| Internet-Draft | DUTF | February 2023 |
| Yang | Expires 23 August 2023 | [Page] |
The Unicode Standard and ISO/IEC 10646 jointly define a coded character set, referred to as Unicode, which encompasses most of the world's writing systems. Characters of the same language are arranged close to each other in the Unicode code table. This memo proposes a dynamic Unicode transformation format(DUTF). DUTF has the characteristic of preserving the full US-ASCII range, and uses XOR to calculate the offset value between the Unicode code point of adjacent non-ASCII characters in the source string, then encodes the result as a variable-length sequence of octets.¶
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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 encoded individually and context-free. In most cases, a string will only contain one or two languages. Characters that belong to the same language are close to each other in the Unicode code table. Therefore, the character encoding can be effectively compressed by exploiting the correlation between adjacent characters.¶
DUTF, the object of this memo, has the capability to preserve the full US-ASCII [US-ASCII] range. For characters outside the US-ASCII range, DUTF calculates the offset value between adjacent characters in the source string using XOR, and then encodes the offset value as a variable-length sequence of octets. The number and value of octets depend on the Unicode code point of the current character and the previous non-ASCII character in the source string. DUTF has the following characteristics (all values are hexadecimal):¶
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶
In DUTF, characters are encoded as sequences of 1 to n octets. For a single-octet sequence, the highest bit is set to 0 and the remaining 7 bits encode the character number. In sequences of n octets (n>1), the highest bit of the initial n-1 octets is set to 1, and the highest bit of the last octet is set to 0, with 7 bits available for encoding the offset value between the Unicode code point of the current character and the previous non-ASCII character in the source string.¶
Table 1 summarizes the format of these different variable-length octets. The letter x indicates bits available for encoding bits of the offset value.¶
| Offset value range(hexadecimal) | DUTF octet sequence(binary) |
|---|---|
| 0000 0000-0000 3FFF | 1xxxxxxx 0xxxxxxx |
| 0000 4000-001F FFFF | 1xxxxxxx 1xxxxxxx 0xxxxxxx |
Encoding a character to DUTF proceeds as follows:¶
Decoding a DUTF character proceeds as follows:¶
Implementations of the decoding algorithm above MUST protect against decoding invalid sequences. For instance, a naive implementation may decode the invalid DUTF sequence 80 00 into the character U+0000. Decoding invalid sequences may have security consequences or cause other problems. See Security Considerations (Section 9) below.¶
For the convenience of implementors using ABNF, a definition of DUTF in ABNF syntax is given here.¶
A DUTF string is a sequence of octets representing a sequence of Unicode characters. An octet sequence is valid DUTF only if it matches the following syntax, which is derived from the rules for encoding DUTF and is expressed in the ABNF of [RFC5234].¶
DUTF-octets = *( DUTF-char ) DUTF-char = DUTF-1 / DUTF-2 / DUTF-3 DUTF-1 = %x00-7F DUTF-2 = %x81-FF DUTF-tail DUTF-3 = %x81-FF %x81-FF DUTF-tail DUTF-tail = %x00-7F¶
ISO/IEC 10646 is updated from time to time by publication of amendments and additional parts; similarly, new versions of the Unicode standard are published over time. 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. In 1996, Amendment 5 to the 1993 edition of ISO/IEC 10646 and Unicode 2.0 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 justification for allowing such an incompatible change was that there were no major implementations and no significant amounts of data containing Hangul. The incident has been dubbed the "Korean mess", and the relevant committees have pledged to never, ever again make such an incompatible change (see Unicode Consortium Policies [UNICODE-POLICIES]).¶
New versions, and in particular any incompatible changes, have consequences regarding MIME charset labels, to be discussed in MIME registration (Section 7).¶
The UCS character U+FEFF "ZERO WIDTH NO-BREAK SPACE" is also known informally as "BYTE ORDER MARK" (abbreviated "BOM"). This character can be used as a genuine "ZERO WIDTH NO-BREAK SPACE" within text, but the BOM name hints at a second possible usage of the character: to prepend a U+FEFF character to a stream of UCS characters as a "signature". A receiver of such a serialized stream may then use the initial character as a hint that the stream consists of UCS characters and also to recognize which UCS encoding is involved and, with encodings having a multi-octet encoding unit, as a way to recognize the serialization order of the octets. DUTF having a single-octet encoding unit, this last function is useless. BOM encoding is not fixed, only at the beginning of the stream, it will always be encoded as the octal sequence FF FD 03.¶
