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UTF-8 revision
Here is the updated draft on UTF-8. The major change is in 6. Security
Considerations (thanks to John Gardiner Myers for the wise suggestion) with
a referencing note in 2. UTF-8 Definition. I'll send an I-D, modified if
necessary, after coming back from a trip next week.
Network Working Group F. Yergeau
Internet Draft Alis Technologies
<draft-yergeau-utf8-rev-01.txt> 30 August 1997
Expires 28 February 1998
[Will obsolete RFC 2044]
UTF-8, a transformation format of Unicode and ISO 10646
Status of this Memo
This document is an Internet-Draft. Internet-Drafts are working doc-
uments of the Internet Engineering Task Force (IETF), its areas, and
its working groups. Note that other groups may also distribute work-
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Distribution of this document is unlimited.
Abstract
ISO/IEC 10646-1 and the Unicode Standard jointly define a multi-octet
character set 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 character-
istic 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.
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1. Introduction
ISO/IEC 10646-1 [ISO-10646] and the Unicode Standard [UNICODE]
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 cur-
rent applications and protocols that assume 8 or even 7 bit charac-
ters. 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 charac-
teristics.
UTF-1 has only historical interest, having been removed from ISO
10646. UTF-7 has the quality of encoding the full Unicode repertoire
using only octets with the high-order bit clear (7 bit US-ASCII val-
ues, [US-ASCII]), and is thus deemed a mail-safe encoding
([RFC1642]). 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 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 charac-
ter 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 val-
ues.
- Round-trip conversion is easy between UTF-8 and either of UCS-4,
UCS-2 or Unicode.
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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 sim-
ple 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 Internationaliza-
tion 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 refer-
ence, 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, UCS-2 val-
ues between D800 and DFFF, being actually UCS-4 characters trans-
formed through UTF-16, need special treatment: the UTF-16 trans-
formation 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 (or equivalently Unicode).
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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
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 previ-
ous one, but implementations, and more significantly data, are not
updated instantly. Similarly, ISO 10646 is updated from time to time
by published amendments, which up to now have tracked the changes in
the Unicode standard, so that the two have remained in sync.
In general, the changes amount to adding new characters, which does
not pose particular problems with old data. Amendment 5 to ISO
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 Uni-
code 1.1. The official justification for allowing such an incompati-
ble change was that no implementations and no data containing Hangul
existed, a statement that is likely to be true but remains unprov-
able. The incident has been dubbed the "Korean mess", and the rele-
vant committees have pledged to never, ever again make such an incom-
patible change.
New versions, and in particular any incompatible changes, have conse-
quences 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 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
The UCS-2 sequence representing the Han characters for the Japanese
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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) [MIME]. The proposed charset
parameter value is "UTF-8". This string would label media types con-
taining text consisting of characters from the repertoire of ISO/IEC
10646 encoded to a sequence of octets using the encoding scheme out-
lined 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 learn-
ing from the tag that newly assigned characters may be received that
one doesn't know about. The tag doesn't teach anything about the new
characters, and they 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 com-
pletely 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 10646 amendment 5) is an incompatible
change, in principle contradicting the appropriateness of a version-
independent MIME charset label as described above. But the compati-
bility problem can only appear with data containing Korean Hangul
characters encoded according to Unicode 1.1 (or equivalently ISO
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.
In practice, then, a version-independent label is warranted. Should
the need ever arise to distinguish data containing Hangul encoded
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according to Unicode 1.1, then a version-dependent label, for that
version only, should be registered (a suggestion would be "UNI-
CODE-1-1-UTF-8"), in order to retain the advantages of a version-
independent label for 2.0 and later versions. Such a version-depen-
dent label could even be registered before actual need arises, pre-
emptively, but it is important to strongly recommend against creating
any new Hangul-containing data without taking Amendment 5 of ISO
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 pro-
hibits the octet sequence 2F 2E 2E 2F ("/../"), yet permits the ille-
gal 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. Dürst David Goldsmith
Edwin F. Hart Kent Karlsson
Markus Kuhn Michael Kung
Alain LaBonté John Gardiner Myers
Murray Sargent Keld Simonsen
Arnold Winkler
Bibliography
[FSS_UTF] X/Open CAE Specification C501 ISBN 1-85912-082-2 28cm.
22p. pbk. 172g. 4/95, X/Open Company Ltd., "File Sys-
tem Safe UCS Transformation Format (FSS_UTF)", X/Open
Preleminary Specification, Document Number P316. Also
published in Unicode Technical Report #4.
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Internet Draft UTF-8 30 August 1998
[ISO-10646] ISO/IEC 10646-1:1993. International Standard -- Infor-
mation technology -- Universal Multiple-Octet Coded
Character Set (UCS) -- Part 1: Architecture and Basic
Multilingual Plane. UTF-8 is described in Annex R,
published as Amendment 2. UTF-16 is described in
Annex Q, published as Amendment 1.
[MIME] N. Freed, N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Mes-
sage Bodies", RFC 2045. N. Freed, N. Borenstein,
"Multipurpose Internet Mail Extensions (MIME) Part
Two: Media Types", RFC 2046. K. Moore, "MIME (Multi-
purpose 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 Proce-
dures", RFC 2048. N. Freed, N. Borenstein, "Multipur-
pose Internet Mail Extensions (MIME) Part Five: Con-
formance Criteria and Examples", RFC 2049. All Novem-
ber 1996.
[RFC1641] D. Goldsmith, M.Davis, "Using Unicode with MIME", RFC
1641, Taligent inc., July 1994.
[RFC1642] D. Goldsmith, M. Davis, "UTF-7: A Mail-safe Transfor-
mation Format of Unicode", RFC 1642, Taligent inc.,
July 1994.
[UNICODE] The Unicode Consortium, "The Unicode Standard -- Ver-
sion 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
François Yergeau
Alis Technologies
100, boul. Alexis-Nihon
Suite 600
Montréal QC H4M 2P2
Canada
Tel: +1 (514) 747-2547
Fax: +1 (514) 747-2561
EMail: fyergeau@alis.com
--
François Yergeau <yergeau@alis.com>
Alis Technologies inc., Montréal
Tél : +1 (514) 747-2547
Fax : +1 (514) 747-2561