RFC2279 - UTF-8, a transformation format of ISO 10646

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.

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 RFC2044, 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

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.

- 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.

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.

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

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 RFC2045, 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.

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. Drst Ned Freed

David Goldsmith Edwin F. Hart

Kent Karlsson Markus Kuhn

Michael Kung Alain LaBonte

John Gardiner Myers Murray Sargent

Keld Simonsen Arnold Winkler

Bibliography

[CHARSET-REG] Freed, N., and J. Postel, "IANA Charset Registration

Procedures", BCP 19, RFC2278, 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", RFC2045. N. Freed, N. Borenstein,

"Multipurpose Internet Mail Extensions (MIME) Part

Two: Media Types", RFC2046. 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", RFC2048. N. Freed, N.

Borenstein, " Multipurpose Internet Mail Extensions

(MIME) Part Five: Conformance Criteria and Examples",

RFC2049. All November 1996.

[RFC2152] Goldsmith, D., and M. Davis, "UTF-7: A Mail-safe

Transformation Format of Unicode", RFC1642, 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.

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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