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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 1 ��� 3 Network Working Group F. Yergeau 4 Internet-Draft Alis Technologies 5 Expires: October 11, 2002 April 12, 2002 7 UTF-8, a transformation format of ISO 10646 8 draft-yergeau-rfc2279bis-00 10 Status of this Memo 12 This document is an Internet-Draft and is in full conformance with 13 all provisions of Section 10 of RFC2026. 15 Internet-Drafts are working documents of the Internet Engineering 16 Task Force (IETF), its areas, and its working groups. Note that 17 other groups may also distribute working documents as Internet- 18 Drafts. 20 Internet-Drafts are draft documents valid for a maximum of six months 21 and may be updated, replaced, or obsoleted by other documents at any 22 time. It is inappropriate to use Internet-Drafts as reference 23 material or to cite them other than as "work in progress." 25 The list of current Internet-Drafts can be accessed at http:// 26 www.ietf.org/ietf/1id-abstracts.txt. 28 The list of Internet-Draft Shadow Directories can be accessed at 29 http://www.ietf.org/shadow.html. 31 This Internet-Draft will expire on October 11, 2002. 33 Copyright Notice 35 Copyright (C) The Internet Society (2002). All Rights Reserved. 37 Abstract 39 <1> 40 ISO/IEC 10646-1 defines a multi-octet character set called the 41 Universal Character Set (UCS) which encompasses most of the world's 42 writing systems. Multi-octet characters, however, are not compatible 43 with many current applications and protocols, and this has led to the 44 development of UTF-8, the object of this memo. UTF-8 has the 45 characteristic of preserving the full US-ASCII range, providing 46 compatibility with file systems, parsers and other software that rely 47 on US-ASCII values but are transparent to other values. This memo 48 updates and replaces RFC 2279. 49 <2> 50 Discussion of this draft should take place on the ietf- 51 charsets@iana.org mailing list. 53 Table of Contents 55 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 56 2. Notational conventions . . . . . . . . . . . . . . . . . . . . 5 57 3. UTF-8 definition . . . . . . . . . . . . . . . . . . . . . . . 6 58 4. Versions of the standards . . . . . . . . . . . . . . . . . . 8 59 5. Byte order mark (BOM) . . . . . . . . . . . . . . . . . . . . 9 60 6. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 61 7. MIME registration . . . . . . . . . . . . . . . . . . . . . . 11 62 8. Security Considerations . . . . . . . . . . . . . . . . . . . 12 63 Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . 13 64 Author's Address . . . . . . . . . . . . . . . . . . . . . . . 13 65 A. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 14 66 B. Changes from RFC 2279 . . . . . . . . . . . . . . . . . . . . 15 67 Full Copyright Statement . . . . . . . . . . . . . . . . . . . 16 69 1. Introduction 70 <3> 71 ISO/IEC 10646 [ISO.10646-1] defines a multi-octet character set 72 called the Universal Character Set (UCS), which encompasses most of 73 the world's writing systems. The same set of characters is defined 74 by the Unicode standard [UNICODE], which further defines additional 75 character properties and other application details of great interest 76 to implementors. Up to the present time, changes in Unicode and 77 amendments and additions to ISO/IEC 10646 have tracked each other, so 78 that the character repertoires and code point assignments have 79 remained in sync. The relevant standardization committees have 80 committed to maintain this very useful synchronism. 81 <4> 82 ISO/IEC 10646 and Unicode define several encoding forms of their 83 common repertoire: UTF-8, UCS-2, UTF-16, UCS-4 and UTF-32. In an 84 encoding form, each character is represented as one or more encoding 85 units. All standard UCS encoding forms except UTF-8 have an encoding 86 unit larger than one octet, making them hard to use in many current 87 applications and protocols that assume 8 or even 7 bit characters. 88 <5> 89 UTF-8, the object of this memo, has a one-octet encoding unit. It 90 uses all bits of an octet, but has the quality of preserving the full 91 US-ASCII [US-ASCII] range: US-ASCII characters are encoded in one 92 octet having the normal US-ASCII value, and any octet with such a 93 value can only stand for an US-ASCII character, and nothing else. 94 <6> 95 UTF-8 encodes UCS characters as a varying number of octets, where the 96 number of octets, and the value of each, depend on the integer value 97 assigned to the character in ISO/IEC 10646 (the character number, 98 a.k.a. code point or Unicode scalar value). This encoding form has 99 the following characteristics (all values are in hexadecimal): 100 <7> 101 o Character numbers from U+0000 to U+007F (US-ASCII repertoire) 102 correspond to octets 00 to 7F (7 bit US-ASCII values). A direct 103 consequence is that a plain ASCII string is also a valid UTF-8 104 string. 