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diff --git a/doc/rfc/rfc3492.txt b/doc/rfc/rfc3492.txt new file mode 100644 index 0000000..e72ad81 --- /dev/null +++ b/doc/rfc/rfc3492.txt @@ -0,0 +1,1963 @@ + + + + + + +Network Working Group A. Costello +Request for Comments: 3492 Univ. of California, Berkeley +Category: Standards Track March 2003 + + + Punycode: A Bootstring encoding of Unicode + for Internationalized Domain Names in Applications (IDNA) + +Status of this Memo + + This document specifies an Internet standards track protocol for the + Internet community, and requests discussion and suggestions for + improvements. Please refer to the current edition of the "Internet + Official Protocol Standards" (STD 1) for the standardization state + and status of this protocol. Distribution of this memo is unlimited. + +Copyright Notice + + Copyright (C) The Internet Society (2003). All Rights Reserved. + +Abstract + + Punycode is a simple and efficient transfer encoding syntax designed + for use with Internationalized Domain Names in Applications (IDNA). + It uniquely and reversibly transforms a Unicode string into an ASCII + string. ASCII characters in the Unicode string are represented + literally, and non-ASCII characters are represented by ASCII + characters that are allowed in host name labels (letters, digits, and + hyphens). This document defines a general algorithm called + Bootstring that allows a string of basic code points to uniquely + represent any string of code points drawn from a larger set. + Punycode is an instance of Bootstring that uses particular parameter + values specified by this document, appropriate for IDNA. + +Table of Contents + + 1. Introduction...............................................2 + 1.1 Features..............................................2 + 1.2 Interaction of protocol parts.........................3 + 2. Terminology................................................3 + 3. Bootstring description.....................................4 + 3.1 Basic code point segregation..........................4 + 3.2 Insertion unsort coding...............................4 + 3.3 Generalized variable-length integers..................5 + 3.4 Bias adaptation.......................................7 + 4. Bootstring parameters......................................8 + 5. Parameter values for Punycode..............................8 + 6. Bootstring algorithms......................................9 + + + +Costello Standards Track [Page 1] + +RFC 3492 IDNA Punycode March 2003 + + + 6.1 Bias adaptation function.............................10 + 6.2 Decoding procedure...................................11 + 6.3 Encoding procedure...................................12 + 6.4 Overflow handling....................................13 + 7. Punycode examples.........................................14 + 7.1 Sample strings.......................................14 + 7.2 Decoding traces......................................17 + 7.3 Encoding traces......................................19 + 8. Security Considerations...................................20 + 9. References................................................21 + 9.1 Normative References.................................21 + 9.2 Informative References...............................21 + A. Mixed-case annotation.....................................22 + B. Disclaimer and license....................................22 + C. Punycode sample implementation............................23 + Author's Address.............................................34 + Full Copyright Statement.....................................35 + +1. Introduction + + [IDNA] describes an architecture for supporting internationalized + domain names. Labels containing non-ASCII characters can be + represented by ACE labels, which begin with a special ACE prefix and + contain only ASCII characters. The remainder of the label after the + prefix is a Punycode encoding of a Unicode string satisfying certain + constraints. For the details of the prefix and constraints, see + [IDNA] and [NAMEPREP]. + + Punycode is an instance of a more general algorithm called + Bootstring, which allows strings composed from a small set of "basic" + code points to uniquely represent any string of code points drawn + from a larger set. Punycode is Bootstring with particular parameter + values appropriate for IDNA. + +1.1 Features + + Bootstring has been designed to have the following features: + + * Completeness: Every extended string (sequence of arbitrary code + points) can be represented by a basic string (sequence of basic + code points). Restrictions on what strings are allowed, and on + length, can be imposed by higher layers. + + * Uniqueness: There is at most one basic string that represents a + given extended string. + + * Reversibility: Any extended string mapped to a basic string can + be recovered from that basic string. + + + +Costello Standards Track [Page 2] + +RFC 3492 IDNA Punycode March 2003 + + + * Efficient encoding: The ratio of basic string length to extended + string length is small. This is important in the context of + domain names because RFC 1034 [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; Bootstring aims at a good balance between them. + + * Readability: Basic code points appearing in the extended string + are represented as themselves in the basic string (although the + main purpose is to improve efficiency, not readability). + + Punycode can also support an additional feature that is not used by + the ToASCII and ToUnicode operations of [IDNA]. When extended + strings are case-folded prior to encoding, the basic string can use + mixed case to tell how to convert the folded string into a mixed-case + string. See appendix A "Mixed-case annotation". + +1.2 Interaction of protocol parts + + Punycode is used by the IDNA protocol [IDNA] for converting domain + labels into ASCII; it is not designed for any other purpose. It is + explicitly not designed for processing arbitrary free text. + +2. Terminology + + The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", + "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this + document are to be interpreted as described in BCP 14, RFC 2119 + [RFC2119]. + + A code point is an integral value associated with a character in a + coded character set. + + 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. + + The operators div and mod perform integer division; (x div y) is the + quotient of x divided by y, discarding the remainder, and (x mod y) + is the remainder, so (x div y) * y + (x mod y) == x. Bootstring uses + these operators only with nonnegative operands, so the quotient and + remainder are always nonnegative. + + The break statement jumps out of the innermost loop (as in C). + + + + +Costello Standards Track [Page 3] + +RFC 3492 IDNA Punycode March 2003 + + + An overflow is an attempt to compute a value that exceeds the maximum + value of an integer variable. + +3. Bootstring description + + Bootstring represents an arbitrary sequence of code points (the + "extended string") as a sequence of basic code points (the "basic + string"). This section describes the representation. Section 6 + "Bootstring algorithms" presents the algorithms as pseudocode. + Sections 7.1 "Decoding traces" and 7.2 "Encoding traces" trace the + algorithms for sample inputs. + + The following sections describe the four techniques used in + Bootstring. "Basic code point segregation" is a very simple and + efficient encoding for basic code points occurring in the extended + string: they are simply copied all at once. "Insertion unsort + coding" encodes the non-basic code points as deltas, and processes + the code points in numerical order rather than in order of + appearance, which typically results in smaller deltas. The deltas + are represented as "generalized variable-length integers", which use + basic code points to represent nonnegative integers. The parameters + of this integer representation are dynamically adjusted using "bias + adaptation", to improve efficiency when consecutive deltas have + similar magnitudes. + +3.1 Basic code point segregation + + All basic code points appearing in the extended string are + represented literally at the beginning of the basic string, in their + original order, followed by a delimiter if (and only if) the number + of basic code points is nonzero. The delimiter is a particular basic + code point, which never appears in the remainder of the basic string. + The decoder can therefore find the end of the literal portion (if + there is one) by scanning for the last delimiter. + +3.2 Insertion unsort coding + + The remainder of the basic string (after the last delimiter if there + is one) represents a sequence of nonnegative integral deltas as + generalized variable-length integers, described in section 3.3. The + meaning of the deltas is best understood in terms of the decoder. + + The decoder builds the extended string incrementally. Initially, the + extended string is a copy of the literal portion of the basic string + (excluding the last delimiter). The decoder inserts non-basic code + points, one for each delta, into the extended string, ultimately + arriving at the final decoded string. + + + + +Costello Standards Track [Page 4] + +RFC 3492 IDNA Punycode March 2003 + + + At the heart of this process is a state machine with two state + variables: an index i and a counter n. The index i refers to a + position in the extended string; it ranges from 0 (the first + position) to the current length of the extended string (which refers + to a potential position beyond the current end). If the current + state is <n,i>, the next state is <n,i+1> if i is less than the + length of the extended string, or <n+1,0> if i equals the length of + the extended string. In other words, each state change causes i to + increment, wrapping around to zero if necessary, and n counts the + number of wrap-arounds. + + Notice that the state always advances monotonically (there is no way + for the decoder to return to an earlier state). At each state, an + insertion is either performed or not performed. At most one + insertion is performed in a given state. An insertion inserts the + value of n at position i in the extended string. The deltas are a + run-length encoding of this sequence of events: they are the lengths + of the runs of non-insertion states preceeding the insertion states. + Hence, for each delta, the decoder performs delta state changes, then + an insertion, and then one more state change. (An implementation + need not perform each state change individually, but can instead use + division and remainder calculations to compute the next insertion + state directly.) It is an error if the inserted code point is a + basic code point (because basic code points were supposed to be + segregated as described in section 3.1). + + The encoder's main task is to derive the sequence of deltas that will + cause the decoder to construct the desired string. It can do this by + repeatedly scanning the extended string for the next code point that + the decoder would need to insert, and counting the number of state + changes the decoder would need to perform, mindful of the fact that + the decoder's extended string will include only those code points + that have already been inserted. Section 6.3 "Encoding procedure" + gives a precise algorithm. + +3.3 Generalized variable-length integers + + In a conventional integer representation the base is the number of + distinct symbols for digits, whose values are 0 through base-1. Let + digit_0 denote the least significant digit, digit_1 the next least + significant, and so on. The value represented is the sum over j of + digit_j * w(j), where w(j) = base^j is the weight (scale factor) for + position j. For example, in the base 8 integer 437, the digits are + 7, 3, and 4, and the weights are 1, 8, and 64, so the value is 7 + + 3*8 + 4*64 = 287. This representation has two disadvantages: First, + there are multiple encodings of each value (because there can be + extra zeros in the most significant positions), which is inconvenient + + + + +Costello Standards Track [Page 