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Date: Tue, 09 Apr 2002 12:16:05 GMT Server: Apache/1.3.20 (Unix) Last-Modified: Thu, 17 Apr 1997 09:45:00 GMT ETag: "3ddc70-44af-3355f11c" Accept-Ranges: bytes Content-Length: 17583 Connection: close Content-Type: text/plain Network Working Group F. YergeauInternet DraftRequest for Comments: 2279 Alis Technologies<draft-yergeau-utf8-rev-00.txt> 14 April 1997 Expires 14 October 1997 [Will obsolete RFC 2044]Obsoletes: 2044 January 1998 Category: Standards Track UTF-8, a transformation format ofUnicode andISO 10646 Status of this Memo This documentisspecifies anInternet-Draft. Internet-Drafts are working doc- uments ofInternet standards track protocol for the InternetEngineering Task Force (IETF), its areas,community, andits working groups. Note that other groups may also distribute work- ing documents as Internet-Drafts. Internet-Drafts are draft documents validrequests discussion and suggestions fora maximum of six months. Internet-Drafts may be updated, replaced, or obsoleted by other documents at any time. It is not appropriate to use Internet- Drafts as reference material orimprovements. Please refer tocite them other than as a "working draft" or "work in progress". To learnthe currentstatusedition ofany Internet-Draft, please checkthe1id-abstracts.txt listing contained in"Internet Official Protocol Standards" (STD 1) for theInternet-Drafts Shadow Directories on ds.internic.net (US East Coast), nic.nordu.net (Europe), ftp.isi.edu (US West Coast), or munnari.oz.au (Pacific Rim).standardization state and status of this protocol. Distribution of thisdocumentmemo is unlimited. Copyright Notice Copyright (C) The Internet Society (1998). All Rights Reserved. Abstract ISO/IEC 10646-1and the Unicode Standard jointly definedefines 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 thecharacter- isticcharacteristic 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.Expires 14 October 1997 [Page 1] Internet Draft UTF-8 14 April 19971. Introduction ISO/IEC 10646-1 [ISO-10646]and the Unicode Standard [UNICODE] jointly definedefines a16-bitmulti-octet characterset, UCS-2,set called the Universal Character Set (UCS), which encompasses most of the world's writing systems.ISO 10646 further definesTwo multi-octet encodings are defined, a31-bitfour-octet per characterset, UCS-4, with currently no assignments outsideencoding called UCS-4 and a two-octet per character encoding called UCS-2, able to address only the first 64K characters of theregion corresponding to UCS-2UCS (the Basic Multilingual Plane,BMP).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 manycur- rentcurrent applications and protocols that assume 8 or even 7 bitcharac- ters.characters. Even newer systems able to deal with 16 bit characters cannot process UCS-4 data. This situation has led to the development ofso- calledso-called UCS transformation formats (UTF), each with differentcharac- teristics.characteristics. UTF-1 has only historical interest, having been removed fromISOISO/IEC 10646. UTF-7 has the quality of encoding the fullUnicodeBMP repertoire using only octets with the high-order bit clear (7 bit US-ASCIIval- ues,values, [US-ASCII]), and is thus deemed a mail-safe encoding([RFC1642]).([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- ASCIIUS-ASCII characters are encoded in one octet having the normalUS-ASCIIUS- 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 inISOISO/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 encodedcharac- tercharacter 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 otherval- ues.values. - Round-trip conversion is easy between UTF-8 and either of UCS-4,UCS-2 or Unicode. Expires 14 October 1997UCS-2. Yergeau Standards Track [Page 2]Internet DraftRFC 2279 UTF-814 April 1997January 1998 - 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 asim- plesimple 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 JointInternationaliza- tionInternationalization 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 definitiverefer- ence,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.Expires 14 October 1997Yergeau Standards Track [Page 3]Internet DraftRFC 2279 UTF-814 April 1997January 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-2val- uesvalues between D800 andDFFF,DFFF (surrogate pairs in Unicode parlance), being actually UCS-4 characterstrans- formedtransformed through UTF-16, need special treatment: the UTF-16trans- formationtransformation 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 toUCS-2 (or equivalently Unicode). AUCS-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],Expires 14 October 1997 [Page 4] Internet Draft UTF-8 14 April 1997[UNICODE] or Annex R to [ISO-10646]. 