105 <8> 106 o US-ASCII octet values do not appear otherwise in a UTF-8 encoded 107 character stream. This provides compatibility with file systems 108 or other software (e.g. the printf() function in C libraries) 109 that parse based on US-ASCII values but are transparent to other 110 values. 111 <9> 112 o Round-trip conversion is easy between UTF-8 and other encoding 113 forms. 114 <10> 115 o The first octet of a multi-octet sequence indicates the number of 116 octets in the sequence. 118 <11> 119 o The octet values FE and FF never appear. 120 <12> 121 o Character boundaries are easily found from anywhere in an octet 122 stream. 123 <13> 124 o The lexicographic sorting order of strings is preserved. Of 125 course this is of limited interest since a sort order based on 126 character numbers is not culturally valid. 127 <14> 128 o The Boyer-Moore fast search algorithm can be used with UTF-8 data. 129 <15> 130 o UTF-8 strings can be fairly reliably recognized as such by a 131 simple algorithm, i.e. the probability that a string of 132 characters in any other encoding appears as valid UTF-8 is low, 133 diminishing with increasing string length. 135 <16> 136 UTF-8 was originally a project of the X/Open Joint 137 Internationalization Group XOJIG with the objective to specify a File 138 System Safe UCS Transformation Format [FSS_UTF] that is compatible 139 with UNIX systems, supporting multilingual text in a single encoding. 140 The original authors were Gary Miller, Greger Leijonhufvud and John 141 Entenmann. Later, Ken Thompson and Rob Pike did significant work for 142 the formal UTF-8. 144 2. Notational conventions 145 <17> 146 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 147 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 148 document are to be interpreted as described in [RFC2119]. 149 <18> 150 UCS characters are designated by the U+HHHH notation, where HHHH is a 151 string of from 4 to 6 hexadecimal digits representing the character 152 number in ISO/IEC 10646. 154 3. UTF-8 definition 155 <19> 156 UTF-8 is defined by Annex D of ISO/IEC 10646-1 [ISO.10646-1]. 157 Descriptions and formulae can also be found in the Unicode Standard 158 [UNICODE] and in [FSS_UTF]. 159 <20> 160 In UTF-8, characters are encoded using sequences of 1 to 6 octets. 161 If the repertoire is restricted to the range U+0000 to U+10FFFF (the 162 Unicode repertoire), then only sequences of one to four octets will 163 occur. The only octet of a "sequence" of one has the higher-order 164 bit set to 0, the remaining 7 bits being used to encode the character 165 number. In a sequence of n octets, n>1, the initial octet has the n 166 higher-order bits set to 1, followed by a bit set to 0. The 167 remaining bit(s) of that octet contain bits from the number of the 168 character to be encoded. The following octet(s) all have the higher- 169 order bit set to 1 and the following bit set to 0, leaving 6 bits in 170 each to contain bits from the character to be encoded. 171 <21> 172 The table below summarizes the format of these different octet types. 173 The letter x indicates bits available for encoding bits of the 174 character number. 176 Char. number range | UTF-8 octet sequence 177 (hexadecimal) | (binary) 178 --------------------+--------------------------------------------- 179 0000 0000-0000 007F | 0xxxxxxx 180 0000 0080-0000 07FF | 110xxxxx 10xxxxxx 181 0000 0800-0000 FFFF | 1110xxxx 10xxxxxx 10xxxxxx 182 0001 0000-001F FFFF | 11110xxx 10xxxxxx 10xxxxxx 10xxxxxx 183 0020 0000-03FF FFFF | 111110xx 10xxxxxx 10xxxxxx 10xxxxxx 10xxxxxx 184 0400 0000-7FFF FFFF | 1111110x 10xxxxxx ... 10xxxxxx 185 <22> 186 Encoding a character to UTF-8 proceeds as follows: 187 <23> 188 1. Determine the number of octets required from the character number 189 and the first column of the table above. It is important to note 190 that the rows of the table are mutually exclusive, i.e. there is 191 only one valid way to encode a given character. 192 <24> 193 2. Prepare the high-order bits of the octets as per the second 194 column of the table. 195 <25> 196 3. Fill in the bits marked x from the bits of the character number, 197 expressed in binary. Start from the lower-order bits of the 198 character number and put them first in the last octet of the 199 sequence, then the next to last, etc. until all x bits are 200 filled in. 202 <26> 203 The definition of UTF-8 prohibits encoding character numbers between 204 U+D800 and U+DFFF, which are reserved for use with the UTF-16 205 encoding form (as surrogate pairs) and do not directly represent 206 characters. When encoding in UTF-8 from UTF-16 data, it is necessary 207 to first decode the UTF-16 data to obtain character numbers, which 208 are then encoded in UTF-8 as described above. 