INTERNET-DRAFT Adam M. Costello
draft-ietf-idn-amc-ace-r-01.txt 2001-May-31
Expires 2001-Nov-30
AMC-ACE-R version 0.2.1
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
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to the author at amc@cs.berkeley.edu, or to the idn working
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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 transformation from a sequence of Unicode
[UNICODE] code points to a sequence of letters, digits, and hyphens
(LDH characters). AMC-ACE-R could be used as an ASCII-Compatible
Encoding (ACE) for internationalized domain names [IDN] [IDNA].
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 hyphens to be
unsafe, a different character could be used to play its role, like
underscore.)
Contents
Technical changes from earlier versions
Features
Name
Terminology
Description
Base-32 characters
Encoding and decoding algorithms
Signature
Mixed-case annotation
Comparison with other ACEs
Example strings
Security considerations
Acknowledgements
References
Author
Example implementation
Technical changes from earlier versions
From 0.0.x to 0.1.x:
In update(), the test "latest - first == 1" was a bug, changed
to "latest == first".
In encode(), "codepoint" was a misspelling of "input[i]".
Initializing refpoint[1] to 0x60 was a design flaw, because this
initial value is useless for nameprep'd strings. The initial
value is changed to 0xE0.
encode() now fails if input[i] is not in 0..10FFFF, in order to
avoid an array bounds error. This does not affect the encoding
of valid Unicode strings.
From 0.1.x to 0.2.x:
The test "latest == first" tests for the first code point, but
the intention was to test for the first non-LDH code point,
or equivalently, for the first time update() is called. The
boolean flag "updated" has been introduced for performing the
proper test. This alters the encoding of some strings, usually
for the better.
The initial value of refpoint[2] has been changed from 0 to
0xA0, which is more useful in light of nameprep's prohibition of
non-LDH code points below 0xA0.
In decode() the number of base-32 characters consumed has been
limited to 5, to avoid an array bounds error on invalid input.
Features
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.
Uniqueness: Every Unicode string maps to at most one LDH string.
Reversibility: Any Unicode string mapped to an LDH string can be
recovered from that LDH string.
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 reasonable balance between them.
Mixed-case annotation: Even if the Unicode string has been
case-folded prior to encoding, it is possible to used mixed case
in the encoded string as an annotation telling how to convert the
folded Unicode string into a mixed-case Unicode string for display
purposes. This feature is optional; see section "Mixed-case
annotation".
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. Most 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:
UniHost
NUDE (Normal Unicode Domain Encoding)
UTF-D ("D" for "domain names")
UTF-37 (there are 37 characters in the output repertoire)
Terminology
LDH characters are the letters A-Z and a-z, the digits 0-9, and
hyphen-minus.
A quartet is a sequence of four bits (also known as a nibble or
nybble).
A quintet is a sequence of five bits.
Hexadecimal values are shown preceeded by "0x". For example, 0x60
is decimal 96.
As in the Unicode Standard [UNICODE], Unicode code points are
denoted by "U+" followed by four to six hexadecimal digits, while a
range of code points is denoted by two hexadecimal numbers separated
by "..", with no prefixes.
"x..y" means the range of integers x through y inclusive.
"x << y" means x left-shifted by y bits (equivalent to x times
2 to the power y), and "x >> y" means x right-shifted by y bits
(equivalent to x divided by 2 to the power y, discarding the
remainder). These operations are used only with nonnegative
integral values.
Description
AMC-ACE-R represents a sequence of Unicode code points as a sequence
of LDH characters, although implementations will also need to
represent the LDH characters somehow, typically as ASCII octets.
The encoder input and decoder output are arrays of Unicode code
points (integral values in the range 0..10FFFF, but not D800..DFFF,
which are reserved for use by UTF-16).
This section describes the representation. Section "Encoding
and decoding algorithms" presents the algorithms as commented
pseudocode. There is also commented C code in section "Example
implementation".