5] + +RFC 3492 IDNA Punycode March 2003 + + + when unique encodings are needed. Second, the integer is not self- + delimiting, so if multiple integers are concatenated the boundaries + between them are lost. + + The generalized variable-length representation solves these two + problems. The digit values are still 0 through base-1, but now the + integer is self-delimiting by means of thresholds t(j), each of which + is in the range 0 through base-1. Exactly one digit, the most + significant, satisfies digit_j < t(j). Therefore, if several + integers are concatenated, it is easy to separate them, starting with + the first if they are little-endian (least significant digit first), + or starting with the last if they are big-endian (most significant + digit first). As before, the value is the sum over j of digit_j * + w(j), but the weights are different: + + w(0) = 1 + w(j) = w(j-1) * (base - t(j-1)) for j > 0 + + For example, consider the little-endian sequence of base 8 digits + 734251... Suppose the thresholds are 2, 3, 5, 5, 5, 5... This + implies that the weights are 1, 1*(8-2) = 6, 6*(8-3) = 30, 30*(8-5) = + 90, 90*(8-5) = 270, and so on. 7 is not less than 2, and 3 is not + less than 3, but 4 is less than 5, so 4 is the last digit. The value + of 734 is 7*1 + 3*6 + 4*30 = 145. The next integer is 251, with + value 2*1 + 5*6 + 1*30 = 62. Decoding this representation is very + similar to decoding a conventional integer: Start with a current + value of N = 0 and a weight w = 1. Fetch the next digit d and + increase N by d * w. If d is less than the current threshold (t) + then stop, otherwise increase w by a factor of (base - t), update t + for the next position, and repeat. + + Encoding this representation is similar to encoding a conventional + integer: If N < t then output one digit for N and stop, otherwise + output the digit for t + ((N - t) mod (base - t)), then replace N + with (N - t) div (base - t), update t for the next position, and + repeat. + + For any particular set of values of t(j), there is exactly one + generalized variable-length representation of each nonnegative + integral value. + + Bootstring uses little-endian ordering so that the deltas can be + separated starting with the first. The t(j) values are defined in + terms of the constants base, tmin, and tmax, and a state variable + called bias: + + t(j) = base * (j + 1) - bias, + clamped to the range tmin through tmax + + + +Costello Standards Track [Page 6] + +RFC 3492 IDNA Punycode March 2003 + + + The clamping means that if the formula yields a value less than tmin + or greater than tmax, then t(j) = tmin or tmax, respectively. (In + the pseudocode in section 6 "Bootstring algorithms", the expression + base * (j + 1) is denoted by k for performance reasons.) These t(j) + values cause the representation to favor integers within a particular + range determined by the bias. + +3.4 Bias adaptation + + After each delta is encoded or decoded, bias is set for the next + delta as follows: + + 1. Delta is scaled in order to avoid overflow in the next step: + + let delta = delta div 2 + + But when this is the very first delta, the divisor is not 2, but + instead a constant called damp. This compensates for the fact + that the second delta is usually much smaller than the first. + + 2. Delta is increased to compensate for the fact that the next delta + will be inserting into a longer string: + + let delta = delta + (delta div numpoints) + + numpoints is the total number of code points encoded/decoded so + far (including the one corresponding to this delta itself, and + including the basic code points). + + 3. Delta is repeatedly divided until it falls within a threshold, to + predict the minimum number of digits needed to represent the next + delta: + + while delta > ((base - tmin) * tmax) div 2 + do let delta = delta div (base - tmin) + + 4. The bias is set: + + let bias = + (base * the number of divisions performed in step 3) + + (((base - tmin + 1) * delta) div (delta + skew)) + + The motivation for this procedure is that the current delta + provides a hint about the likely size of the next delta, and so + t(j) is set to tmax for the more significant digits starting with + the one expected to be last, tmin for the less significant digits + up through the one expected to be third-last, and somewhere + between tmin and tmax for the digit expected to be second-last + + + +Costello Standards Track [Page 7] + +RFC 3492 IDNA Punycode March 2003 + + + (balancing the hope of the expected-last digit being unnecessary + against the danger of it being insufficient). + +4. Bootstring parameters + + Given a set of basic code points, one needs to be designated as the + delimiter. The base cannot be greater than the number of + distinguishable basic code points remaining. The digit-values in the + range 0 through base-1 need to be associated with distinct non- + delimiter basic code points. In some cases multiple code points need + to have the same digit-value; for example, uppercase and lowercase + versions of the same letter need to be equivalent if basic strings + are case-insensitive. + + The initial value of n cannot be greater than the minimum non-basic + code point that could appear in extended strings. + + The remaining five parameters (tmin, tmax, skew, damp, and the + initial value of bias) need to satisfy the following constraints: + + 0 <= tmin <= tmax <= base-1 + skew >= 1 + damp >= 2 + initial_bias mod base <= base - tmin + + Provided the constraints are satisfied, these five parameters affect + efficiency but not correctness. They are best chosen empirically. + + If support for mixed-case annotation is desired (see appendix A), + make sure that the code points corresponding to 0 through tmax-1 all + have both uppercase and lowercase forms. + +5. Parameter values for Punycode + + Punycode uses the following Bootstring parameter values: + + base = 36 + tmin = 1 + tmax = 26 + skew = 38 + damp = 700 + initial_bias = 72 + initial_n = 128 = 0x80 + + Although the only restriction Punycode imposes on the input integers + is that they