209 <27> 210 Decoding a UTF-8 character proceeds as follows: 211 <28> 212 1. Initialize a binary number with all bits set to 0. Up to 31 bits 213 may be needed (up to 21 if the repertoire is known to be 214 restricted to the Unicode repertoire). 215 <29> 216 2. Determine which bits encode the character number from the number 217 of octets in the sequence and the second column of the table 218 above (the bits marked x). 219 <30> 220 3. Distribute the bits from the sequence to the binary number, first 221 the lower-order bits from the last octet of the sequence and 222 proceeding to the left until no x bits are left. The binary 223 number is now equal to the character number. 225 <31> 226 Implementations of the decoding algorithm above MUST protect against 227 decoding invalid sequences. For instance, a naive implementation may 228 decode the overlong UTF-8 sequence C0 80 into the character U+0000, 229 or the surrogate pair ED A1 8C ED BE B4 into U+233B4. Decoding 230 invalid sequences may have security consequences or cause other 231 problems. See Security Considerations (Section 8) below. 233 4. Versions of the standards 234 <32> 235 ISO/IEC 10646 is updated from time to time by publication of 236 amendments and additional parts; similarly, different versions of the 237 Unicode standard are published over time. Each new version obsoletes 238 and replaces the previous one, but implementations, and more 239 significantly data, are not updated instantly. 240 <33> 241 In general, the changes amount to adding new characters, which does 242 not pose particular problems with old data. Amendment 5 to ISO/IEC 243 10646, however, has moved and expanded the Korean Hangul block, 244 thereby making any previous data containing Hangul characters invalid 245 under the new version. Unicode 2.0 has the same difference from 246 Unicode 1.1. The official justification for allowing such an 247 incompatible change was that no implementations and no data 248 containing Hangul existed, a statement that is likely to be true but 249 remains unprovable. The incident has been dubbed the "Korean mess", 250 and the relevant committees have pledged to never, ever again make 251 such an incompatible change (see Unicode Consortium Policies [1]). 252 <34> 253 New versions, and in particular any incompatible changes, have 254 consequences regarding MIME character encoding labels, to be 255 discussed in section 5. 257 5. Byte order mark (BOM) 258 <35> 259 The Unicode Standard and ISO 10646 define the character "ZERO WIDTH 260 NO-BREAK SPACE" (U+FEFF), which is also known informally as "BYTE 261 ORDER MARK" (abbreviated "BOM"). The latter name hints at a second 262 possible usage of the character, in addition to its normal use as a 263 genuine "ZERO WIDTH NO-BREAK SPACE" within text. This usage, 264 suggested by Unicode section 2.7 and ISO/IEC 10646 Annex H 265 (informative), is to prepend a U+FEFF character to a stream of 266 Unicode characters as a "signature"; a receiver of such a serialized 267 stream may then use the initial character both as a hint that the 268 stream consists of Unicode characters, as a way to recognize which 269 UCS encoding is involved and, with encodings having a multi-octet 270 encoding unit, as a way to recognize the serialization order of the 271 octets. UTF-8 having a single-octet encoding unit, this last 272 function is useless and the BOM will always appear as the octet 273 sequence EF BB BF. 274 <36> 275 It is important to understand that the character U+FEFF appearing at 276 any position other than the beginning of a stream MUST be interpreted 277 with the semantics for the zero-width non-breaking space, and MUST 278 NOT be interpreted as a byte-order mark. The contrapositive of that 279 statement is not always true: the character U+FEFF in the first 280 position of a stream MAY be interpreted as a zero-width non-breaking 281 space, and is not always a byte-order mark. For example, if a 282 process splits a UCS string into many parts, a part might begin with 283 U+FEFF because there was a zero-width non-breaking space at the 284 beginning of that substring. 285 <37> 286 The Unicode standard further suggests than an initial U+FEFF 287 character may be stripped before processing the text, the rationale 288 being that such a character in initial position may be an artifact of 289 the encoding (an encoding signature), not a genuine intended "ZERO 290 WIDTH NO-BREAK SPACE". Note that such stripping might affect an 291 external process at a different layer (such as a digital signature or 292 a count of the characters) that is relying on the presence of all 293 characters in the stream. 