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 as one or more LDH
characters using base-32, in which each character of the encoded
string represents a quintet according to the table in section
"Base-32 characters". A mode change is indicated by an unpaired
hyphen-minus. A pair of consecutive hyphen-minuses represents a
hyphen-minus and does not change the mode.
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. The delta is represented by the lowest four
bits of each quintet. The highest bit of each quintet is 1, except
for the last quintet, where it is 0, allowing the decoder to detect
the end of the sequence.
Code sequences:
delta from reference point 1: 0xxxx
delta from reference point 2: 1xxxx 0xxxx
delta from reference point 3: 1xxxx 1xxxx 0xxxx
delta from reference point 4: 1xxxx 1xxxx 1xxxx 0xxxx
delta from reference point 5: 1xxxx 1xxxx 1xxxx 1xxxx 0xxxx
For reference point k, the delta is constrained by the available
bits to range from 0 to (1 << (4*k)) - 1, so each reference point is
the bottom of a window of 1 << (4*k) code points. A code point is
encoded as an offset into the smallest window that contains it.
Reference points 4 and 5 are fixed at 0 and 0x10000 respectively,
so that windows 4 and 5 always cover the entire Unicode code space
0..10FFFF. The other reference points are updated whenever a code
point has been encoded or decoded in base-32 mode, using following
heuristic.
The latest code point is rounded down to a multiple of 0x10 to
obtain a candidate for replacing reference point 1. If a non-LDH
code point falling within the candidate window has appeared more
recently than one falling within the current window, then the
reference point is changed. Otherwise a similar check is performed
for reference point 2 using 0x100 as the divisor, and failing that,
reference point 3 is checked using 0x1000. At most one window is
changed each time, except that after the very first non-LDH code
point (when there is no useful history), all three windows are
changed.
The initial values of the state variables are:
mode: base-32
reference point 1: 0xE0
reference point 2: 0xA0
reference point 3: 0
reference point 4: 0
reference point 5: 0x10000
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 (including mixtures of both forms).
An encoder should output only lowercase forms or only uppercase
forms unless it uses the feature described in section "Mixed-case
annotation").
Encoding and decoding algorithms
All ordering of bits, quartets, and quintets is big-endian (most
significant first). When subroutines alter variables that are
passed in as arguments, those changes are seen by the caller after
the subroutine returns. As in C, "continue" means terminate the
current iteration of the innermost loop, and "break" means terminate
the innermost loop.
procedure initialize(refpoint,literal,updated):
let refpoint[1..5] = (0xE0, 0xA0, 0, 0, 0x10000)
let literal = updated = false
procedure update(refpoint, updated, history[first..latest]):
# Update the reference points based on the history.
for k = 1 to 3 do begin
let b = 4 * k
# The first time here change all the windows:
if not updated
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 a code point falling in the existing window has appeared
# at least as recently as one falling in the candidate window,
# then leave this window unchanged and go on to the next one:
if (refpoint[k] XOR history[i]) >> b == 0 then break
if (history[latest] XOR history[i]) >> b == 0 then begin
# A code point falling in the candidate window has appeared
# more recently than one falling in the existing window, so
# change this window (and no others):
let refpoint[k] = (history[latest] >> b) << b
goto update_end
end
end
end
update_end: let updated = true
procedure encode(input[first..last]):
initialize(refpoint,literal,updated)
for i = first to last do begin
# Check code point range to avoid array bounds errors later:
if input[i] is not in 0..10FFFF then fail
if input[i] == 0x2D then output two hyphen-minuses
else if input[i] represents an LDH character then begin
# Letter/digit is encoded literally, so get into literal mode.
if not literal then output hyphen-minus
let literal = true
output the character represented by input[i]
