INTERNET-DRAFT Adam M. Costello draft-ietf-idn-amc-ace-r-00.txt 2001-Mar-27 Expires 2001-Sep-27 AMC-ACE-R version 0.0.0 Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html Distribution of this document is unlimited. Please send comments to the author at amc@cs.berkeley.edu, or to the idn working group at idn@ops.ietf.org. A non-paginated (and possibly newer) version of this specification may be available at http://www.cs.berkeley.edu/~amc/charset/amc-ace-r Abstract AMC-ACE-R is a reversible map from a sequence of Unicode [UNICODE] code points to a sequence of letters (A-Z, a-z), digits (0-9), and hyphen-minus (-), henceforth called LDH characters. Such a map might be useful for an "ASCII-Compatible Encoding" (ACE) for internationalized domain names [IDN], because host name labels are currently restricted to LDH characters by [RFC952] and [RFC1123]. AMC-ACE-R is similar to AMC-ACE-O [AMCACEO00] but is simpler and not quite as efficient. Besides domain names, there might also be other contexts where it is useful to transform Unicode characters into "safe" (delimiter-free) ASCII characters. (If other contexts consider hyphen-minus to be unsafe, a different character could be used to play its role, like underscore.) Contents Features Name Overview Base-32 characters Encoding and decoding algorithms Signature Case sensitivity models Comparison with RACE, BRACE, LACE, AltDUDE, AMC-ACE-M, AMC-ACE-O Example strings Security considerations Credits References Author Example implementation Features Uniqueness: Every Unicode string maps to at most one LDH string. Completeness: Every Unicode string maps to an LDH string. Restrictions on which Unicode strings are allowed, and on length, may be imposed by higher layers. Efficient encoding: The ratio of encoded size to original size is small for all Unicode strings. This is important in the context of domain names because [RFC1034] restricts the length of a domain label to 63 characters. Simplicity: The encoding and decoding algorithms are reasonably simple to implement. The goals of efficiency and simplicity are at odds; AMC-ACE-R aims at a good balance between them. Case-preservation: If the Unicode string has been case-folded prior to encoding, it is possible to record the case information in the case of the letters in the encoding, allowing a mixed-case Unicode string to be recovered if desired, but a case-insensitive comparison of two encoded strings is equivalent to a case-insensitive comparison of the Unicode strings. This feature is optional; see section "Case sensitivity models". Readability: The letters A-Z and a-z and the digits 0-9 appearing in the Unicode string are represented as themselves in the label. This comes for free because it usually the most efficient encoding anyway. Name AMC-ACE-R is a working name that should be changed if it is adopted. (The R merely indicates that it is the eighteenth ACE devised by this author. BRACE was the third. D-L, N, P, and Q were not worth releasing.) Rather than waste good names on experimental proposals, let's wait until one proposal is chosen, then assign it a good name. Suggestions (assuming the primary use is in domain names): UniHost UTF-D ("D" for "domain names") NUDE (Normal Unicode Domain Encoding) A name that makes no reference to domain names: UTF-37 (there are 37 characters in the output repertoire) Overview AMC-ACE-R maps a sequence of Unicode code points to a sequence of LDH characters. The encoder input and decoder output are arrays of code points, not characters, bytes, or code units (in particular, not UTF-16 surrogates). Formally, the encoder output and decoder input are character strings, not code points, code units, or bytes, although implementations will of course need to represent the characters somehow, usually as bytes or other code units. Each Unicode code point is represented by an integral number of characters in the encoded string. There is no intermediate bit string or octet string. The encoded string alternates between two modes: literal mode and base-32 mode. Unicode code points representing LDH characters are encoded as those LDH characters, except that hyphen-minus is doubled. Other Unicode code points are encoded using base-32, in which each character of the encoded string represents five bits (a "quintet"). A non-paired hyphen-minus in the encoded string indicates a mode change. In base-32 mode a variable-length code sequence of one to five quintets represents a delta, which is added to a reference point to yield a Unicode code point. There are five reference points, one for each code length, three of which continually change during the encoding/decoding process. Base-32 characters "a" = 0 = 0x00 = 00000 "s" = 16 = 0x10 = 10000 "b" = 1 = 0x01 = 00001 "t" = 17 = 0x11 = 10001 "c" = 2 = 0x02 = 00010 "u" = 18 = 0x12 = 10010 "d" = 3 = 0x03 = 00011 "v" = 19 = 0x13 = 10011 "e" = 4 = 0x04 = 00100 "w" = 20 = 0x14 = 10100 "f" = 5 = 0x05 = 00101 "x" = 21 = 0x15 = 10101 "g" = 6 = 0x06 = 00110 "y" = 22 = 0x16 = 10110 "h" = 7 = 0x07 = 00111 "z" = 23 = 0x17 = 10111 "i" = 8 = 0x08 = 01000 "2" = 24 = 0x18 = 11000 "j" = 9 = 0x09 = 01001 "3" = 25 = 0x19 = 11001 "k" = 10 = 0x0A = 01010 "4" = 26 = 0x1A = 11010 "m" = 11 = 0x0B = 01011 "5" = 27 = 0x1B = 11011 "n" = 12 = 0x0C = 01100 "6" = 28 = 0x1C = 11100 "p" = 13 = 0x0D = 01101 "7" = 29 = 0x1D = 11101 "q" = 14 = 0x0E = 01110 "8" = 30 = 0x1E = 11110 "r" = 15 = 0x0F = 01111 "9" = 31 = 0x1F = 11111 The digits "0" and "1" and the letters "o" and "l" are not used, to avoid transcription errors. All decoders must recognize both the uppercase and lowercase forms of the base-32 characters. The case may or may not convey information, as described in section "Case sensitivity models". Encoding and decoding algorithms The algorithms are given below as commented pseudocode. All ordering of bits and quintets is big-endian (most significant first). The >> and << operators used below mean bit shift, as in C. For >> there is no question of logical versus arithmetic shift because AMC-ACE-R never needs to right-shift a negative value. As in C, "continue" means terminate the current iteration of the innermost loop, "break" means terminate the innermost loop, and "return" means terminate the current function. shared variables: # All others are local to each function. array refpoint[1..5] # refpoint[k] is for sequences of length k function update_refpoints(history[first..latest]): # Adapt refpoint[1..3] based on the code points seen so far. for k = 1 to 3 do begin let b = k << 2 if latest - first == 1 then let refpoint[k] = (history[latest] >> b) << b else for i = latest - 1 down to first do begin if history[i] represents an LDH character then continue if (refpoint[k] XOR history[i]) >> b == 0 then break if (history[latest] XOR history[i]) >> b == 0 then begin let refpoint[k] = (history[latest] >> b) << b