be nonnegative, these parameters are especially designed + to work well with Unicode [UNICODE] code points, which are integers + in the range 0..10FFFF (but not D800..DFFF, which are reserved for + + + +Costello Standards Track [Page 8] + +RFC 3492 IDNA Punycode March 2003 + + + use by the UTF-16 encoding of Unicode). The basic code points are + the ASCII [ASCII] code points (0..7F), of which U+002D (-) is the + delimiter, and some of the others have digit-values as follows: + + code points digit-values + ------------ ---------------------- + 41..5A (A-Z) = 0 to 25, respectively + 61..7A (a-z) = 0 to 25, respectively + 30..39 (0-9) = 26 to 35, respectively + + Using hyphen-minus as the delimiter implies that the encoded string + can end with a hyphen-minus only if the Unicode string consists + entirely of basic code points, but IDNA forbids such strings from + being encoded. The encoded string can begin with a hyphen-minus, but + IDNA prepends a prefix. Therefore IDNA using Punycode conforms to + the RFC 952 rule that host name labels neither begin nor end with a + hyphen-minus [RFC952]. + + A decoder MUST recognize the letters in both uppercase and lowercase + forms (including mixtures of both forms). An encoder SHOULD output + only uppercase forms or only lowercase forms, unless it uses mixed- + case annotation (see appendix A). + + Presumably most users will not manually write or type encoded strings + (as opposed to cutting and pasting them), but those who do will need + to be alert to the potential visual ambiguity between the following + sets of characters: + + G 6 + I l 1 + O 0 + S 5 + U V + Z 2 + + Such ambiguities are usually resolved by context, but in a Punycode + encoded string there is no context apparent to humans. + +6. Bootstring algorithms + + Some parts of the pseudocode can be omitted if the parameters satisfy + certain conditions (for which Punycode qualifies). These parts are + enclosed in {braces}, and notes immediately following the pseudocode + explain the conditions under which they can be omitted. + + + + + + + +Costello Standards Track [Page 9] + +RFC 3492 IDNA Punycode March 2003 + + + Formally, code points are integers, and hence the pseudocode assumes + that arithmetic operations can be performed directly on code points. + In some programming languages, explicit conversion between code + points and integers might be necessary. + +6.1 Bias adaptation function + + function adapt(delta,numpoints,firsttime): + if firsttime then let delta = delta div damp + else let delta = delta div 2 + let delta = delta + (delta div numpoints) + let k = 0 + while delta > ((base - tmin) * tmax) div 2 do begin + let delta = delta div (base - tmin) + let k = k + base + end + return k + (((base - tmin + 1) * delta) div (delta + skew)) + + It does not matter whether the modifications to delta and k inside + adapt() affect variables of the same name inside the + encoding/decoding procedures, because after calling adapt() the + caller does not read those variables before overwriting them. + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +Costello Standards Track [Page 10] + +RFC 3492 IDNA Punycode March 2003 + + +6.2 Decoding procedure + + let n = initial_n + let i = 0 + let bias = initial_bias + let output = an empty string indexed from 0 + consume all code points before the last delimiter (if there is one) + and copy them to output, fail on any non-basic code point + if more than zero code points were consumed then consume one more + (which will be the last delimiter) + while the input is not exhausted do begin + let oldi = i + let w = 1 + for k = base to infinity in steps of base do begin + consume a code point, or fail if there was none to consume + let digit = the code point's digit-value, fail if it has none + let i = i + digit * w, fail on overflow + let t = tmin if k <= bias {+ tmin}, or + tmax if k >= bias + tmax, or k - bias otherwise + if digit < t then break + let w = w * (base - t), fail on overflow + end + let bias = adapt(i - oldi, length(output) + 1, test oldi is 0?) + let n = n + i div (length(output) + 1), fail on overflow + let i = i mod (length(output) + 1) + {if n is a basic code point then fail} + insert n into output at position i + increment i + end + + The full statement enclosed in braces (checking whether n is a basic + code point) can be omitted if initial_n exceeds all basic code points + (which is true for Punycode), because n is never less than initial_n. + + In the assignment of t, where t is clamped to the range tmin through + tmax, "+ tmin" can always be omitted. This makes the clamping + calculation incorrect when bias < k < bias + tmin, but that cannot + happen because of the way bias is computed and because of the + constraints on the parameters. + + Because the decoder state can only advance monotonically, and there + is only one representation of any delta, there is therefore only one + encoded string that can represent a given sequence of integers. The + only error conditions are invalid code points, unexpected end-of- + input, overflow, and basic code points encoded using deltas instead + of appearing literally. If the decoder fails on these errors as + shown above, then it cannot produce the same output for two distinct + inputs. Without this property it would have been necessary to re- + + + +Costello Standards Track [Page 11] + +RFC 3492 IDNA Punycode March 2003 + + + encode the output and verify that it matches the input in order to + guarantee the uniqueness of the encoding. + +6.3 Encoding procedure + + let n = initial_n + let delta = 0 + let bias = initial_bias + let h = b = the number of basic code points in the input + copy them to the output in order, followed by a delimiter if b > 0 + {if the input contains a non-basic code point < n then fail} + while h < length(input) do begin + let m = the minimum {non-basic} code point >= n in the input + let delta = delta + (m - n) * (h + 1), fail on overflow + let n = m + for each code point c in the input (in order) do begin + if c < n {or c is basic} then increment delta, fail on overflow + if c == n then begin + let q = delta + for k = base to infinity in steps of base do begin + let t = tmin if k <= bias {+ tmin}, or + tmax if k >= bias + tmax, or k - bias otherwise + if q < t then break + output the code point for digit t + ((q - t) mod (base - t)) + let q = (q - t) div (base - t) + end + output the code point for digit q + let bias = adapt(delta, h + 1, test h equals b?) + let delta = 0 + increment h + end + end + increment delta and n + end + + The full statement enclosed in braces (checking whether the input + contains a non-basic code point less than n) can be omitted if all + code points less than initial_n are basic code points (which is true + for Punycode if code points are unsigned). + + The brace-enclosed conditions "non-basic" and "or c is basic" can be + omitted if initial_n exceeds all basic code points (which is true for + Punycode), because the code point being tested is never less than + initial_n. + + In the assignment of t, where t is clamped to the range tmin through + tmax, "+ tmin" can always be omitted. This makes the clamping + calculation incorrect when bias < k < bias + tmin, but that cannot + + + +Costello Standards Track [Page 12] + +RFC 3492 IDNA Punycode March 2003 + + + happen because of the way bias is computed and because of the + constraints on the parameters. + + The checks for overflow are necessary to avoid producing invalid + output when the input contains very large values or is very long. + + The increment of delta at the bottom of the outer loop cannot + overflow because delta < length(input) before the increment, and + length(input) is already assumed to be representable. The increment + of n could overflow, but only if h == length(input), in which case + the procedure is finished anyway. + +6.4 Overflow handling + + For IDNA, 26-bit unsigned integers are sufficient to handle all valid + IDNA labels without overflow, because any string that needed a 27-bit + delta would have to exceed either the code point limit (0..10FFFF) or + the label length limit (63 characters). However, overflow handling + is necessary because the inputs are not necessarily valid IDNA + labels. + + If the programming language does not provide overflow detection, the + following technique can be used. Suppose A, B, and C are + representable nonnegative integers and C is nonzero. Then A + B + overflows if and only if B > maxint - A, and A + (B * C) overflows if + and only if B > (maxint - A) div C, where maxint is the greatest + integer for which maxint + 1 cannot be represented. Refer to + appendix C "Punycode sample implementation" for demonstrations of + this technique in the C language. + + The decoding and encoding algorithms shown in sections 6.2 and 6.3 + handle overflow by detecting it whenever it happens. Another + approach is to enforce limits on the inputs that prevent overflow + from happening. For example, if the encoder were to verify that no + input code points exceed M and that the input length does not exceed + L, then no delta could ever exceed (M - initial_n) * (L + 1), and + hence no overflow could occur if integer variables were capable of + representing values that large. This prevention approach would + impose more restrictions on the input than the detection approach + does, but might be considered simpler in some programming languages. + + In theory, the decoder could use an analogous approach, limiting the + number of digits in a variable-length integer (that is, limiting the + number of iterations in the innermost loop). However, the number of + digits that suffice to represent a given delta can sometimes + represent much larger deltas (because of the adaptation), and hence + this approach would probably need integers wider than 32 bits. + + + + +Costello Standards Track [Page 13] + +RFC 3492 IDNA Punycode March 2003 + + + Yet another approach for the decoder is to allow overflow to occur, + but to check the final output string by re-encoding it and comparing + to the decoder input. If and only if they do not match (using a + case-insensitive ASCII comparison) overflow has occurred. This + delayed-detection approach would not impose any more restrictions on + the input than the immediate-detection approach does, and might be + considered simpler in some programming languages. + + In fact, if the decoder is used only inside the IDNA ToUnicode + operation [IDNA], then it need not check for overflow at all, because + ToUnicode performs a higher level re-encoding and comparison, and a + mismatch has the same consequence as if the Punycode decoder had + failed. + +7. Punycode examples + +7.1 Sample strings + + In the Punycode encodings below, the ACE prefix is not shown. + 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 <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 + Punycode: egbpdaj6bu4bxfgehfvwxn + + (B) Chinese (simplified): + u+4ED6 u+4EEC u+4E3A u+4EC0 u+4E48 u+4E0D u+8BF4 u+4E2D u+6587 + Punycode: ihqwcrb4cv8a8dqg056pqjye + + (C) Chinese (traditional): + u+4ED6 u+5011 u+7232 u+4EC0 u+9EBD u+4E0D u+8AAA u+4E2D u+6587 + Punycode: ihqwctvzc91f659drss3x8bo0yb + + (D) Czech: Pro<ccaron>prost<ecaron>nemluv<iacute><ccaron>esky + 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 + Punycode: Proprostnemluvesky-uyb24dma41a + + + + + + +Costello Standards Track [Page 14] + +RFC 3492 IDNA Punycode March 2003 + + + (E) 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 + Punycode: 4dbcagdahymbxekheh6e0a7fei0b + + (F) 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 + Punycode: i1baa7eci9glrd9b2ae1bj0hfcgg6iyaf8o0a1dig0cd + + (G) 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 + Punycode: n8jok5ay5dzabd5bym9f0cm5685rrjetr6pdxa + + (H) 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 + Punycode: 989aomsvi5e83db1d2a355cv1e0vak1dwrv93d5xbh15a0dt30a5j\ + psd879ccm6fea98c + + (I) 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 + Punycode: b1abfaaepdrnnbgefbaDotcwatmq2g4l + + (J) Spanish: Porqu<eacute>nopuedensimplementehablarenEspa<ntilde>ol + 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 + Punycode: PorqunopuedensimplementehablarenEspaol-fmd56a + + (K) Vietnamese: + T<adotbelow>isaoh<odotbelow>kh<ocirc>ngth<ecirchookabove>ch\ + <ihookabove>n<oacute>iti<ecircacute>ngVi<ecircdotbelow>t + U+0054 u+1EA1 u+0069 u+0073 u+0061 u+006F u+0068 u+1ECD u+006B + u+0068 u+00F4 u+006E u+0067 u+0074 u+0068 u+1EC3 u+0063 u+0068 + u+1EC9 u+006E u+00F3 u+0069 u+0074 u+0069 u+1EBF u+006E u+0067 + U+0056 u+0069 u+1EC7 u+0074 + Punycode: TisaohkhngthchnitingVit-kjcr8268qyxafd2f1b9g + + + +Costello Standards Track [Page 15] + +RFC 3492 IDNA Punycode March 2003 + + + 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> + u+0033 u+5E74 U+0042 u+7D44 u+91D1 u+516B u+5148 u+751F + Punycode: 3B-ww4c5e180e575a65lsy2b + + (M) <amuro><namie>-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 + Punycode: -with-SUPER-MONKEYS-pc58ag80a8qai00g7n9n + + (N) Hello-Another-Way-<sorezore><no><basho> + 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 + Punycode: Hello-Another-Way--fc4qua05auwb3674vfr0b + + (O) <hitotsu><yane><no><shita>2 + u+3072 u+3068 u+3064 u+5C4B u+6839 u+306E u+4E0B u+0032 + Punycode: 2-u9tlzr9756bt3uc0v + + (P) Maji<de>Koi<suru>5<byou><mae> + 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 + Punycode: MajiKoi5-783gue6qz075azm5e + + (Q) <pafii>de<runba> + u+30D1 u+30D5 u+30A3 u+30FC u+0064 u+0065 u+30EB u+30F3 u+30D0 + Punycode: de-jg4avhby1noc0d + + (R) <sono><supiido><de> + u+305D u+306E u+30B9 u+30D4 u+30FC u+30C9 u+3067 + Punycode: d9juau41awczczp + + The last example is an ASCII string that breaks the existing rules + for host name labels. (It is not a realistic example for IDNA, + because IDNA never encodes pure ASCII labels.) + + (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 + Punycode: -> $1.00 <-- + + + + +Costello Standards Track [Page 16] + +RFC 3492 IDNA Punycode March 2003 + + +7.2 Decoding traces + + In the following traces, the evolving state of the decoder is shown + as a sequence of hexadecimal values, representing the code points in + the extended string. An asterisk appears just after the most + recently inserted code point, indicating both n (the value preceeding + the asterisk) and i (the position of the value just after the + asterisk). Other numerical values are decimal. + + Decoding trace of example B from section 7.1: + + n is 128, i is 0, bias is 72 + input is "ihqwcrb4cv8a8dqg056pqjye" + there is no delimiter, so extended string starts empty + delta "ihq" decodes to 19853 + bias becomes 21 + 4E0D * + delta "wc" decodes to 64 + bias becomes 20 + 4E0D 4E2D * + delta "rb" decodes to 37 + bias becomes 13 + 4E3A * 4E0D 4E2D + delta "4c" decodes to 56 + bias becomes 17 + 4E3A 4E48 * 4E0D 4E2D + delta "v8a" decodes to 599 + bias becomes 32 + 4E3A 4EC0 * 4E48 4E0D 4E2D + delta "8d" decodes to 130 + bias becomes 23 + 4ED6 * 4E3A 4EC0 4E48 4E0D 4E2D + delta "qg" decodes to 154 + bias becomes 25 + 4ED6 4EEC * 4E3A 4EC0 4E48 4E0D 4E2D + delta "056p" decodes to 46301 + bias becomes 84 + 4ED6 4EEC 4E3A 4EC0 4E48 4E0D 4E2D 6587 * + delta "qjye" decodes to 88531 + bias becomes 90 + 4ED6 4EEC 4E3A 4EC0 4E48 4E0D 8BF4 * 4E2D 6587 + + + + + + + + + + +Costello Standards Track [Page 17] + +RFC 3492 IDNA Punycode March 2003 + + + Decoding trace of example L from section 7.1: + + n is 128, i is 0, bias is 72 + input is "3B-ww4c5e180e575a65lsy2b" + literal portion is "3B-", so extended string starts as: + 0033 0042 + delta "ww4c" decodes to 62042 + bias becomes 27 + 0033 0042 5148 * + delta "5e" decodes to 139 + bias becomes 24 + 0033 0042 516B * 5148 + delta "180e" decodes to 16683 + bias becomes 67 + 0033 5E74 * 0042 516B 5148 + delta "575a" decodes to 34821 + bias becomes 82 + 0033 5E74 0042 516B 5148 751F * + delta "65l" decodes to 14592 + bias becomes 67 + 0033 5E74 0042 7D44 * 516B 5148 751F + delta "sy2b" decodes to 42088 + bias becomes 84 + 0033 5E74 0042 7D44 91D1 * 516B 5148 751F + + + + + + + + + + + + + + + + + + + + + + + + + + + +Costello Standards Track [Page 18] + +RFC 3492 IDNA Punycode March 2003 + + +7.3 Encoding traces + + In the following traces, code point values are hexadecimal, while + other numerical values are decimal. + + Encoding trace of example B from section 7.1: + + bias is 72 + input is: + 4ED6 4EEC 4E3A 4EC0 4E48 4E0D 8BF4 4E2D 6587 + there are no basic code points, so no literal portion + next code point to insert is 4E0D + needed delta is 19853, encodes as "ihq" + bias becomes 21 + next code point to insert is 4E2D + needed delta is 64, encodes as "wc" + bias becomes 20 + next code point to insert is 4E3A + needed delta is 37, encodes as "rb" + bias becomes 13 + next code point to insert is 4E48 + needed delta is 56, encodes as "4c" + bias becomes 17 + next code point to insert is 4EC0 + needed delta is 599, encodes as "v8a" + bias becomes 32 + next code point to insert is 4ED6 + needed delta is 130, encodes as "8d" + bias becomes 23 + next code point to insert is 4EEC + needed delta is 154, encodes as "qg" + bias becomes 25 + next code point to insert is 6587 + needed delta is 46301, encodes as "056p" + bias becomes 84 + next code point to insert is 8BF4 + needed delta is 88531, encodes as "qjye" + bias becomes 90 + output is "ihqwcrb4cv8a8dqg056pqjye" + + + + + + + + + + + + +Costello Standards Track [Page 19] + +RFC 3492 IDNA Punycode March 2003 + + + Encoding trace of example L from section 7.1: + + bias is 72 + input is: + 0033 5E74 0042 7D44 91D1 516B 5148 751F + basic code points (0033, 0042) are copied to literal portion: "3B-" + next code point to insert is 5148 + needed delta is 62042, encodes as "ww4c" + bias becomes 27 + next code point to insert is 516B + needed delta is 139, encodes as "5e" + bias becomes 24 + next code point to insert is 5E74 + needed delta is 16683, encodes as "180e" + bias becomes 67 + next code point to insert is 751F + needed delta is 34821, encodes as "575a" + bias becomes 82 + next code point to insert is 7D44 + needed delta is 14592, encodes as "65l" + bias becomes 67 + next code point to insert is 91D1 + needed delta is 42088, encodes as "sy2b" + bias becomes 84 + output is "3B-ww4c5e180e575a65lsy2b" + +8. 