end
else begin
# Non-LDH code point is encoded in base-32.
# Compute the number of base-32 characters to use:
for k = 1 to infinity do begin
let delta = input[i] - refpoint[k]
if delta >= 0 and delta >> (4*k) == 0 then break
end
# Switch to base-32 mode if necessary:
if literal then output hyphen-minus
let literal = false
represent delta in base 16 as k quartets
prepend 0 to the last quartet and 1 to each of the others
output a base-32 character corresponding to each quintet
update(refpoint, updated, input[first..i])
end
end
procedure decode(input string):
initialize(refpoint,literal,updated)
let history = the empty array
while the input string is not exhausted do begin
read the next character into c
# Unpaired hyphen-minus toggles the mode:
if c is hyphen-minus and the next character is not
then read the next character into c and toggle literal
# Double hyphen-minus represents 0x2D:
if c is hyphen-minus
then read the next character and append 0x2D to history
else if literal then append the code point of c to history
else begin
# Decode a base-32 sequence.
convert c to a quintet
while a quintet beginning with 0 has not been seen
do read and convert up to four more characters
concatenate the lowest four bits of each quintet to form delta
append refpoint[number of quintets] + delta to history
update(refpoint,updated,history)
end
end
# Enforce the uniqueness of the encoding:
encode history and compare it to the input string
fail if they are not equal
output history
The decoder must always be prepared for premature end-of-input or
invalid input characters, and must either fail immediately or forge
ahead and let the comparison at the end fail. The comparison must
be case-insensitive if ACEs are always compared case-insensitively
(which is true of domain names), case-sensitive otherwise. This
check is necessary to guarantee the uniqueness property (there
cannot be two distinct encoded strings representing the same
sequence of integers). (If the decoder is one step of a larger
decoding process, it may be possible to defer the re-encoding and
comparison to the end of that larger decoding process.)
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.
Mixed-case annotation
In order to use AMC-ACE-R to represent case-insensitive Unicode
strings, higher layers need to case-fold the Unicode strings prior
to AMC-ACE-R encoding. The encoded string can, however, use
mixed-case base-32 (rather than all-lowercase or all-uppercase
as recommended in section "Base-32 characters") as an annotation
telling how to convert the folded Unicode string into a mixed-case
Unicode string for display purposes.
Each non-LDH code point is represented by a sequence of base-32
characters, the last of which is always a letter (as opposed to
a digit). If that letter is uppercase, it is a suggestion that
the Unicode character be mapped to uppercase (if possible); if the
letter is lowercase, it is a suggestion that the Unicode character
be mapped to lowercase (if possible).
AMC-ACE-R encoders and decoders are not required to support these
annotations, and higher layers need not use them.
Comparison with other ACEs
Please refer to the comparison in [AMCACEW].
Example strings
In the ACE encodings below, no signatures are shown. AMC-ACE-R is
abbreviated AMC-R. Backslashes show where line breaks have been
inserted in strings too long for one line.