return end end end function encode(input[first..last]): let refpoint[1..5] = 0x60, 0, 0, 0, 0x10000 let output = the empty string let literal = false for i = first to last do begin if input[i] == 0x2D then append two hyphen-minuses to output else if input[i] represents an LDH character then begin if not literal then append hyphen-minus to output let literal = true append the character represented by input[i] to output end else begin if literal then append hyphen-minus to output let literal = false for k = 1 to infinity do begin let delta = codepoint - refpoint[k] if delta >= 0 and delta >> (4*k) == 0 then break end extract the k least significant nybbles of delta prepend 0 to the last nybble and 1 to the rest output base-32 characters corresponding to the quintets update_refpoints(input[first..i]) end end return output function decode(input string): let refpoint[1..5] = 0x60, 0, 0, 0, 0x10000 let output = the empty array let literal = false while not end-of-input do begin if the next character is hyphen-minus then begin consume the character if the next character is also hyphen-minus then consume it and append 0x2D to output else toggle literal end else if literal then consume the character and output it else begin consume characters and convert them to quintets until encountering a quintet beginning with 0 fail upon encountering a non-base-32 character or end-of-input let k = the number of quintets obtained strip the first bit of each quintet concatenate the resulting nybbles to form delta append refpoint[k] + delta to output update_refpoints(output) end end let check = encode(output) if check != the input string then fail return output The comparison at the end of decode() must be case-insensitive if ACEs are always compared case-insensitively (which is true of domain names), case-sensitive otherwise (see also section "Case sensitivity models"). This check is necessary to guarantee the uniqueness property, that there cannot be two distinct encoded strings representing the same sequence of integers. This check also frees the decoder from having to check for overflow while decoding the base-32 characters. Signature The issue of how to distinguish ACE strings from unencoded strings is largely orthogonal to the encoding scheme itself, and is therefore not specified here. In the context of domain name labels, a standard prefix and/or suffix (chosen to be unlikely to occur naturally) would presumably be attached to ACE labels. In order to use AMC-ACE-R in domain names, the choice of signature must be mindful of the requirement in [RFC952] that labels never begin or end with hyphen-minus. Since the raw encoded string sometimes begins with a hyphen-minus, the signature must include a prefix that does not begin with hyphen-minus. If the Unicode strings are forbidden from ending with hyphen-minus (which seems prudent anyway), then the raw encoded string will never end with hyphen-minus; otherwise, the signature must include a suffix as well as a prefix. It appears that "---" is extremely rare in domain names; among the four-character prefixes of all the second-level domains under .com, .net, and .org, "---" never appears at all. Therefore, perhaps the signature should be of the form "?---", where ? could be "u" for Unicode, or "i" for internationalized, or "a" for ACE, or maybe "q" or "z" because they are rare. Case sensitivity models The higher layer must choose one of the following four models. Models suitable for domain names: * Case-insensitive: Before a string is encoded, all its non-LDH characters must be case-folded so that any strings differing only in case become the same string (for example, strings could be forced to lowercase). Folding LDH characters is optional. The case of base-32 characters and literal-mode characters is arbitrary and not significant. Comparisons between encoded strings must be case-insensitive. The original case of non-LDH characters cannot be recovered from the encoded string. * Case-preserving: The case of the Unicode characters is not considered significant, but it can be preserved and recovered, just like in non-internationalized host names. Before a string is encoded, all its non-LDH characters must be case-folded as in the previous model. LDH characters are naturally able to retain their case attributes because they are encoded literally. The case attribute of a non-LDH character is recorded in the last of the base-32 characters that represent it, which is guaranteed to be a letter rather than a digit. If the base-32 character is uppercase, it means the Unicode character is caseless or should be forced to uppercase after being decoded (which is a no-op if the case folding already forces to uppercase). If the base-32 character is lowercase, it means the Unicode character is caseless or should be forced to lowercase after being decoded (which is a no-op if the case folding already forces to lowercase). The case of the other base-32 characters in a multi-quintet encoding is arbitrary and not significant. Only uppercase and lowercase attributes can be recorded, not titlecase. Comparisons between encoded strings must be case-insensitive, and are equivalent to case-insensitive comparisons between the Unicode strings. The intended mixed-case Unicode string can be recovered as long as the encoded characters are unaltered, but altering the case of the encoded characters is not harmful--it merely alters the case of the Unicode characters, and such a change is not considered significant. In this model, the input to the encoder and the output of the decoder can be the unfolded Unicode string (in which case the encoder and decoder are responsible for performing the case folding and recovery), or can be the folded Unicode string accompanied by separate case information (in which case the higher layer is responsible for performing the case folding and recovery). Whichever layer performs the case recovery must first verify that the Unicode string is properly folded, to guarantee the uniqueness of the encoding. It should not be very difficult to extend the nameprep algorithm [NAMEPREP03] to remember case information; it could be done by adding flags to the mapping tables. The case-insensitive and case-preserving models are interoperable. If a domain name passes from a case-preserving entity to a case-insensitive entity, the case information may be lost, but the domain name will still be equivalent. This phenomenon already occurs with non-internationalized domain names. Models unsuitable for domain names, but possibly useful in other contexts: * Case-sensitive: Unicode strings may contain both uppercase and lowercase characters, which are not folded. Base-32 characters must be lowercase. Comparisons between encoded strings must be case-sensitive. * Case-flexible: Like case-preserving, except that the choice of whether the case of the