RFC2279: UTF-8, a transformation format of ISO 10646
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Obsoletes:
RFC2044
Obsoleted By:
RFC3629
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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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
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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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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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: fyergeau@alis.com
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RFC 2279 UTF-8 January 1998
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