The first several examples are all translations of the sentence "Why
can't they just speak in ?" (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
AMC-R: ywekhfuhuikwdwefivevjbuiwktr
(B) Chinese (simplified):
u+4ED6 u+4EEC u+4E3A u+4EC0 u+4E48 u+4E0D u+8BF4 u+4E2D u+6587
AMC-R: w87g8nvk6awisp259eupyx2h
(C) Czech: Proprostnemluvesky
U+0050 u+0072 u+006F u+010D u+0070 u+0072 u+006F u+0073 u+0074
u+011B u+006E u+0065 u+006D u+006C u+0075 u+0076 u+00ED u+010D
u+0065 u+0073 u+006B u+0079
AMC-R: -Pro-yp-prost-tm-nemluv-s8pp-esky
(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-R: x7nqeep8e8j7f7inaqdb8ijp8cb8ij8k
(E) Hindi (Devanagari):
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
AMC-R: 3urvjvcwmthjruiwpugwatfwpurmscuivjascunmvcvitfuewhjwisc
(F) Japanese (kanji and hiragana):
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
AMC-R: vsykxnzr3dkyx8fyzun243q3c24zbxhgwr2nkweqwm
(G) Korean (Hangul syllables):
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
AMC-R: 6tvi466ezxi544i5w8a6s4nz2nw8e6zze7xxn47yp6x5e53znze7xze\
7xxn5u8e54ze6x5n36is3i622m6zwe48wn
(H) Russian (Cyrillic):
U+043F 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
AMC-R: wvRqwhfnwdgfqpipfdqcqwawrcvrvqwawdbbvkvi
(I) Spanish: PorqunopuedensimplementehablarenEspaol
U+0050 u+006F u+0072 u+0071 u+0075 u+00E9 u+006E u+006F u+0070
u+0075 u+0065 u+0064 u+0065 u+006E u+0073 u+0069 u+006D u+0070
u+006C u+0065 u+006D u+0065 u+006E u+0074 u+0065 u+0068 u+0061
u+0062 u+006C u+0061 u+0072 u+0065 u+006E U+0045 u+0073 u+0070
u+0061 u+00F1 u+006F u+006C
AMC-R: -Porqu-j-nopuedensimplementehablarenEspa-9b-ol
(J) Taiwanese:
u+4ED6 u+5011 u+7232 u+4EC0 u+9EBD u+4E0D u+8AAA u+4E2D u+6587
AMC-R: w87gxstbzuvc6a385psp244kupyx2h
(K) Vietnamese:
Taisaohokhngthchi\
noitingVit
U+0054 u+0061 u+0323 u+0069 u+0073 u+0061 u+006F u+0068 u+006F
u+0323 u+006B u+0068 u+00F4 u+006E u+0067 u+0074 u+0068 u+00EA
u+0309 u+0063 u+0068 u+0069 u+0309 u+006E u+006F u+0301 u+0069
u+0074 u+0069 u+00EA u+0301 u+006E u+0067 U+0056 u+0069 u+00EA
u+0323 u+0074
AMC-R: -Ta-vud-isaoho-d-kh-s9e-ngth-s8kvsj-chi-vsj-no-b-iti-s8\
kb-ngVi-s8kud-t
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) 3B
u+0033 u+5E74 U+0042 u+7D44 u+91D1 u+516B u+5148 u+751F
AMC-R: -3-x8ze-B-z7we3t7btymtwizxtr
(M) -with-SUPER-MONKEYS
u+5B89 u+5BA4 u+5948 u+7F8E u+6075 u+002D u+0077 u+0069 u+0074
u+0068 u+002D U+0053 U+0055 U+0050 U+0045 U+0052 u+002D U+004D
U+004F U+004E U+004B U+0045 U+0059 U+0053
AMC-R: x52j4e3wiz92qyszf---with--SUPER--MONKEYS
(N) Hello-Another-Way-
U+0048 u+0065 u+006C u+006C u+006F u+002D U+0041 u+006E u+006F
u+0074 u+0068 u+0065 u+0072 u+002D U+0057 u+0061 u+0079 u+002D
u+305D u+308C u+305E u+308C u+306E u+5834 u+6240
AMC-R: -Hello--Another--Way---vsxp2nq2nyqx2veyuwa
(O) 2
u+3072 u+3068 u+3064 u+5C4B u+6839 u+306E u+4E0B u+0032
AMC-R: vszcyiyex6wmy2vjqw8sm-2
(P) MajiKoi5
U+004D u+0061 u+006A u+0069 u+3067 U+004B u+006F u+0069 u+3059
u+308B u+0035 u+79D2 u+524D
AMC-R: -Maji-vsyh-Koi-xj2m-5-z37cxuwp
(Q) de
u+30D1 u+30D5 u+30A3 u+30FC u+0064 u+0065 u+30EB u+30F3 u+30D0
AMC-R: vs7bf4d9n-de-8m9d7a
(R)
u+305D u+306E u+30B9 u+30D4 u+30FC u+30C9 u+3067
AMC-R: vsxpyq5j7e9n6jyh
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 <-
u+002D u+003E u+0020 u+0024 u+0031 u+002E u+0030 u+0030 u+0020
u+003C u+002D
AMC-R: --svquaue-1-q-00-avn--
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].