Unicode characters is considered significant is deferred. Therefore, base-32 characters must be lowercase, except for those used to indicate uppercase Unicode characters. Comparisons between encoded strings may be case-sensitive or case-insensitive, and such comparisons are equivalent to the corresponding comparisons between the Unicode strings. Comparison with RACE, BRACE, LACE, AltDUDE, AMC-ACE-M, AMC-ACE-O In this section we compare AMC-ACE-R and six other ACEs: RACE [RACE03], BRACE [BRACE00], LACE [LACE01], AltDUDE [AltDUDE00], AMC-ACE-M [AMCACEM00], and AMC-ACE-O [AMCACEO00]. We do not include SACE [SACE], UTF-5 [UTF5], UTF-6 [UTF6], or DUDE [DUDE01] in the comparison, because SACE appears obviously too complex, UTF-5 appears obviously too inefficient, UTF-6 can never be more efficient than its similarly simple successor DUDE, and DUDE is almost identical to AltDUDE. Complexity is hard to measure. This author would subjectively describe the complexity of the algorithms as: LACE, AltDUDE: simple but not trivial RACE, AMC-ACE-R: less simple AMC-ACE-O: moderate AMC-ACE-M: fairly complex BRACE: complex AMC-ACE-R is similar to AMC-ACE-O, but is considerably simpler because it does not calculate the most useful reference points beforehand, encode them, and decode them. Instead, it uses a simple heuristic to set the reference points adaptively based on the code points that have been seen so far. Implementations can be long and straightforward, or short and subtle, but for whatever it's worth, here are the code sizes of four of the algorithms that were implemented by this author in similar styles: AltDUDE: 130 lines @@@@@@@@@@@@@@@@@@@ AMC-ACE-R: 171 lines @@@@@@@@@@@@@@@@@@@@@@@@ AMC-ACE-O: 232 lines @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ AMC-ACE-M: 324 lines @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ (Not counted in the code sizes are blank lines, lines containing only comments or only a single brace, and wrapper code for testing. BRACE was also implemented by this author, but it was a less general implementation, with bounded input and output sizes.) If a different implementation style were to alter the code sizes additively, or multiplicatively, or a combination thereof, AMC-ACE-O would remain about halfway between AltDUDE and AMC-ACE-M, and AMC-ACE-R would remain closer to AltDUDE than to AMC-ACE-O. Case preservation support: AltDUDE, AMC-ACE-M/O/R: all characters BRACE: only the letters A-Z, a-z RACE, LACE: none RACE, BRACE, and LACE transform the Unicode string to an intermediate bit string, then into a base-32 string, so there is no particular alignment between the base-32 characters and the Unicode characters. AltDUDE and AMC-ACE-M/O/R do not have this intermediate stage, and enforce alignment between the base-32 characters and the Unicode characters, which facilitates the case preservation. The relative efficiency of the various algorithms is suggested by the sizes of the encodings in section "Example strings". The lengths of examples A-K (which are the same sentence translated into a languages from a variety of language families using a variety of scripts) are shown graphically below for each ACE, scaled by a factor of 0.4 so they fit on one line, and sorted so they look like a cummulative distribution. The fictional "Super-ACE" encodes its input using whichever of the other seven ACEs is shortest for that input. RACE: A Arabic 29 @@@@@@@@@@@@ B Chinese 31 @@@@@@@@@@@@ J Taiwanese 31 @@@@@@@@@@@@ D Hebrew 37 @@@@@@@@@@@@@@@ H Russian 47 @@@@@@@@@@@@@@@@@@@ E Hindi 50 @@@@@@@@@@@@@@@@@@@@ F Japanese 60 @@@@@@@@@@@@@@@@@@@@@@@@ I Spanish 66 @@@@@@@@@@@@@@@@@@@@@@@@@@ C Czech 68 @@@@@@@@@@@@@@@@@@@@@@@@@@@ G Korean 79 @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ K Vietnamese 112 @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ LACE: B Chinese 28 @@@@@@@@@@@ A Arabic 31 @@@@@@@@@@@@ J Taiwanese 31 @@@@@@@@@@@@ D Hebrew 39 @@@@@@@@@@@@@@@@ H Russian 48 @@@@@@@@@@@@@@@@@@@ E Hindi 52 @@@@@@@@@@@@@@@@@@@@@ F Japanese 52 @@@@@@@@@@@@@@@@@@@@@ C Czech 58 @@@@@@@@@@@@@@@@@@@@@@@ I Spanish 68 @@@@@@@@@@@@@@@@@@@@@@@@@@@ G Korean 79 @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ K Vietnamese 109 @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ AltDUDE: A Arabic 25 @@@@@@@@@@ B Chinese 26 @@@@@@@@@@ D Hebrew 33 @@@@@@@@@@@@@ J Taiwanese 36 @@@@@@@@@@@@@@ H Russian 38 @@@@@@@@@@@@@@@ C Czech 43 @@@@@@@@@@@@@@@@@ F Japanese 49 @@@@@@@@@@@@@@@@@@@@ E Hindi 58 @@@@@@@@@@@@@@@@@@@@@@@ I Spanish 59 @@@@@@@@@@@@@@@@@@@@@@@@ K Vietnamese 81 @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ G Korean 89 @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ AMC-ACE-R: B Chinese 24 @@@@@@@@@@ A Arabic 28 @@@@@@@@@@@ J Taiwanese 30 @@@@@@@@@@@@ D Hebrew 32 @@@@@@@@@@@@@ C Czech 36 @@@@@@@@@@@@@@ H Russian 40 @@@@@@@@@@@@@@@@ F Japanese 42 @@@@@@@@@@@@@@@@@ I Spanish 47 @@@@@@@@@@@@@@@@@@@ E Hindi 55 @@@@@@@@@@@@@@@@@@@@@@ K Vietnamese 70 @@@@@@@@@@@@@@@@@@@@@@@@@@@@ G Korean 89 @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ AMC-ACE-O: B Chinese 24 @@@@@@@@@@ A Arabic 28 @@@@@@@@@@@ J Taiwanese 30 @@@@@@@@@@@@ D Hebrew 31 @@@@@@@@@@@@ C Czech 34 @@@@@@@@@@@@@@ H Russian 40 @@@@@@@@@@@@@@@@ F Japanese 41 @@@@@@@@@@@@@@@@ I Spanish 49 @@@@@@@@@@@@@@@@@@@@ E Hindi 54 @@@@@@@@@@@@@@@@@@@@@@ K Vietnamese 69 @@@@@@@@@@@@@@@@@@@@@@@@@@@@ G Korean 80 @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ BRACE: B Chinese 22 @@@@@@@@@ A Arabic 26 @@@@@@@@@@ J Taiwanese 27 @@@@@@@@@@@ D Hebrew 33 @@@@@@@@@@@@@ C Czech 36 @@@@@@@@@@@@@@ F Japanese 40 @@@@@@@@@@@@@@@@ H Russian 42 @@@@@@@@@@@@@@@@@ E Hindi 45 @@@@@@@@@@@@@@@@@@ I Spanish 48 @@@@@@@@@@@@@@@@@@@ K Vietnamese 72 @@@@@@@@@@@@@@@@@@@@@@@@@@@@@ G Korean 78 @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ AMC-ACE-M: B Chinese 23 @@@@@@@@@ J Taiwanese 27 @@@@@@@@@@@ A Arabic 28 @@@@@@@@@@@ D Hebrew 31 @@@@@@@@@@@@ C Czech 34 @@@@@@@@@@@@@@ H Russian 38 @@@@@@@@@@@@@@@ F Japanese 42 @@@@@@@@@@@@@@@@@ I Spanish 48 @@@@@@@@@@@@@@@@@@@ E Hindi 54 @@@@@@@@@@@@@@@@@@@@@@ K Vietnamese 69 @@@@@@@@@@@@@@@@@@@@@@@@@@@@ G Korean 71 @@@@@@@@@@@@@@@@@@@@@@@@@@@@ Super-ACE: B Chinese 22 @@@@@@@@@ A Arabic 25 @@@@@@@@@@ J Taiwanese 27 @@@@@@@@@@@ D Hebrew 31 @@@@@@@@@@@@ C Czech 34 @@@@@@@@@@@@@@ H Russian 38 @@@@@@@@@@@@@@@ F Japanese 40 @@@@@@@@@@@@@@@@ E Hindi 45 @@@@@@@@@@@@@@@@@@ I Spanish 47 @@@@@@@@@@@@@@@@@@@ K Vietnamese 69 @@@@@@@@@@@@@@@@@@@@@@@@@@@@ G Korean 71 @@@@@@@@@@@@@@@@@@@@@@@@@@@@ totals: RACE: 610 @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ LACE: 595 @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ AltDUDE: 537 @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ AMC-ACE-R: 493 @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ AMC-ACE-O: 480 @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ BRACE: 469 @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ AMC-ACE-M: 465 @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ Super-ACE: 449 @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ worst cases: RACE: 112 @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ LACE: 109 @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ AltDUDE: 89 @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ AMC-ACE-R: 89 @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ AMC-ACE-O: 80 @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ BRACE: 78 @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ AMC-ACE-M: 71 @@@@@@@@@@@@@@@@@@@@@@@@@@@@ Super-ACE: 71 @@@@@@@@@@@@@@@@@@@@@@@@@@@@ The totals and worst cases above give more weight to languages that produce longer encodings, which arguably yields a good metric (because being efficient for easy languages is arguably less important than being efficient for difficult languages). We can alternatively give each language equal weight by dividing each output length by the corresponding Super-ACE output length. This method yields: totals: RACE: 14.9 @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ LACE: 14.5 @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ AltDUDE: 13.0 @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ AMC-ACE-R: 12.0 @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ AMC-ACE-O: 11.8 @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ AMC-ACE-M: 11.4 @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ BRACE: 11.4 @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ Super-ACE: 11.0 @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ worst cases: RACE: 2.00 @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ LACE: 1.71 @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ AltDUDE: 1.33 @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ AMC-ACE-R: 1.25 @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ AMC-ACE-O: 1.20 @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ AMC-ACE-M: 1.20 @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ BRACE: 1.11 @@@@@@@@@@@@@@@@@@@@@@@@@@@@ Super-ACE: 1.00 @@@@@@@@@@@@@@@@@@@@@@@@@ No matter which way we average, the results suggest that AltDUDE is preferrable to RACE and LACE, because it is no more complex, is more efficient, and has better support for case preservation. The results also suggest that AMC-ACE-M is preferrable to BRACE, because it has similar efficiency, is a little simpler, and has better support for case preservation. AltDUDE, AMC-ACE-R, AMC-ACE-O, and AMC-ACE-M are progressively more complex and more efficient, and have equal support for case preservation. The choice depends on how much efficiency is required and how much complexity is acceptable. The efficiency gaps between AMC-ACE-M, AMC-ACE-O, and AMC-ACE-R are mostly due to the Korean (Hangul) string. Of the 15 characters by which the AMC-ACE-M total beats the AMC-ACE-O total, 9 come from the Korean string. Similarly, of the 13 characters by which the AMC-ACE-O total beats the AMC-ACE-R total, 9 come from the Korean string. The large increases in complexity from AMC-ACE-R to -O to -M yield significant efficiency gains for Korean, but only very small gains for the other languages. More sample strings from more languages need to be tried before one can conclude that Korean is the only significant beneficiary, but if it is, then this author would suggest that AMC-ACE-R is preferable to -O and -M, with apologies to Korean speakers. That would leave a choice between AltDUDE and AMC-ACE-R, the latter being somewhat more complex and somewhat more efficient. Example strings In the ACE encodings below, signatures (like "bq--" for RACE) are not shown. Non-LDH characters in the Unicode string are forced to lowercase before being encoded. For RACE, LACE, and AltDUDE, the letters A-Z are likewise forced to lowercase. UTF-8 and UTF-16 are included for length comparisons, with non-ASCII bytes shown as "?". AMC-ACE-* and AltDUDE are abbreviated AMC-* and ADUDE. Backslashes show where line breaks have been inserted in ACE strings too long for one line. The RACE and LACE encodings are courtesy of Mark Davis's online UTF converter [UTFCONV] (slightly modified to remove the length restrictions). The first several examples are all translations of the sentence "Why can't they just speak in <language>?" (courtesy of Michael Kaplan's "provincial" page [PROVINCIAL]). Word breaks and punctuation have been removed, as is often done in domain names. (A) Arabic (Egyptian): U+0644 U+064A U+0647 U+0645 U+0627 U+0628 U+062A U+0643 U+0644 U+0645 U+0648 U+0634 U+0639 U+0631 U+0628 U+064A U+061F ADUDE: yueqpcycrcyjhbpznpitjycxf BRACE: 28akcjwcmp3ciwb4t3ngd4nbaz AMC-R: ywekhfuhuikwdwefivevjbuiwktr AMC-O: ageekhfuhuiukdefivevjvbuiktr AMC-M: agiekhfuhuiukdefivevjvbuiktr RACE: azceur2fe4ucuq2eivediojrfbfb6 LACE: cedeisshiutsqksdircuqnbzgeueuhy UTF-16: ?????????????????????????????????? UTF-8: ?????????????????????????????????? (B) Chinese (simplified): U+4ED6 U+4EEC U+4E3A U+4EC0 U+4E48 U+4E0D U+8BF4 U+4E2D U+6587 UTF-16: ?????????????????? BRACE: kgcqqsgp26i5h4zn7req5i AMC-M: uqj7g8nvk6awispn9wupdnh AMC-R: w87g8nvk6awisp259eupyx2h AMC-O: eqpg8nvk6awisp259eupyx2h ADUDE: w85gvk7g9k2iwf6x9j6x7ju54k UTF-8: ??????????????????????????? LACE: azhnn3b2ybea2aml6qau4libmwdq RACE: 3bhnmtxmjy5e5qcojbha3c7ujywwlby (C) Czech: Pro<ccaron>prost<ecaron>nemluv<iacute><ccaron>esky <ccaron> = U+010D <ecaron> = U+011B <iacute> = U+00ED UTF-8: Pro??prost??nemluv????esky AMC-O: piq-Pro-p-prost-9m-nemluv-6pp-esky AMC-M: g26-Pro-p-prost-9m-nemluv-6pp-esky AMC-R: -Pro-tsp-prost-ttm-nemluv-s8psp-esky BRACE: i32-Pro-u-prost-8y-nemluv-29f3n-esky ADUDE: tActptyctzpctptnhtyrtzfmibtjd3mt8atyitgtitc UTF-16: ???????????????????????????????????????????? LACE: amaha4tpaeaq2biaobzg643uaearwbyanzsw23dvo3wqcainaqagk43\ lpe RACE: ah7xb73s75xq373q75zp6377op7xig77n37wl73n75wp65p7o3762dp\ 7mx7xh73l754q (D) Hebrew: U+05DC U+05DE U+05D4 U+05D4 U+05DD U+05E4 U+05E9 U+05D5 U+05D8 U+05DC U+05D0 U+05DE U+05D3 U+05D1 U+05E8 U+05D9 U+05DD U+05E2 U+05D1 U+05E8 U+05D9 U+05EA AMC-O: afpnqeep8e8jfinaqdb8ijp8cb8ij8k AMC-M: af4nqeep8e8jfinaqdb8ijp8cb8ij8k AMC-R: x7nqeep8e8j7f7inaqdb8ijp8cb8ij8k ADUDE: x5nckajvjpvnpenqpcvjvbevrvdvjvbvd BRACE: 27vkyp7bgwmbpfjgc4ynx5nd8xsp5nd9c RACE: axon5vgu3xsotvoy3tin5u6r5dm53ywr5dm6u LACE: cyc5zxwu2to6j2ov3donbxwt2huntxpc2hunt2q UTF-8: ???????????????????????????????????????????? UTF-16: ???????????????????????????????????????????? (E) Hindi: U+092F U+0939 U+0932 U+094B U+0917 U+0939 U+093F U+0928 U+094D U+0926 U+0940 U+0915 U+094D U+092F U+094B U+0902 U+0928 U+0939 U+0940 U+0902 U+092C U+094B U+0932 U+0938 U+0915 U+0924 U+0947 U+0939 U+0948 U+0902 (Devanagari) BRACE: 2b7xtenqdr7zc6uma2pmcz7ibage237kdemicnk9gei32 RACE: bextsmslc44t6kcnezabktjpjmbcqokaaiwewmrycuseookiai LACE: dyes6ojsjmltspzijuteafknf5fqekbziabcyszshaksirzzjaba AMC-O: ajeurvjvcmthvjvruipugatfpurmscuivjascunmvcvitfuehvjisc AMC-M: ajhurbvcwmthbhuiwpugitfwpurwmscuibiscunwmvcatfuerbwisc AMC-R: 3urvjvcwmthjruiwpugwatfwpurmscuivjascunmvcvitfuewhjwisc ADUDE: 3wrtgmzjxnuqgthyfymygxfxiycyewjuktbzjwcuqyhzjkupvbydzqz\ bwk UTF-16: ???????????????????????????????????????????????????????