Network Working Group F. Yergeau Request for Comments: 3629 Alis Technologies STD: 63 November 2003 Obsoletes: 2279 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 (2003). All Rights Reserved. Abstract ISO/IEC 10646-1 defines a large character set called the Universal Character Set (UCS) which encompasses most of the world's writing systems. The originally proposed encodings of the UCS, however, were not compatible with many current applications and protocols, and this has led to the development of UTF-8, the object of this memo. UTF-8 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 obsoletes and replaces RFC 2279. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2 2. Notational conventions . . . . . . . . . . . . . . . . . . . . 3 3. UTF-8 definition . . . . . . . . . . . . . . . . . . . . . . . 4 4. Syntax of UTF-8 Byte Sequences . . . . . . . . . . . . . . . . 5 5. Versions of the standards . . . . . . . . . . . . . . . . . . 6 6. Byte order mark (BOM) . . . . . . . . . . . . . . . . . . . . 6 7. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 8. MIME registration . . . . . . . . . . . . . . . . . . . . . . 9 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10 10. Security Considerations . . . . . . . . . . . . . . . . . . . 10 11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 11 12. Changes from RFC 2279 . . . . . . . . . . . . . . . . . . . . 11 13. Normative References . . . . . . . . . . . . . . . . . . . . . 12 Yergeau Standards Track [Page 1]
RFC 3629 UTF-8 November 2003 14. Informative References . . . . . . . . . . . . . . . . . . . . 12 15. URI's . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 16. Intellectual Property Statement . . . . . . . . . . . . . . . 13 17. Author's Address . . . . . . . . . . . . . . . . . . . . . . . 13 18. Full Copyright Statement . . . . . . . . . . . . . . . . . . . 14 1. Introduction ISO/IEC 10646 [ISO.10646] defines a large character set called the Universal Character Set (UCS), which encompasses most of the world's writing systems. 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 implementers. Up to the present time, changes in Unicode and amendments and additions 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. ISO/IEC 10646 and Unicode define several encoding forms of their common repertoire: UTF-8, UCS-2, UTF-16, UCS-4 and UTF-32. In an encoding form, each character is represented as one or more encoding units. All standard UCS encoding forms except UTF-8 have an encoding unit larger than one octet, making them hard to use in many current applications and protocols that assume 8 or even 7 bit characters. UTF-8, the object of this memo, has a one-octet encoding unit. It uses all bits of an octet, but has the quality of preserving the full US-ASCII [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 a US-ASCII character, and nothing else. UTF-8 encodes UCS 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 (the character number, a.k.a. code position, code point or Unicode scalar value). This encoding form has the following characteristics (all values are in hexadecimal): o Character numbers from U+0000 to U+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. Yergeau Standards Track [Page 2]
RFC 3629 UTF-8 November 2003 o US-ASCII octet 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. o Round-trip conversion is easy between UTF-8 and other encoding forms. o The first octet of a multi-octet sequence indicates the number of octets in the sequence. o The octet values C0, C1, F5 to FF never appear. o Character boundaries are easily found from anywhere in an octet stream. o The byte-value lexicographic sorting order of UTF-8 strings is the same as if ordered by character numbers. Of course this is of limited interest since a sort order based on character numbers is almost never culturally valid. o The Boyer-Moore fast search algorithm can be used with UTF-8 data. o 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 devised in September 1992 by Ken Thompson, guided by design criteria specified by Rob Pike, with the objective of defining a UCS transformation format usable in the Plan9 operating system in a non- disruptive manner. Thompson's design was stewarded through standardization by the X/Open Joint Internationalization Group XOJIG (see [FSS_UTF]), bearing the names FSS-UTF (variant FSS/UTF), UTF-2 and finally UTF-8 along the way. 2. Notational conventions The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119]. UCS characters are designated by the U+HHHH notation, where HHHH is a string of from 4 to 6 hexadecimal digits representing the character number in ISO/IEC 10646. Yergeau Standards Track [Page 3]
RFC 3629 UTF-8 November 2003 3. UTF-8 definition UTF-8 is defined by the Unicode Standard [UNICODE]. Descriptions and formulae can also be found in Annex D of ISO/IEC 10646-1 [ISO.10646] In UTF-8, characters from the U+0000..U+10FFFF range (the UTF-16 accessible range) are encoded using sequences of 1 to 4 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 number. 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 number 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 character number. Char. number range | UTF-8 octet sequence (hexadecimal) | (binary) --------------------+--------------------------------------------- 0000 0000-0000 007F | 0xxxxxxx 0000 0080-0000 07FF | 110xxxxx 10xxxxxx 0000 0800-0000 FFFF | 1110xxxx 10xxxxxx 10xxxxxx 0001 0000-0010 FFFF | 11110xxx 10xxxxxx 10xxxxxx 10xxxxxx Encoding a character to UTF-8 proceeds as follows: 1. Determine the number of octets required from the character number 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 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 number, expressed in binary. Start by putting the lowest-order bit of the character number in the lowest-order position of the last octet of the sequence, then put the next higher-order bit of the character number in the next higher-order position of that octet, etc. When the x bits of the last octet are filled in, move on to the next to last octet, then to the preceding one, etc. until all x bits are filled in. Yergeau Standards Track [Page 4]
RFC 3629 UTF-8 November 2003 The definition of UTF-8 prohibits encoding character numbers between U+D800 and U+DFFF, which are reserved for use with the UTF-16 encoding form (as surrogate pairs) and do not directly represent characters. When encoding in UTF-8 from UTF-16 data, it is necessary to first decode the UTF-16 data to obtain character numbers, which are then encoded in UTF-8 as described above. This contrasts with CESU-8 [CESU-8], which is a UTF-8-like encoding that is not meant for use on the Internet. CESU-8 operates similarly to UTF-8 but encodes the UTF-16 code values (16-bit quantities) instead of the character number (code point). This leads to different results for character numbers above 0xFFFF; the CESU-8 encoding of those characters is NOT valid UTF-8. Decoding a UTF-8 character proceeds as follows: 1. Initialize a binary number with all bits set to 0. Up to 21 bits may be needed. 2. Determine which bits encode the character number 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 binary number, first the lower-order bits from the last octet of the sequence and proceeding to the left until no x bits are left. The binary number is now equal to the character number. Implementations of the decoding algorithm above MUST protect against decoding invalid sequences. For instance, a naive implementation may decode the overlong UTF-8 sequence C0 80 into the character U+0000, or the surrogate pair ED A1 8C ED BE B4 into U+233B4. Decoding invalid sequences may have security consequences or cause other problems. See Security Considerations (Section 10) below. 4. Syntax of UTF-8 Byte Sequences For the convenience of implementors using ABNF, a definition of UTF-8 in ABNF syntax is given here. A UTF-8 string is a sequence of octets representing a sequence of UCS characters. An octet sequence is valid UTF-8 only if it matches the following syntax, which is derived from the rules for encoding UTF-8 and is expressed in the ABNF of [