Acknowledgements
AMC-ACE-R reuses a number of preexisting techniques.
The basic encoding of integers to quartets to quintets to base-32
comes from UTF-5 [UTF5], and the particular variant used here comes
from AMC-ACE-M [AMCACEM].
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 [BRACE].
The general idea of using the alphabetic case of base-32 characters
to indicate 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
[AMCACEM] Adam Costello, "AMC-ACE-M version 0.1.4", 2001-Apr-01,
update of draft-ietf-idn-amc-ace-m-00, latest version at
http://www.cs.berkeley.edu/~amc/charset/amc-ace-m.
[AMCACEW] Adam Costello, "AMC-ACE-W version 0.1.0",
2001-May-31, draft-ietf-idn-amc-ace-w-00, latest version at
http://www.cs.berkeley.edu/~amc/charset/amc-ace-w.
[BRACE] Adam Costello, "BRACE: Bi-mode Row-based
ASCII-Compatible Encoding for IDN version 0.1.2",
2000-Sep-19, draft-ietf-idn-brace-00, version at
http://www.cs.berkeley.edu/~amc/charset/brace.
[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.
[IDNA] Patrik Faltstrom, Paul Hoffman, "Internationalizing Host
Names In Applications (IDNA)", draft-ietf-idn-idna-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.
[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-*.
[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/.
Author
Adam M. Costello
http://www.cs.berkeley.edu/~amc/
Example implementation
/******************************************/
/* amc-ace-r.c 0.2.1 (2001-May-31-Thu) */
/* Adam M. Costello */
/******************************************/
/* This is ANSI C code (C89) implementing AMC-ACE-R version 0.2.*. */
/************************************************************/
/* Public interface (would normally go in its own .h file): */
#include
enum amc_ace_status {
amc_ace_success,
amc_ace_bad_input,
amc_ace_big_output /* Output would exceed the space provided. */
};
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
/* is_ldh(n) returns 1 if the code point n represents an LDH */
/* character (ASCII letter, digit, or hyphen-minus), 0 otherwise. */
static int is_ldh(u_code_point n)
{
return n <= 122 && ( n >= 97 || n == 45 ||
(n >= 48 && n <= 57) || (n >= 65 && n <= 90) );
}
/* base32[q] is the lowercase base-32 character representing */
/* the number q 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(refpoint,updated,history,latest) updates refpoint[1..3] */
/* based on the updated flag and history[0..latest]. */
static void update( u_code_point refpoint[6], unsigned int *updated,
const u_code_point history[], unsigned int latest )
{
unsigned int k, b, i;
for (k = 1; k <= 3; ++k) {
b = k << 2;
/* The first time here change all the windows: */
if (!*updated) refpoint[k] = (history[latest] >> b) << b;
else for (i = latest; i-- > 0; ) {
if (is_ldh(history[i])) continue;
/* If a code point falling in the existing window has appeared */
/* at least as recently as one falling in the candidate window, */
/* then leave this window unchanged and go on to the next one: */
if ((refpoint[k] ^ history[i]) >> b == 0) break;
if ((history[latest] ^ history[i]) >> b == 0) {
/* A code point falling in the candidate window has appeared */
/* more recently than one falling in the existing window, so */
/* change this window (and no others): */
refpoint[k] = (history[latest] >> b) << b;
goto update_end;
}
}
}
update_end: *updated = 1;
}
/* 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 literal, updated, max_out, in, out, k, j;
u_code_point n, delta;
char shift;
/* Initialize the state: */
u_code_point refpoint[6] = {0, 0xE0, 0xA0, 0, 0, 0x10000};
literal = updated = 0;
max_out = *output_size;
for (in = out = 0; in < input_length; ++in) {
/* At the start of each iteration, in and out are the number of */
/* items already input/output, or equivalently, the indices of */
/* the next items to be input/output. */
n = input[in];
/* Check the code point range to avoid array bounds errors later: */