\ ????? UTF-8: ???????????????????????????????????????????????????????\ ??????????????????????????????????? (F) Japanese: U+306A U+305C U+307F U+3093 U+306A U+65E5 U+672C U+8A9E U+3092 U+8A71 U+3057 U+3066 U+304F U+308C U+306A U+3044 U+306E U+304B (kanji and hiragana) UTF-16: ???????????????????????????????????? BRACE: ji8nr5zj8uqth7v97mjchakwcg7dqemw88nj5gbe AMC-O: gvagkxnzr3dkx8fzun243q3c24zbxhgwr2nkweqwm AMC-R: vsykxnzr3dkyx8fyzun243q3c24zbxhgwr2nkweqwm AMC-M: bsnkxnzr3dkyx8fyzun243q3c24zbxhgwr2nkweqwm ADUDE: vsskvgud8n9jxx2ru6j875c54sn548d54ugvbuj6d8guqukuf LACE: auyguxd7snvaczpfaftsyamktyatbeqbrjyqqmcxmzhyy2senzfq UTF-8: ?????????????????????????????????????????????????????? RACE: 3aygumc4gb7tbezqnjs6kzzmrkpdbeukoeyfomdggbhtbdbqniyeimd\ ogbfq (G) Korean: U+C138 U+ACC4 U+C758 U+BAA8 U+B4E0 U+C0AC U+B78C U+B4E4 U+C774 U+D55C U+AD6D U+C5B4 U+B97C U+C774 U+D574 U+D55C U+B2E4 U+BA74 U+C5BC U+B9C8 U+B098 U+C88B U+C744 U+AE4C (Hangul syllables) UTF-16: ???????????????????????????????????????????????? UTF-8: ???????????????????????????????????????????????????????\ ????????????????? AMC-M: yhxcj2w6exiaxi68acfn92n68ezehk6xypdpwam6zehmwhk648eavwd\ p6aqi23ieemweywn BRACE: y394qebjusrcndbs82pkvstf96sxufcr7ffr4vbgdwsxufcx8pdktgb\ gmnsqydmk7im56arju6pt82 LACE: 77atrlgey5mlvkfu4dakzn4mwtsmo5gvlsww3rnuxf6mo5gvotkvzmx\ exj2mlpfzzcyjrsely5ck4ta RACE: 3datrlgey5mlvkfu4dakzn4mwtsmo5gvlsww3rnuxf6mo5gvotkvzmx\ exj2mlpfzzcyjrsely5ck4ta AMC-O: m6hwq6tvi466exi44ia6s4nz2neze7xxn47yp6x5e3znze7xze7xxnu\ 8e4ze6x5n36is3i622mwe48wn ADUDE: 6txiy79ny53nz79a8wizwwnzzuavyizv3atuuiz2vby27jz66iz8sit\ usauiyz5i23az96iz6ze3xaz2td96ry3si AMC-R: 6tvi466ezxi544i5w8a6s4nz2nw8e6zze7xxn47yp6x5e53znze7xze\ 7xxn5u8e54ze6x5n36is3i622m6zwe48wn (H) Russian: U+041F U+043E U+0447 U+0435 U+043C U+0443 U+0436 U+0435 U+043E U+043D U+0438 U+043D U+0435 U+0433 U+043E U+0432 U+043E U+0440 U+044F U+0442 U+043F U+043E U+0440 U+0443 U+0441 U+0441 U+043A U+0438 (Cyrillic) ADUDE: wxRbzjzcjzrzfdmdffigpnnzqrpzpbzqdcazmc AMC-M: aehHgrvfemvgvfgfafvfvdgvcgiwrkhgimjjca AMC-R: wvRqwhfnwdgfqpipfdqcqwawrcvrvqwawdbbvkvi AMC-O: aedRqwhfnwdgfqpipfdqcqwawrwcrqwawdwbwbki BRACE: 269xyjvcyafqfdwyr3xfd8z8byi6z39xyi692s7ug2 RACE: aq7t4rzvhrbtmnj6hu4d2njthyzd4qcpii7t4qcdifatuoa LACE: dqcd6pshgu6egnrvhy6tqpjvgm7depsaj5bd6psainaucory UTF-16: ???????????????????????????????????????????????????????\ ??? UTF-8: ??????????????????????????????????????????????????????? ??? (I) Spanish: Porqu<eacute>nopuedensimplementehablarenEspa<ntilde>ol <eacute> = U+00E9 <ntilde> = U+00F1 UTF-8: Porqu??nopuedensimplementehablarenEspa??ol AMC-R: -Porqu-8j-nopuedensimplementehablarenEspa-9b-ol AMC-M: aa7-Porqu-b-nopuedensimplementehablarenEspa-j-ol BRACE: 22x-Porqu-9-nopuedensimplementehablarenEspa-j-ol AMC-O: aaq-Porqu-j-nopuedensimplementehablarenEspa-9b-ol ADUDE: tAtrtpde3n2hbtrftabbmtptketptnjiimtktbpjdqptdthmMtgdtb3\ a3qd RACE: abyg64troxuw433qovswizloonuw24dmmvwwk3tumvugcytmmfzgk3t\ fonygd4lpnq LACE: faaha33sof26s3tpob2wkzdfnzzws3lqnrsw2zloorswqylcnrqxezl\ omvzxayprn5wa UTF-16: ???????????????????????????????????????????????????????\ ????????????????????????? (J) Taiwanese: U+4ED6 U+5011 U+7232 U+4EC0 U+9EBD U+4E0D U+8AAA U+4E2D U+6587 UTF-16: ?????????????????? UTF-8: ??????????????????????????? AMC-M: uqj7g2tbgtu6a385pspnxkupdnh BRACE: kgcqui49gatc2wyrn8y7cndgte9 AMC-R: w87gxstbzuvc6a385psp244kupyx2h AMC-O: eqpgxstbzuvc6a385psp244kupyx2h RACE: 3bhnmuaroize5qe6xvha3cvkjywwlby LACE: 75hnmuaroize5qe6xvha3cvkjywwlby ADUDE: w85gt86huuudv69c7szp7s5a6w4h6w2hu54k (K) Vietnamese: Ta<dotbelow>isaoho<dotbelow>kh<ocirc>ngth<ecirc><hookabove>chi\ <hookabove>no<acute>iti<ecirc><acute>ngVi<ecirc><dotbelow>t <dotbelow> = U+0323 <ocirc> = U+00F4 <ecirc> = U+00EA <hookabove> = U+0309 <acute> = U+0301 UTF-8: Ta??isaoho??kh??ngth????chi??no??iti????ngVi????t AMC-O: aava-Ta-vud-isaoho-vud-kh-9e-ngth-8kj-chi-j-no-b-iti-8k\ b-ngVi-8kvud-t AMC-M: ada-Ta-ud-isaoho-ud-kh-s9e-ngth-s8kj-chi-j-no-b-iti-s8k\ b-ngVi-s8kud-t AMC-R: -Ta-vud-isaoho-vud-kh-9e-ngth-8kvsj-chi-vsj-no-b-iti-s8\ kb-ngVi-s8kud-t BRACE: i54-Ta-8-isaoho-ay-kh-29n-ngth-s2xa6i-chi-k-no-2g-iti-2\ 9c29-ngVi-25p48-t UTF-16: ???????????????????????????????????????????????????????\ ????????????????????? ADUDE: tEtfvwcvwktktcqhhvwnvwid3n3kjtdtn2cv8dvykmbvyavyhbvyqvy\ itptp2dv8mvyrjtBtr2dv6jvxh LACE: aiahiyibamrqmadjonqw62dpaebsgcaannupi3thoruouaidbebqay3\ ineaqgcicabxg6aidaecaa2lunhvacaybauag4z3wnhvacazdaeahi RACE: ap7xj73bep7wt73t75q76377nd7w6i77np7wr77u75xp6z77ot7wr77\ kbh7wh73i75uqt73o75xqd73j752p62p75ia763x7m77xn73j77vch7\ 3u The next several examples are all names of Japanese music artists, song titles, and TV programs, just because the author happens to have them handy (but Japanese is useful for providing examples of single-row text, two-row text, ideographic text, and various mixtures thereof). (L) 3<nen>B<gumi><kinpachi><sensei> (Japanese TV program title) <nen> = U+5E74 (kanji) <gumi> = U+7D44 (kanji) <kinpachi><sensei> = U+91D1 U+516B U+5148 U+751F (kanji) UTF-16: ???????????????? UTF-8: 3???B??????????????? AMC-M: utk-3-8ze-B-hkenqtymwifi9 BRACE: u-3-ygj-b-ynb6gjc7pp4k5p5w AMC-O: fb8h-3-e-B-z7we3t7bymwizxtr ADUDE: xdx8whx8tGz7ug863f6s5kuduwxh RACE: 3aadgxtuabrh2rer2fiwwukioupq LACE: 74adgxtuabrh2rer2fiwwukioupq AMC-R: -3-x8ze-B-z7we3t7bxtymtwizxtr (M) <amuro><namie>-with-SUPER-MONKEYS (Japanese music group name) <amuro><namie> = U+5B89 U+5BA4 U+5948 U+7F8E U+6075 (kanji) UTF-8: ??????????????????-with-SUPER-MONKEYS AMC-M: u5m2j4etwif6q2zf---with--SUPER--MONKEYS AMC-R: x52j4e3wiz92qyszf---with--SUPER--MONKEYS AMC-O: fmij4e3wiz92qyszf---with--SUPER--MONKEYS BRACE: uvj7fuaqcahy982xa---with--SUPER--MONKEYS ADUDE: x58jupu8nuy6gt99m-yssctqtptn-tMGFtFtH-tRCBFQtNK UTF-16: ???????????????????????????????????????????????? LACE: ajnytjablfeac74oafqhkeyafv3qm5difvzxk4dfoiww233onnsxs4y RACE: 3bnysw5elfeh7dtaouac2adxabuqa5aanaac2adtab2qa4aamuaheab\ nabwqa3yanyagwadfab4qa4y (N) Hello-Another-Way-<sorezore><no><basho> (Japanese song title) <sorezore><no> = U+305D U+308C U+305E U+308C U+306E (hiragana) <basho> = U+5834 U+6240 (kanji) UTF-8: Hello-Another-Way-????????????????????? BRACE: ji7-Hello--Another--Way---v3jhaefvd2ufj62 AMC-O: daf-Hello--Another--Way---p2nq2nyqx2veyuwa AMC-M: bsk-Hello--Another--Way---p2nq2nyqx2veyuwa ADUDE: Ipjad-Qrbtmtnpth-Ftgti-vsue7b7c7c8cy2xkv4ze AMC-R: -Hello--Another--Way---vsxpvs2nxq2nyqx2veyuwa UTF-16: ?????????????????????????????????????????????????? LACE: ciagqzlmnrxs2ylon52gqzlsfv3wc6jnauyf3dc6rrxacwbuafrea RACE: 