294 <38> 295 In particular, in UTF-8 plain text it is likely, but not certain, 296 that an initial octet sequence of EF BB BF is a signature. When 297 concatenating two strings, it is important to strip out those 298 signatures, because otherwise the resulting string may contain an 299 unintended "ZERO WIDTH NO-BREAK SPACE" at the connection point. 301 6. Examples 302 <39> 303 The character sequence "A." (U+0041, U+2262, 304 U+0391, U+002E) is encoded in UTF-8 as follows: 306 --+--------+-----+-- 307 41 E2 89 A2 CE 91 2E 308 --+--------+-----+-- 309 <40> 310 The character sequence representing the Hangul characters for the 311 Korean word "hangugo" (U+D55C, U+AD6D, U+C5B4) is encoded in UTF-8 as 312 follows: 314 --------+--------+-------- 315 ED 95 9C EA B5 AD EC 96 B4 316 --------+--------+-------- 317 <41> 318 The character sequence representing the Han characters for the 319 Japanese word "nihongo" (U+65E5, U+672C, U+8A9E) is encoded in UTF-8 320 as follows: 322 --------+--------+-------- 323 E6 97 A5 E6 9C AC E8 AA 9E 324 --------+--------+-------- 325 <42> 326 The character U+233B4 (a Chinese character meaning 'stump of tree'), 327 prepended with a UTF-8 BOM, is encoded in UTF-8 as follows: 329 --------+----------- 330 EF BB BF F0 A3 8E B4 331 --------+----------- 333 7. MIME registration 334 <43> 335 This memo is meant to serve as the basis for registration of a MIME 336 character set parameter (charset) [RFC2978]. The proposed charset 337 parameter value is "UTF-8". This string labels media types 338 containing text consisting of characters from the repertoire of ISO/ 339 IEC 10646 including all amendments at least up to amendment 5 (Korean 340 block), encoded to a sequence of octets using the encoding scheme 341 outlined above. UTF-8 is suitable for use in MIME content types 342 under the "text" top-level type. 343 <44> 344 It is noteworthy that the label "UTF-8" does not contain a version 345 identification, referring generically to ISO/IEC 10646. This is 346 intentional, the rationale being as follows: 347 <45> 348 A MIME charset label is designed to give just the information needed 349 to interpret a sequence of bytes received on the wire into a sequence 350 of characters, nothing more (see [RFC2045], section 2.2). As long as 351 a character set standard does not change incompatibly, version 352 numbers serve no purpose, because one gains nothing by learning from 353 the tag that newly assigned characters may be received that one 354 doesn't know about. The tag itself doesn't teach anything about the 355 new characters, which are going to be received anyway. 356 <46> 357 Hence, as long as the standards evolve compatibly, the apparent 358 advantage of having labels that identify the versions is only that, 359 apparent. But there is a disadvantage to such version-dependent 360 labels: when an older application receives data accompanied by a 361 newer, unknown label, it may fail to recognize the label and be 362 completely unable to deal with the data, whereas a generic, known 363 label would have triggered mostly correct processing of the data, 364 which may well not contain any new characters. 365 <47> 366 Now the "Korean mess" (ISO/IEC 10646 amendment 5) is an incompatible 367 change, in principle contradicting the appropriateness of a version 368 independent MIME charset label as described above. But the 369 compatibility problem can only appear with data containing Korean 370 Hangul characters encoded according to Unicode 1.1 (or equivalently 371 ISO/IEC 10646 before amendment 5), and there is arguably no such data 372 to worry about, this being the very reason the incompatible change 373 was deemed acceptable. 374 <48> 375 In practice, then, a version-independent label is warranted, provided 376 the label is understood to refer to all versions after Amendment 5, 377 and provided no incompatible change actually occurs. Should 378 incompatible changes occur in a later version of ISO/IEC 10646, the 379 MIME charset label defined here will stay aligned with the previous 380 version until and unless the IETF specifically decides otherwise. 382 8. Security Considerations 383 <49> 384 Implementors of UTF-8 need to consider the security aspects of how 385 they handle illegal UTF-8 sequences. It is conceivable that in some 386 circumstances an attacker would be able to exploit an incautious UTF- 387 8 parser by sending it an octet sequence that is not permitted by the 388 UTF-8 syntax. 