3. Versions of the standardsDifferentISO/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 theprevi- ousprevious 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 toISOISO/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 fromUni- codeUnicode 1.1. The official justification for allowing such anincompati- bleincompatible change was that no implementations and no data containing Hangul existed, a statement that is likely to be true but remainsunprov- able.unprovable. The incident has been dubbed the "Korean mess", and therele- vantrelevant committees have pledged to never, ever again make such anincom- patibleincompatible change. New versions, and in particular any incompatible changes, haveconse- quencesq 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 9EExpires 14 October 1997 [Page 5] Internet Draft UTF-8 14 April 19975. MIME registration This memo is meant to serve as the basis for registration of a MIME character set parameter (charset)[MIME].[CHARSET-REG]. The proposed charset parameter value is "UTF-8". This stringwould labellabels media typescon- tainingcontaining 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 schemeout- linedoutlined 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 bylearn- inglearning 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,and theywhich 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 becom- pletelycompletely 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(ISO/IEC 10646 amendment 5) is an incompatible change, in principle contradicting the appropriateness of aversion-version independent MIME charset label as described above. But thecompati- bilitycompatibility problem can only appear with data containing Korean Hangul characters encoded according to Unicode 1.1 (or equivalentlyISOISO/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 iswarranted.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, theneed ever ariseMIME charset label defined here will stay aligned with the previous version until and unless the IETF specifically decides otherwise. It is also proposed todistinguishregister the charset parameter value "UNICODE-1-1-UTF-8", for the exclusive purpose of labelling text data containing Hangul syllables encodedaccordingtoUnicode 1.1, then a version-dependentUTF-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,for that version only, should be registered (a suggestion would be "UNI- CODE-1-1-UTF-8"),inorder to retain the advantages of a version- independent label for 2.0particular data not containing any Hangul syllables, andlater versions. Such a version-depen- dent label could even be registered before actual need arises, pre- Expires 14 October 1997 [Page 6] Internet Draft UTF-8 14 April 1997 emptively, butit is felt important to strongly recommend against creating any new Hangul-containing data without taking Amendment 5 ofISOISO/IEC 10646 into account. 6. Security ConsiderationsSecurity issues are not discussedImplementors 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 thismemo.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.DürstD|rst Ned Freed David Goldsmith Edwin F. Hart Kent Karlsson Markus Kuhn Michael Kung AlainLaBontéLaBonte John Gardiner Myers Murray Sargent Keld Simonsen Arnold Winkler Yergeau Standards Track [Page 7] RFC 2279 UTF-8 January 1998 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., "FileSys- temSystem 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 --Infor- mationInformation 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]N.Freed, N., and N. Borenstein, "Multipurpose Internet Mail Extensions (MIME) Part One: Format of InternetMes- sageMessage Bodies", RFC 2045. N. Freed, N. Borenstein, "Multipurpose Internet Mail Extensions (MIME) Part Two: Media Types", RFC 2046. K. Moore, "MIME(Multi- purpose(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: RegistrationProce- dures",Procedures", RFC 2048. N. Freed, N. Borenstein,"Multipur- pose" Multipurpose Internet Mail Extensions (MIME) Part Five:Con- formanceConformance Criteria and Examples", RFC 2049. AllExpires 14 October 1997 [Page 7] Internet Draft UTF-8 14 April 1997November 1996.[RFC1641] D. Goldsmith, M.Davis, "Using Unicode with MIME", RFC 1641, Taligent inc., July 1994. [RFC1642] D.[RFC2152] Goldsmith, D., and M. Davis, "UTF-7: A Mail-safeTransfor- mationTransformation Format of Unicode", RFC 1642, Taligent inc.,July 1994.May 1997. (Obsoletes RFC1642) [UNICODE] The Unicode Consortium, "The Unicode Standard --Ver- sionVersion 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 AddressFrançoisFrancois Yergeau Alis Technologies 100, boul. Alexis-Nihon Suite 600MontréalMontreal QC H4M 2P2 CanadaTel:Phone: +1 (514) 747-2547 Fax: +1 (514) 747-2561 EMail: fyergeau@alis.comExpires 14 October 1997Yergeau Standards Track [Page8]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 English. 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 "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Yergeau Standards Track [Page 10] ----