found from anywhere in an octet


  1. 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.
  1. The Boyer-Moore fast search algorithm can be used with UTF-8 data.
  1. 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.

Yergeau Standards Track [Page 3] RFC 2279 UTF-8 January 1998

 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
    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.

Yergeau Standards Track [Page 4] RFC 2279 UTF-8 January 1998

      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

Yergeau Standards Track [Page 5] 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.

Yergeau Standards Track [Page 6] RFC 2279 UTF-8 January 1998

 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.


 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

Yergeau Standards Track [Page 7] RFC 2279 UTF-8 January 1998


 [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.

Yergeau Standards Track [Page 8] RFC 2279 UTF-8 January 1998

Author's Address

 Francois Yergeau
 Alis Technologies
 100, boul. Alexis-Nihon
 Suite 600
 Montreal  QC  H4M 2P2
 Phone: +1 (514) 747-2547
 Fax:   +1 (514) 747-2561

Yergeau Standards Track [Page 9] RFC 2279 UTF-8 January 1998

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
 the copyright notice or references to the Internet Society or other
 Internet organizations, except as needed for the purpose of
 developing Internet standards in which case the procedures for
 copyrights defined in the Internet Standards process must be
 followed, or as required to translate it into languages other than
 The limited permissions granted above are perpetual and will not be
 revoked by the Internet Society or its successors or assigns.
 This document and the information contained herein is provided on an

Yergeau Standards Track [Page 10]

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