389 <50> 390 A particularly subtle form of this attack could be carried out 391 against a parser which performs security-critical validity checks 392 against the UTF-8 encoded form of its input, but interprets certain 393 illegal octet sequences as characters. For example, a parser might 394 prohibit the NUL character when encoded as the single-octet sequence 395 00, but allow the illegal two-octet sequence C0 80 and interpret it 396 as a NUL character. Another example might be a parser which 397 prohibits the octet sequence 2F 2E 2E 2F ("/../"), yet permits the 398 illegal octet sequence 2F C0 AE 2E 2F. This last exploit has 399 actually been used in a widespread virus attacking Web servers in 400 2001; the security threat is thus very real. 402 Bibliography 404 [FSS_UTF] X/Open Company Ltd., "X/Open CAE Specification C501 -- 405 File System Safe UCS Transformation Format (FSS_UTF)", 406 ISBN 1-85912-082-2, April 1995. 408 [ISO.10646-1] International Organization for Standardization, 409 "Information Technology - Universal Multiple-octet 410 coded Character Set (UCS) - Part 1: Architecture and 411 Basic Multilingual Plane", ISO Standard 10646-1, 2000. 413 [RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet 414 Mail Extensions (MIME) Part One: Format of Internet 415 Message Bodies", RFC 2045, November 1996. 417 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 418 Requirement Levels", BCP 14, RFC 2119, March 1997. 420 [RFC2978] Freed, N. and J. Postel, "IANA Charset Registration 421 Procedures", BCP 19, RFC 2978, October 2000. 423 [UNICODE] The Unicode Consortium, "The Unicode Standard -- 424 Version 3.0", ISBN 0-201-61633-5, 2000, . 428 [US-ASCII] American National Standards Institute, "Coded 429 Character Set - 7-bit American Standard Code for 430 Information Interchange", ANSI X3.4, 1986. 432 [1] 434 Author's Address 436 FranȺois Yergeau 437 Alis Technologies 438 100, boul. Alexis-Nihon, bureau 600 439 MontrȨal, QC H4M 2P2 440 Canada 442 Phone: +1 514 747 2547 443 Fax: +1 514 747 2561 444 EMail: fyergeau@alis.com 446 Appendix A. Acknowledgements 447 <59> 448 The following have participated in the drafting and discussion of 449 this memo: James E. Agenbroad, Andries Brouwer, Martin J. Dȭrst, 450 Ned Freed, David Goldsmith, Edwin F. Hart, Kent Karlsson, Markus 451 Kuhn, Michael Kung, Alain LaBontȨ, John Gardiner Myers, Murray 452 Sargent, Keld Simonsen and Arnold Winkler. 454 Appendix B. Changes from RFC 2279 455 <60> 456 <61> 457 o Significantly shortened Introduction. No more mention of UTF-1 or 458 UTF-7, of Transformation Formats. 459 <62> 460 o Straightened out terminology. UTF-8 now described in terms of an 461 encoding form of the character number. UCS-2 and UCS-4 almost 462 disappeared. 463 <63> 464 o Note warning against decoding of invalid sequences turned into a 465 normative MUST NOT. 466 <64> 467 o New section about the BOM, mostly extracted and slightly adapted 468 from RFC 2781. 469 <65> 470 o Updated a couple of references (10646-1:2000, Unicode 3, RFC 471 2978). 472 <66> 473 o Added TOC. 474 <67> 475 o Removed suggested UNICODE-1-1-UTF-8 MIME charset registration. 476 <68> 477 o New "Notational conventions" section about RFC 2119 and U+HHHH 478 notation. 479 <69> 480 o Pointer to Unicode Consortium Policies added in "Versions of the 481 standards" section. 482 <70> 483 o Added a fourth example with a non-BMP character and a BOM. 485 Full Copyright Statement 487 Copyright (C) The Internet Society (2002). All Rights Reserved. 489 This document and translations of it may be copied and furnished to 490 others, and derivative works that comment on or otherwise explain it 491 or assist in its implementation may be prepared, copied, published 492 and distributed, in whole or in part, without restriction of any 493 kind, provided that the above copyright notice and this paragraph are 494 included on all such copies and derivative works. However, this 495 document itself may not be modified in any way, such as by removing 496 the copyright notice or references to the Internet Society or other 497 Internet organizations, except as needed for the purpose of 498 developing Internet standards in which case the procedures for 499 copyrights defined in the Internet Standards process must be 500 followed, or as required to translate it into languages other than 501 English. 503 The limited permissions granted above are perpetual and will not be 504 revoked by the Internet Society or its successors or assigns. 506 This document and the information contained herein is provided on an 507 "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING 508 TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING 509 BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION 510 HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF 511 MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. 513 Acknowledgement 515 Funding for the RFC Editor function is currently provided by the 516 Internet Society.