if (n > 0x10FFFF) return amc_ace_bad_input;
if (n == 0x2D) {
/* Hyphen-minus is doubled. */
if (max_out - out < 2) return amc_ace_big_output;
output[out++] = 0x2D;
output[out++] = 0x2D;
}
else if (is_ldh(n)) {
/* Encode an LDH character literally. */
if (max_out - out < 1 + !literal) return amc_ace_big_output;
/* Switch to literal mode if necessary: */
if (!literal) output[out++] = 0x2D;
literal = 1;
output[out++] = n;
}
else {
/* Encode a non-LDH character using base-32. */
/* First compute the number of base-32 characters (k): */
for (k = 1; ; ++k) {
delta = n - refpoint[k];
if (delta >> (4*k) == 0) break;
}
if (max_out - out < k + literal) return amc_ace_big_output;
/* Switch to base-32 mode if necessary: */
if (literal) output[out++] = 0x2D;
literal = 0;
shift = uppercase_flags && uppercase_flags[in] ? 32 : 0;
/* Each quintet has the form 1xxxx except the last is 0xxxx. */
/* Computing the base-32 digits in reverse order is easiest. */
out += k;
output[out - 1] = base32[delta & 0xF] - shift;
for (j = 2; j <= k; ++j) {
delta >>= 4;
output[out - j] = base32[0x10 | (delta & 0xF)];
}
update(refpoint, &updated, input, in);
}
}
/* Append the null terminator: */
if (max_out - out < 1) return amc_ace_big_output;
output[out++] = 0;
*output_size = 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;
char c;
unsigned int literal, updated, max_out, in, out, k, scratch_size;
enum amc_ace_status status;
/* Initialize the state: */
u_code_point refpoint[6] = {0, 0xE0, 0xA0, 0, 0, 0x10000};
literal = updated = 0;
max_out = *output_length;
for (c = input[in = 0], out = 0; c != 0; c = input[++in], ++out) {
/* At the start of each iteration, in and out are the number of */
/* items already input/output, or equivalently, the indices of */
/* the next items to be input/output. c is the same as input[in]. */
if (c == 0x2D && input[in + 1] != 0x2D) {
/* Unpaired hyphen-minus toggles mode. */
literal = !literal;
c = input[++in];
}
if (max_out - out < 1) return amc_ace_big_output;
if (c == 0x2D) {
/* Double hyphen-minus represents a hyphen-minus. */
++in;
output[out] = 0x2D;
}
else {
if (literal) output[out] = c;
else {
/* Decode a base-32 sequence. */
/* First decode quintets until 0xxxx is found: */
for (delta = 0, k = 1; ; c = input[++in], ++k) {
q = base32_decode(c);
if (q == base32_invalid || k > 5) return amc_ace_bad_input;
delta = (delta << 4) | (q & 0xF);
if (q >> 4 == 0) break;
}
output[out] = refpoint[k] + delta;
update(refpoint, &updated, output, out);
}
}
/* Case of last character determines uppercase flag: */
if (uppercase_flags) uppercase_flags[out] = c >= 65 && c <= 90;
}
/* Enforce the uniqueness of the encoding by re-encoding */
/* the output and comparing the result to the input: */
scratch_size = ++in;
status = amc_ace_r_encode(out, output, uppercase_flags,
&scratch_size, scratch_space);
if (status != amc_ace_success || scratch_size != in ||
unequal(case_sensitivity, scratch_space, input)
) return amc_ace_bad_input;
*output_length = out;
return amc_ace_success;
}
/******************************************************************/
/* Wrapper for testing (would normally go in a separate .c file): */
#include
#include
#include
#include
/* 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
/* suitable for host names */
};
static void usage(char **argv)
{
fprintf(stderr,
"%s -e reads code points and writes an AMC-ACE-R string.\n"
"%s -d reads an AMC-ACE-R string and writes code points.\n"
"Input and output are plain text in the native character set.\n"
"Code points are in the form u+hex separated by whitespace.\n"
"An AMC-ACE-R string is a newline-terminated sequence of LDH\n"
"characters (without any signature).\n"
"The case of the u in u+hex is the 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";
/* The following string is used to convert LDH */
/* characters between ASCII and the native charset: */
static const char ldh_ascii[] =
"................"