3aagqadfabwaa3aan4ac2adbabxaa3yaoqagqadfabzaaliao4agcad\ zaawtaxjqrqyf4memgbxfqndcia (O) <hitotsu><yane><no><shita>2 (Japanese TV program title) <hitotsu> = U+3072 U+3068 U+3064 (hiragana) <yane> = U+5C4B U+6839 (kanji) <no> = U+306E (hiragana) <shita> = U+4E0B (kanji) UTF-16: ???????????????? UTF-8: ?????????????????????2 AMC-O: dagzciex6wmy2vjqw8sm-2 AMC-M: bsnzciex6wmy2vjqw8sm-2 BRACE: ji96u56uwbhf2wqxnw4s-2 AMC-R: vszcyiyex6wmy2vjqw8sm-2 ADUDE: vstctkny6urvwzcx2xhz8yfw8vj RACE: 3ayhemdigbsfys3iheyg4tqlaaza LACE: 74yhemdigbsfys3iheyg4tqlaaza (P) Maji<de>Koi<suru>5<byou><mae> (Japanese song title) <de> = U+3067 (hiragana) <suru> = U+3059 U+308B (hiragana) <byou><mae> = U+79D2 U+524D (kanji) UTF-8: Maji???Koi??????5?????? UTF-16: ?????????????????????????? AMC-M: bsm-Maji-r-Koi-b2m-5-z37cxuwp BRACE: ji8-Maji-g-Koi-qe7x-5-wx7p6ma AMC-O: dag-Maji-h-Koi-xj2m-5-z37cxuwp ADUDE: PnmdvssqvssNegvsva7cvs5qz38hu53r AMC-R: -Maji-vsyh-Koi-vsxj2m-5-z37cxuwp RACE: 3aag2adbabvaa2jqm4agwadpabutawjqrmadk6oskjgq LACE: 74ag2adbabvaa2jqm4agwadpabutawjqrmadk6oskjgq (Q) <pafii>de<runba> (Japanese song title) <pafii> = U+30D1 U+30D5 U+30A3 U+30FC (katakana) <runba> = U+30EB U+30F3 U+30D0 (katakana) UTF-16: ?????????????? BRACE: 3iu8pazt-de-pygi AMC-O: dapbf4d9n-de-8m9da AMC-M: bs3jp4d9n-de-8m9di AMC-R: vs7bf4d9n-de-8m9d7a RACE: gdi5li7475sp6zpl6pia ADUDE: vs5bezgxrvs3ibvs2qtiud UTF-8: ????????????de????????? LACE: aqyndvnd7qbaazdfamyox46q (R) <sono><supiido><de> (Japanese song title) <sono> = U+305D U+306E (hiragana) <supiido> = U+30B9 U+30D4 U+30FC U+30C9 (katakana) <de> = U+3067 (hiragana) RACE: gbow5oou7tewo UTF-16: ?????????????? BRACE: bidprdmp9wt7mi LACE: a4yf23vz2t6mszy AMC-O: dagxpq5j7e9n6jh AMC-M: bsmfyq5j7e9n6jr ADUDE: vsvpvd7hypuivf4q AMC-R: vsxpyq5j7e9n6jyh UTF-8: ????????????????????? The last example is an ASCII string that breaks not only the existing rules for host name labels but also the rules proposed in [NAMEPREP03] for internationalized domain names. (S) -> $1.00 <- UTF-8: -> $1.00 <- ADUDE: -xqtqetftrtqatatn- RACE: aawt4ibegexdambahqwq LACE: bmac2praeqys4mbqea6c2 AMC-R: --vquaue-1-q-00-avn-- UTF-16: ?????????????????????? AMC-O: aac--vqae-1-q-00-avn-- AMC-M: aae--vqae-1-q-00-avn-- BRACE: 229--t2b4-1-w-00-i9i-- Security considerations Users expect each domain name in DNS to be controlled by a single authority. If a Unicode string intended for use as a domain label could map to multiple ACE labels, then an internationalized domain name could map to multiple ACE domain names, each controlled by a different authority, some of which could be spoofs that hijack service requests intended for another. Therefore AMC-ACE-R is designed so that each Unicode string has a unique encoding. However, there can still be multiple Unicode representations of the "same" text, for various definitions of "same". This problem is addressed to some extent by the Unicode standard under the topic of canonicalization, and this work is leveraged for domain names by "nameprep" [NAMEPREP03]. Credits AMC-ACE-R reuses a number of preexisting techniques. The basic encoding of integers to nybbles to quintets to base-32 comes from UTF-5 [UTF5], and the particular variant used here comes from AMC-ACE-M [AMCACEM00]. The idea of avoiding 0, 1, o, and l in base-32 strings was taken from SFS [SFS]. The idea of encoding deltas from reference points was taken from RACE (of which the latest version is [RACE03]), which may have gotten the idea from Unicode Technical Standard #6 [UTS6]. The idea of switching between literal mode and base-32 mode comes from BRACE [BRACE00]. The general idea of using the alphabetic case of base-32 characters to record the desired case of the Unicode characters was suggested by this author, and first applied to the UTF-5-style encoding in DUDE (of which the latest version is [DUDE01]). The heuristic used to adapt the reference points based on past code points is new in AMC-ACE-R. References [AltDUDE00] Adam Costello, "AltDUDE version 0.0.2", 2001-Mar-19, draft-ietf-idn-altdude-00. [AMCACEM00] Adam Costello, "AMC-ACE-M version 0.1.0", 2001-Feb-12, draft-ietf-idn-amc-ace-m-00. [AMCACEO00] Adam Costello, "AMC-ACE-O version 0.0.3", 2001-Mar-19, draft-ietf-idn-amc-ace-o-00. [BRACE00] Adam Costello, "BRACE: Bi-mode Row-based ASCII-Compatible Encoding for IDN version 0.1.2", 2000-Sep-19, draft-ietf-idn-brace-00. [DUDE01] Mark Welter, Brian Spolarich, "DUDE: Differential Unicode Domain Encoding", 2001-Mar-02, draft-ietf-idn-dude-01. [IDN] Internationalized Domain Names (IETF working group), http://www.i-d-n.net/, idn@ops.ietf.org. [LACE01] Paul Hoffman, Mark Davis, "LACE: Length-based ASCII Compatible Encoding for IDN", 2001-Jan-05, draft-ietf-idn-lace-01. [NAMEPREP03] Paul Hoffman, Marc Blanchet, "Preparation of Internationalized Host Names", 2001-Feb-24, draft-ietf-idn-nameprep-03. [PROVINCIAL] Michael Kaplan, "The 'anyone can be provincial!' page", http://www.trigeminal.com/samples/provincial.html. [RACE03] Paul Hoffman, "RACE: Row-based ASCII Compatible Encoding for IDN", 2000-Nov-28, draft-ietf-idn-race-03. [RFC952] K. Harrenstien, M. Stahl, E. Feinler, "DOD Internet Host Table Specification", 1985-Oct, RFC 952. [RFC1034] P. Mockapetris, "Domain Names - Concepts and Facilities", 1987-Nov, RFC 1034. [RFC1123] Internet Engineering Task Force, R. Braden (editor), "Requirements for Internet Hosts -- Application and Support", 1989-Oct, RFC 1123. [SACE] Dan Oscarsson, "Simple ASCII Compatible Encoding (SACE)", draft-ietf-idn-sace-*. [SFS] David Mazieres et al, "Self-certifying File System", http://www.fs.net/. [UNICODE] The Unicode Consortium, "The Unicode Standard", http://www.unicode.org/unicode/standard/standard.html. [UTF5] James Seng, Martin Duerst, Tin Wee Tan, "UTF-5, a Transformation Format of Unicode and ISO 10646", draft-jseng-utf5-*. [UTF6] Mark Welter, Brian W. Spolarich, "UTF-6 - Yet Another ASCII-Compatible Encoding for IDN", draft-ietf-idn-utf6-*. [UTS6] Misha Wolf, Ken Whistler, Charles Wicksteed, Mark Davis, Asmus Freytag, "Unicode Technical Standard #6: A Standard Compression Scheme for Unicode", http://www.unicode.org/unicode/reports/tr6/. [UTFCONV] Mark Davis, "UTF Converter", http://www.macchiato.com/unicode/convert.html. Author Adam M. Costello <amc@cs.berkeley.edu> http://www.cs.berkeley.edu/~amc/ Example implementation /******************************************/ /* amc-ace-r.c 0.0.0 (2001-Mar-27-Tue) */ /* Adam M. Costello <amc@cs.berkeley.edu> */ /******************************************/ /* This is ANSI C code (C89) implementing AMC-ACE-R version 0.0.