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 ASCII domain names, each controlled by a + different authority, some of which could be spoofs that hijack + service requests intended for another. Therefore Punycode 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 [NAMEPREP]. + + + + + + + + + + +Costello Standards Track [Page 20] + +RFC 3492 IDNA Punycode March 2003 + + +9. References + +9.1 Normative References + + [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate + Requirement Levels", BCP 14, RFC 2119, March 1997. + +9.2 Informative References + + [RFC952] Harrenstien, K., Stahl, M. and E. Feinler, "DOD Internet + Host Table Specification", RFC 952, October 1985. + + [RFC1034] Mockapetris, P., "Domain Names - Concepts and + Facilities", STD 13, RFC 1034, November 1987. + + [IDNA] Faltstrom, P., Hoffman, P. and A. Costello, + "Internationalizing Domain Names in Applications + (IDNA)", RFC 3490, March 2003. + + [NAMEPREP] Hoffman, P. and M. Blanchet, "Nameprep: A Stringprep + Profile for Internationalized Domain Names (IDN)", RFC + 3491, March 2003. + + [ASCII] Cerf, V., "ASCII format for Network Interchange", RFC + 20, October 1969. + + [PROVINCIAL] Kaplan, M., "The 'anyone can be provincial!' page", + http://www.trigeminal.com/samples/provincial.html. + + [UNICODE] The Unicode Consortium, "The Unicode Standard", + http://www.unicode.org/unicode/standard/standard.html. + + + + + + + + + + + + + + + + + + + + +Costello Standards Track [Page 21] + +RFC 3492 IDNA Punycode March 2003 + + +A. Mixed-case annotation + + In order to use Punycode to represent case-insensitive strings, + higher layers need to case-fold the strings prior to Punycode + encoding. The encoded string can use mixed case as an annotation + telling how to convert the folded string into a mixed-case string for + display purposes. Note, however, that mixed-case annotation is not + used by the ToASCII and ToUnicode operations specified in [IDNA], and + therefore implementors of IDNA can disregard this appendix. + + Basic code points can use mixed case directly, because the decoder + copies them verbatim, leaving lowercase code points lowercase, and + leaving uppercase code points uppercase. Each non-basic code point + is represented by a delta, which is represented by a sequence of + basic code points, the last of which provides the annotation. If it + is uppercase, it is a suggestion to map the non-basic code point to + uppercase (if possible); if it is lowercase, it is a suggestion to + map the non-basic code point to lowercase (if possible). + + These annotations do not alter the code points returned by decoders; + the annotations are returned separately, for the caller to use or + ignore. Encoders can accept annotations in addition to code points, + but the annotations do not alter the output, except to influence the + uppercase/lowercase form of ASCII letters. + + Punycode encoders and decoders need not support these annotations, + and higher layers need not use them. + +B. Disclaimer and license + + Regarding this entire document or any portion of it (including the + pseudocode and C code), the author makes no guarantees and is not + responsible for any damage resulting from its use. The author grants + irrevocable permission to anyone to use, modify, and distribute it in + any way that does not diminish the rights of anyone else to use, + modify, and distribute it, provided that redistributed derivative + works do not contain misleading author or version information. + Derivative works need not be licensed under similar terms. + + + + + + + + + + + + + +Costello Standards Track [Page 22] + +RFC 3492 IDNA Punycode March 2003 + + +C. Punycode sample implementation + +/* +punycode.c from RFC 3492 +http://www.nicemice.net/idn/ +Adam M. Costello +http://www.nicemice.net/amc/ + +This is ANSI C code (C89) implementing Punycode (RFC 3492). + +*/ + + +/************************************************************/ +/* Public interface (would normally go in its own .h file): */ + +#include <limits.h> + +enum punycode_status { + punycode_success, + punycode_bad_input, /* Input is invalid. */ + punycode_big_output, /* Output would exceed the space provided. */ + punycode_overflow /* Input needs wider integers to process. */ +}; + +#if UINT_MAX >= (1 << 26) - 1 +typedef unsigned int punycode_uint; +#else +typedef unsigned long punycode_uint; +#endif + +enum punycode_status punycode_encode( + punycode_uint input_length, + const punycode_uint input[], + const unsigned char case_flags[], + punycode_uint *output_length, + char output[] ); + + /* punycode_encode() converts Unicode to Punycode. The input */ + /* is represented as an array of Unicode code points (not code */ + /* units; surrogate pairs are not allowed), and the output */ + /* will be represented as an array of ASCII code points. The */ + /* output string is *not* null-terminated; it will contain */ + /* zeros if and only if the input contains zeros. (Of course */ + /* the caller can leave room for a terminator and add one if */ + /* needed.) The input_length is the number of code points in */ + /* the input. The output_length is an in/out argument: the */ + /* caller passes in the maximum number of code points that it */ + + + +Costello Standards Track [Page 23] + +RFC 3492 IDNA Punycode March 2003 + + + /* can receive, and on successful return it will contain the */ + /* number of code points actually output. The case_flags array */ + /* holds input_length boolean values, where nonzero suggests that */ + /* the corresponding Unicode character be forced to uppercase */ + /* after being decoded (if possible), and zero suggests that */ + /* it be forced to lowercase (if possible). ASCII code points */ + /* are encoded literally, except that ASCII letters are forced */ + /* to uppercase or lowercase according to the corresponding */ + /* uppercase flags. If case_flags is a null pointer then ASCII */ + /* letters are left as they are, and other code points are */ + /* treated as if their uppercase flags were zero. The return */ + /* value can be any of