It is important to understand that the character U+FEFF appearing at any position other than the beginning of a stream MUST be interpreted with the semantics for the zero-width non-breaking space, and MUST NOT be interpreted as a signature. When interpreted as a signature, the Unicode standard suggests than an initial U+FEFF character may be stripped before processing the text. Such stripping is necessary in some cases (e.g., when concatenating two strings, because otherwise the resulting string may contain an unintended "ZERO WIDTH NO-BREAK SPACE" at the connection point), but might affect an external process at a different layer (such as a digital signature or a count of the characters) that is relying on the presence of all characters in the stream. It is therefore RECOMMENDED to avoid stripping an initial U+FEFF interpreted as a signature without a good reason, to ignore it instead of stripping it when appropriate (such as for display) and to strip it only when really necessary.¶
U+FEFF in the first position of a stream MAY be interpreted as a zero-width non-breaking space, and is not always a signature. In an attempt at diminishing this uncertainty, Unicode 3.2 adds a new character, U+2060 "WORD JOINER", with exactly the same semantics and usage as U+FEFF except for the signature function, and strongly recommends its exclusive use for expressing word-joining semantics. Eventually, following this recommendation will make it all but certain that any initial U+FEFF is a signature, not an intended "ZERO WIDTH NO-BREAK SPACE".¶
In the meantime, the uncertainty unfortunately remains and may affect Internet protocols. Protocol specifications MAY restrict usage of U+FEFF as a signature in order to reduce or eliminate the potential ill effects of this uncertainty. In the interest of striking a balance between the advantages (reduction of uncertainty) and drawbacks (loss of the signature function) of such restrictions, it is useful to distinguish a few cases:¶
When a protocol forbids use of U+FEFF as a signature for a certain protocol element, then any initial U+FEFF in that protocol element MUST be interpreted as a "ZERO WIDTH NO-BREAK SPACE". When a protocol does NOT forbid use of U+FEFF as a signature for a certain protocol element, then implementations SHOULD be prepared to handle a signature in that element and react appropriately: using the signature to identify the character encoding as necessary and stripping or ignoring the signature as appropriate.¶
The character sequence U+0041 U+2262 U+0391 U+002E "A<NOT IDENTICAL TO><ALPHA>." is encoded in DUTF as Figure 1:¶
--+-----+-----+--
41 E2 44 F3 43 2E
--+-----+-----+--
The character sequence U+D55C U+AD6D U+C5B4 (Korean "hangugeo", meaning "the Korean language") is encoded in DUTF as Figure 2:¶
--------+--------+--------
DC AA 03 B1 F0 01 D9 D1 01
--------+--------+--------
The character sequence U+65E5 U+672C U+8A9E (Japanese "nihongo", meaning "the Japanese language") is encoded in DUTF as Figure 3:¶
--------+-----+--------
E5 CB 01 C9 05 B2 DB 03
--------+-----+--------
The character sequence U+233B4 (a Chinese character meaning 'stump of tree'), prepended with a DUTF BOM, is encoded in DUTF as Figure 4:¶
--------+--------
FF FD 03 B4 E7 08
--------+--------
This memo serves as the basis for registration of the MIME charset parameter for DUTF, according to [RFC2978]. The charset parameter value is "DUTF". 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 of the 1993 edition (Korean block), encoded to a sequence of octets using the encoding scheme outlined above. DUTF is suitable for use in MIME content types under the "text" top-level type.¶
It is noteworthy that the label "DUTF" 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 octets received on the wire into a sequence of characters, nothing more (see [RFC2045], section 2.2). 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.¶
IANA is to register the charset found in Appendix A according to [RFC2978], using registration template found in this appendix.¶
Implementers of DUTF need to consider the security aspects of how they handle illegal DUTF sequences. It is conceivable that in some circumstances an attacker would be able to exploit an incautious DUTF parser by sending it an octet sequence that is not permitted by the DUTF syntax.¶
A particularly subtle form of this attack could be carried out against a parser which performs security-critical validity checks against the DUTF encoded form of its input, but interprets certain illegal octet sequences as characters. For example, a parser might prohibit the ACK character when encoded as the single-octet sequence 06, but allow the illegal two-octet sequence 86 00 and interpret it as a ACK character. Another example might be a parser which prohibits the octet sequence 2F 2E 2E 2F ("/../"), yet permits the illegal octet sequence AF 00 2E 2E 2F.¶
Some of the text in this specification was copied from [RFC3629] and [RFC2781].¶
To: ietf-charsets@iana.org Subject: Registration of new charset DUTF Charset name: DUTF Charset aliases: dutf Suitability for use in MIME text: Body: ASCII compatible Published specification(s): This specification ISO 10646 equivalency table: This specification Person & email address to contact for further information: Yao Yang <yao.yang.sy@foxmail.com> Intended usage: COMMON¶