*. */ /************************************************************/ /* Public interface (would normally go in its own .h file): */ #include <limits.h> enum amc_ace_status { amc_ace_success, amc_ace_invalid_input, amc_ace_big_output }; enum case_sensitivity { case_sensitive, case_insensitive }; #if UINT_MAX >= 0x10FFFF typedef unsigned int u_code_point; #else typedef unsigned long u_code_point; #endif enum amc_ace_status amc_ace_r_encode( unsigned int input_length, const u_code_point *input, const unsigned char *uppercase_flags, unsigned int *output_size, char *output ); /* amc_ace_r_encode() converts Unicode to AMC-ACE-R (without */ /* any signature). The input must be represented as an array */ /* of Unicode code points (not code units; surrogate pairs */ /* are not allowed), and the output will be represented as */ /* null-terminated ASCII. The input_length is the number of */ /* code points in the input. The output_size is an in/out */ /* argument: the caller must pass in the maximum number of */ /* characters that may be output (including the terminating */ /* null), and on successful return it will contain the number of */ /* characters actually output (including the terminating null, */ /* so it will be one more than strlen() would return, which is */ /* why it is called output_size rather than output_length). The */ /* uppercase_flags array must hold input_length boolean values, */ /* where nonzero means the corresponding Unicode character should */ /* be forced to uppercase after being decoded, and zero means it */ /* is caseless or should be forced to lowercase. Alternatively, */ /* uppercase_flags may be a null pointer, which is equivalent */ /* to all zeros. The letters a-z and A-Z are always encoded */ /* literally, regardless of the corresponding flags. The encoder */ /* always outputs lowercase base-32 characters except when */ /* nonzero values of uppercase_flags require otherwise. The */ /* return value may be any of the amc_ace_status values defined */ /* above; if not amc_ace_success, then output_size and output may */ /* contain garbage. On success, the encoder will never need to */ /* write an output_size greater than input_length*5+1, because of */ /* how the encoding is defined. */ enum amc_ace_status amc_ace_r_decode( enum case_sensitivity case_sensitivity, char *scratch_space, const char *input, unsigned int *output_length, u_code_point *output, unsigned char *uppercase_flags ); /* amc_ace_r_decode() converts AMC-ACE-R (without any signature) */ /* to Unicode. The input must be represented as null-terminated */ /* ASCII, and the output will be represented as an array of */ /* Unicode code points. The case_sensitivity argument influences */ /* the check on the well-formedness of the input string; it */ /* must be case_sensitive if case-sensitive comparisons are */ /* allowed on encoded strings, case_insensitive otherwise. */ /* The scratch_space must point to space at least as large */ /* as the input, which will get overwritten (this allows the */ /* decoder to avoid calling malloc()). The output_length is */ /* an in/out argument: the caller must pass in the maximum */ /* number of code points that may be output, and on successful */ /* return it will contain the actual number of code points */ /* output. The uppercase_flags array must have room for at */ /* least output_length values, or it may be a null pointer */ /* if the case information is not needed. A nonzero flag */ /* indicates that the corresponding Unicode character should */ /* be forced to uppercase by the caller, while zero means it */ /* is caseless or should be forced to lowercase. The letters */ /* a-z and A-Z are output already in the proper case, but their */ /* flags will be set appropriately so that applying the flags */ /* would be harmless. The return value may be any of the */ /* amc_ace_status values defined above; if not amc_ace_success, */ /* then output_length, output, and uppercase_flags may contain */ /* garbage. On success, the decoder will never need to write */ /* an output_length greater than the length of the input (not */ /* counting the null terminator), because of how the encoding is */ /* defined. */ /**********************************************************/ /* Implementation (would normally go in its own .c file): */ #include <string.h> static int is_ldh(u_code_point codept) { return codept > 122 ? 0 : codept >= 97 ? 1 : codept > 90 ? 0 : codept >= 65 ? 1 : codept > 57 ? 0 : codept >= 48 ? 1 : codept == 45 ; } /* is_AtoZ(c) returns 1 if c is an */ /* uppercase ASCII letter, zero otherwise. */ static unsigned char is_AtoZ(char c) { return c >= 65 && c <= 90; } /* base32[n] is the lowercase base-32 character representing */ /* the number n from the range 0 to 31. Note that we cannot */ /* use string literals for ASCII characters because an ANSI C */ /* compiler does not necessarily use ASCII. */ static const char base32[] = { 97, 98, 99, 100, 101, 102, 103, 104, 105, 106, 107, /* a-k */ 109, 110, /* m-n */ 112, 113, 114, 115, 116, 117, 118, 119, 120, 121, 122, /* p-z */ 50, 51, 52, 53, 54, 55, 56, 57 /* 2-9 */ }; /* base32_decode(c) returns the value of a base-32 character, in the */ /* range 0 to 31, or the constant base32_invalid if c is not a valid */ /* base-32 character. */ enum { base32_invalid = 32 }; static unsigned int base32_decode(char c) { if (c < 50) return base32_invalid; if (c <= 57) return c - 26; if (c < 97) c += 32; if (c < 97 || c == 108 || c == 111 || c > 122) return base32_invalid; return c - 97 - (c > 108) - (c > 111); } /* unequal(case_sensitivity,s1,s2) returns 0 if the strings s1 and s2 */ /* are equal, 1 otherwise. If case_sensitivity is case_insensitive, */ /* then ASCII A-Z are considered equal to a-z respectively. */ static int unequal( enum case_sensitivity case_sensitivity, const char *s1, const char *s2 ) { char c1, c2; if (case_sensitivity != case_insensitive) return strcmp(s1,s2) != 0; for (;;) { c1 = *s1; c2 = *s2; if (c1 >= 65 && c1 <= 90) c1 += 32; if (c2 >= 65 && c2 <= 90) c2 += 32; if (c1 != c2) return 1; if (c1 == 0) return 0; ++s1, ++s2; } } /* update_refpoints(refpoint,history,latest) updates refpoint[1..3] */ /* based on the history, where history[latest] is the latest code */ /* point. */ void update_refpoints( u_code_point *refpoint, const u_code_point *history, unsigned int latest ) { unsigned int k, b, i; for (k = 1; k <= 3; ++k) { b = k << 2; if (latest == 0) refpoint[k] = (history[0] >> b) << b; else for (i = latest; i-- > 0; ) { if (is_ldh(history[i])) continue; if ((refpoint[k] ^ history[i]) >> b == 0) break; if ((history[latest] ^ history[i]) >> b == 0) { refpoint[k] = (history[latest] >> b) << b; return; } } } } /* Main encode function: */ enum amc_ace_status amc_ace_r_encode( unsigned int input_length, const u_code_point *input, const unsigned char *uppercase_flags, unsigned int *output_size, char *output ) { unsigned int max_out, next_out, literal, i, k, out; u_code_point codept, delta; char shift; u_code_point refpoint[6] = {0, 0x60, 0, 0, 0, 0x10000}; max_out = *output_size; next_out = 0; literal = 0; for (i = 0; i < input_length; ++i) { codept = input[i]; if (codept > 0x10FFFF) return amc_ace_invalid_input; if (codept == 0x2D) { /* hyphen-minus is doubled */ if (max_out - next_out < 1) return amc_ace_big_output; output[next_out++] = 0x2D; output[next_out++] = 0x2D; } else if (is_ldh(codept)) { /* encode LDH character literally */ if (max_out - next_out < 1 + !literal) return amc_ace_big_output; /* switch to literal mode if necessary: */ if (!literal) output[next_out++] = 0x2D; literal = 1; output[next_out++] = codept; } else { /* encode non-LDH character using base-32 */ shift = uppercase_flags && uppercase_flags[i] ? 