"................"
".............-.."
"0123456789......"
".ABCDEFGHIJKLMNO"
"PQRSTUVWXYZ....."
".abcdefghijklmno"
"pqrstuvwxyz";
int main(int argc, char **argv)
{
enum amc_ace_status status;
int r;
char *p;
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 long codept;
unsigned char uppercase_flags[unicode_max_length];
char output[ace_max_size], uplus[3];
unsigned int input_length, output_size, i;
/* Read the input code points: */
input_length = 0;
for (;;) {
r = scanf("%2s%lx", uplus, &codept);
if (ferror(stdin)) fail(io_error);
if (r == EOF || r == 0) break;
if (r != 2 || uplus[1] != '+' || codept > (u_code_point)-1) {
fail(invalid_input);
}
if (input_length == unicode_max_length) fail(too_big);
if (uplus[0] == 'u') uppercase_flags[input_length] = 0;
else if (uplus[0] == 'U') uppercase_flags[input_length] = 1;
else fail(invalid_input);
input[input_length++] = codept;
}
/* Encode: */
output_size = ace_max_size;
status = amc_ace_r_encode(input_length, input, uppercase_flags,
&output_size, output);
if (status == amc_ace_bad_input) fail(invalid_input);
if (status == amc_ace_big_output) fail(too_big);
assert(status == amc_ace_success);
/* Convert to native charset and output: */
for (p = output; *p != 0; ++p) {
i = *p;
assert(i <= 122 && ldh_ascii[i] != '.');
*p = ldh_ascii[i];
}
r = puts(output);
if (r == EOF) fail(io_error);
return EXIT_SUCCESS;
}
if (argv[1][1] == 'd') {
char input[ace_max_size], scratch[ace_max_size], *pp;
u_code_point output[unicode_max_length];
unsigned char uppercase_flags[unicode_max_length];
unsigned int input_length, output_length, i;
/* Read the AMC-ACE-R input string and convert to ASCII: */
fgets(input, ace_max_size, stdin);
if (ferror(stdin)) fail(io_error);
if (feof(stdin)) fail(invalid_input);
input_length = strlen(input);
if (input[input_length - 1] != '\n') fail(too_big);
input[--input_length] = 0;
for (p = input; *p != 0; ++p) {
pp = strchr(ldh_ascii, *p);
if (pp == 0) fail(invalid_input);
*p = pp - ldh_ascii;
}
/* Decode: */
output_length = unicode_max_length;
status = amc_ace_r_decode(test_case_sensitivity, scratch, input,
&output_length, output, uppercase_flags);
if (status == amc_ace_bad_input) fail(invalid_input);
if (status == amc_ace_big_output) fail(too_big);
assert(status == amc_ace_success);
/* Output the result: */
for (i = 0; i < output_length; ++i) {
r = printf("%s+%04lX\n",
uppercase_flags[i] ? "U" : "u",
(unsigned long) output[i] );
if (r < 0) fail(io_error);
}
return EXIT_SUCCESS;
}
usage(argv);
return EXIT_SUCCESS; /* not reached, but quiets compiler warning */
}
INTERNET-DRAFT expires 2001-Nov-30