32 : 0; /* shift will determine the case of the last base-32 digit */ for (k = 1; ; ++k) { delta = codept - refpoint[k]; if (delta >> (4*k) == 0) break; } /* We will encode delta as k base-32 digits. */ if (max_out - next_out < k + literal) return amc_ace_big_output; /* switch to base-32 mode if necessary: */ if (literal) output[next_out++] = 0x2D; literal = 0; /* Computing the base-32 digits in reverse order is easiest. */ /* Only the last base-32 digit has the high bit clear. */ out = next_out + k - 1; output[out] = base32[delta & 0xF] - shift; while (out > next_out) { delta >>= 4; output[--out] = base32[0x10 | (delta & 0xF)]; } next_out += k; update_refpoints(refpoint,input,i); } } /* null terminator: */ if (max_out - next_out < 1) return amc_ace_big_output; output[next_out++] = 0; *output_size = next_out; return amc_ace_success; } /* Main decode function: */ enum amc_ace_status amc_ace_r_decode( enum case_sensitivity case_sensitivity, char *scratch_space, const char *input, unsigned int *output_length, u_code_point *output, unsigned char *uppercase_flags ) { u_code_point q, delta; const char *in, *first; char c; unsigned int next_out, max_out, literal, input_size, scratch_size; enum amc_ace_status status; u_code_point refpoint[6] = {0, 0x60, 0, 0, 0, 0x10000}; max_out = *output_length; next_out = 0; in = input; literal = 0; for (c = *in; c != 0; ) { if (c == 45 && in[1] != 45) { /* unpaired hyphen-minus toggles mode */ literal = !literal; c = *++in; continue; } if (max_out - next_out < 1) return amc_ace_big_output; if (c == 45) { /* double hyphen-minus represents a hyphen-minus */ if (uppercase_flags) uppercase_flags[next_out] = 0; output[next_out] = 45; c = *(in += 2); } else { if (literal) { /* decode one base-32 code point */ output[next_out] = c; c = *++in; } else { /* Base-32 sequence: */ delta = 0; first = in; do { q = base32_decode(c); if (q == base32_invalid) return amc_ace_invalid_input; delta = (delta << 4) | (q & 0xF); c = *++in; } while (q >> 4 == 1); output[next_out] = refpoint[in - first] + delta; update_refpoints(refpoint, output, next_out); } /* case of last digit determines uppercase flag: */ if (uppercase_flags) uppercase_flags[next_out] = is_AtoZ(in[-1]); } ++next_out; } /* Re-encode the output and compare to the input: */ input_size = in - input + 1; scratch_size = input_size; status = amc_ace_r_encode(next_out, output, uppercase_flags, &scratch_size, scratch_space); if (status != amc_ace_success || scratch_size != input_size || unequal(case_sensitivity, scratch_space, input) ) return amc_ace_invalid_input; *output_length = next_out; return amc_ace_success; } /******************************************************************/ /* Wrapper for testing (would normally go in a separate .c file): */ #include <assert.h> #include <stdio.h> #include <stdlib.h> #include <string.h> /* For testing, we'll just set some compile-time limits rather than */ /* use malloc(), and set a compile-time option rather than using a */ /* command-line option. */ enum { unicode_max_length = 256, ace_max_size = 256, test_case_sensitivity = case_insensitive /* good for host names */ }; static void usage(char **argv) { fprintf(stderr, "%s -e reads big-endian UTF-32 and writes AMC-ACE-R ASCII.\n" "%s -d reads AMC-ACE-R ASCII and writes big-endian UTF-32.\n" "UTF-32 is extended: bit 31 is used as force-to-uppercase flag.\n" , argv[0], argv[0]); exit(EXIT_FAILURE); } static void fail(const char *msg) { fputs(msg,stderr); exit(EXIT_FAILURE); } static const char too_big[] = "input or output is too large, recompile with larger limits\n"; static const char invalid_input[] = "invalid input\n"; static const char io_error[] = "I/O error\n"; int main(int argc, char **argv) { enum amc_ace_status status; int r; if (argc != 2) usage(argv); if (argv[1][0] != '-') usage(argv); if (argv[1][2] != '\0') usage(argv); if (argv[1][1] == 'e') { u_code_point input[unicode_max_length]; unsigned char uppercase_flags[unicode_max_length]; char output[ace_max_size]; unsigned int input_length, output_size; int c0, c1, c2, c3; /* Read the UTF-32 input string: */ input_length = 0; for (;;) { c0 = getchar(); c1 = getchar(); c2 = getchar(); c3 = getchar(); if (ferror(stdin)) fail(io_error); if (c1 == EOF || c2 == EOF || c3 == EOF) { if (c0 != EOF) fail("input not a multiple of 4 bytes\n"); break; } if (input_length == unicode_max_length) fail(too_big); if ((c0 != 0 && c0 != 0x80) || c1 < 0 || c1 > 0x10 || c2 < 0 || c2 > 0xFF || c3 < 0 || c3 > 0xFF ) { fail(invalid_input); } input[input_length] = ((u_code_point) c1 << 16) | ((u_code_point) c2 << 8) | (u_code_point) c3 ; uppercase_flags[input_length] = (c0 >> 7); ++input_length; } /* Encode, and output the result: */ output_size = ace_max_size; status = amc_ace_r_encode(input_length, input, uppercase_flags, &output_size, output); if (status == amc_ace_invalid_input) fail(invalid_input); if (status == amc_ace_big_output) fail(too_big); assert(status == amc_ace_success); r = fputs(output,stdout); if (r == EOF) fail(io_error); return EXIT_SUCCESS; } if (argv[1][1] == 'd') { char input[ace_max_size], scratch[ace_max_size]; u_code_point output[unicode_max_length], codept; unsigned char uppercase_flags[unicode_max_length]; unsigned int output_length, i; /* Read the AMC-ACE-R ASCII input string: */ fgets(input, ace_max_size, stdin); if (ferror(stdin)) fail(io_error); if (!feof(stdin)) fail(too_big); /* Decode, and output the result: */ output_length = unicode_max_length; status = amc_ace_r_decode(test_case_sensitivity, scratch, input, &output_length, output, uppercase_flags); if (status == amc_ace_invalid_input) fail(invalid_input); if (status == amc_ace_big_output) fail(too_big); assert(status == amc_ace_success); for (i = 0; i < output_length; ++i) { r = putchar(uppercase_flags[i] ? 0x80 : 0); if (r == EOF) fail(io_error); codept = output[i]; r = putchar(codept >> 16); if (r == EOF) fail(io_error); r = putchar((codept >> 8) & 0xFF); if (r == EOF) fail(io_error); r = putchar(codept & 0xFF); if (r == EOF) fail(io_error); } return EXIT_SUCCESS; } usage(argv); return EXIT_SUCCESS; /* not reached, but quiets compiler warning */ } INTERNET-DRAFT expires 2001-Sep-27
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draft-ietf-idn-amc-ace-r-00
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