the punycode_status values defined above */ + /* except punycode_bad_input; if not punycode_success, then */ + /* output_size and output might contain garbage. */ + +enum punycode_status punycode_decode( + punycode_uint input_length, + const char input[], + punycode_uint *output_length, + punycode_uint output[], + unsigned char case_flags[] ); + + /* punycode_decode() converts Punycode to Unicode. The input is */ + /* represented as an array of ASCII code points, and the output */ + /* will be represented as an array of Unicode code points. The */ + /* input_length is the number of code points in the input. The */ + /* output_length is an in/out argument: the caller passes in */ + /* the maximum number of code points that it can receive, and */ + /* on successful return it will contain the actual number of */ + /* code points output. The case_flags array needs room for at */ + /* least output_length values, or it can be a null pointer if the */ + /* case information is not needed. A nonzero flag suggests that */ + /* the corresponding Unicode character be forced to uppercase */ + /* by the caller (if possible), while zero suggests that it be */ + /* forced to lowercase (if possible). ASCII code points 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 can be any of the punycode_status values */ + /* defined above; if not punycode_success, then output_length, */ + /* output, and case_flags might contain garbage. On success, the */ + /* decoder will never need to write an output_length greater than */ + /* input_length, because of how the encoding is defined. */ + +/**********************************************************/ +/* Implementation (would normally go in its own .c file): */ + +#include <string.h> + + + + +Costello Standards Track [Page 24] + +RFC 3492 IDNA Punycode March 2003 + + +/*** Bootstring parameters for Punycode ***/ + +enum { base = 36, tmin = 1, tmax = 26, skew = 38, damp = 700, + initial_bias = 72, initial_n = 0x80, delimiter = 0x2D }; + +/* basic(cp) tests whether cp is a basic code point: */ +#define basic(cp) ((punycode_uint)(cp) < 0x80) + +/* delim(cp) tests whether cp is a delimiter: */ +#define delim(cp) ((cp) == delimiter) + +/* decode_digit(cp) returns the numeric value of a basic code */ +/* point (for use in representing integers) in the range 0 to */ +/* base-1, or base if cp is does not represent a value. */ + +static punycode_uint decode_digit(punycode_uint cp) +{ + return cp - 48 < 10 ? cp - 22 : cp - 65 < 26 ? cp - 65 : + cp - 97 < 26 ? cp - 97 : base; +} + +/* encode_digit(d,flag) returns the basic code point whose value */ +/* (when used for representing integers) is d, which needs to be in */ +/* the range 0 to base-1. The lowercase form is used unless flag is */ +/* nonzero, in which case the uppercase form is used. The behavior */ +/* is undefined if flag is nonzero and digit d has no uppercase form. */ + +static char encode_digit(punycode_uint d, int flag) +{ + return d + 22 + 75 * (d < 26) - ((flag != 0) << 5); + /* 0..25 map to ASCII a..z or A..Z */ + /* 26..35 map to ASCII 0..9 */ +} + +/* flagged(bcp) tests whether a basic code point is flagged */ +/* (uppercase). The behavior is undefined if bcp is not a */ +/* basic code point. */ + +#define flagged(bcp) ((punycode_uint)(bcp) - 65 < 26) + +/* encode_basic(bcp,flag) forces a basic code point to lowercase */ +/* if flag is zero, uppercase if flag is nonzero, and returns */ +/* the resulting code point. The code point is unchanged if it */ +/* is caseless. The behavior is undefined if bcp is not a basic */ +/* code point. */ + +static char encode_basic(punycode_uint bcp, int flag) +{ + + + +Costello Standards Track [Page 25] + +RFC 3492 IDNA Punycode March 2003 + + + bcp -= (bcp - 97 < 26) << 5; + return bcp + ((!flag && (bcp - 65 < 26)) << 5); +} + +/*** Platform-specific constants ***/ + +/* maxint is the maximum value of a punycode_uint variable: */ +static const punycode_uint maxint = -1; +/* Because maxint is unsigned, -1 becomes the maximum value. */ + +/*** Bias adaptation function ***/ + +static punycode_uint adapt( + punycode_uint delta, punycode_uint numpoints, int firsttime ) +{ + punycode_uint k; + + delta = firsttime ? delta / damp : delta >> 1; + /* delta >> 1 is a faster way of doing delta / 2 */ + delta += delta / numpoints; + + for (k = 0; delta > ((base - tmin) * tmax) / 2; k += base) { + delta /= base - tmin; + } + + return k + (base - tmin + 1) * delta / (delta + skew); +} + +/*** Main encode function ***/ + +enum punycode_status punycode_encode( + punycode_uint input_length, + const punycode_uint input[], + const unsigned char case_flags[], + punycode_uint *output_length, + char output[] ) +{ + punycode_uint n, delta, h, b, out, max_out, bias, j, m, q, k, t; + + /* Initialize the state: */ + + n = initial_n; + delta = out = 0; + max_out = *output_length; + bias = initial_bias; + + /* Handle the basic code points: */ + + + + +Costello Standards Track [Page 26] + +RFC 3492 IDNA Punycode March 2003 + + + for (j = 0; j < input_length; ++j) { + if (basic(input[j])) { + if (max_out - out < 2) return punycode_big_output; + output[out++] = + case_flags ? encode_basic(input[j], case_flags[j]) : input[j]; + } + /* else if (input[j] < n) return punycode_bad_input; */ + /* (not needed for Punycode with unsigned code points) */ + } + + h = b = out; + + /* h is the number of code points that have been handled, b is the */ + /* number of basic code points, and out is the number of characters */ + /* that have been output. */ + + if (b > 0) output[out++] = delimiter; + + /* Main encoding loop: */ + + while (h < input_length) { + /* All non-basic code points < n have been */ + /* handled already. Find the next larger one: */ + + for (m = maxint, j = 0; j < input_length; ++j) { + /* if (basic(input[j])) continue; */ + /* (not needed for Punycode) */ + if (input[j] >= n && input[j] < m) m = input[j]; + } + + /* Increase delta enough to advance the decoder's */ + /* <n,i> state to <m,0>, but guard against overflow: */ + + if (m - n > (maxint - delta) / (h + 1)) return punycode_overflow; + delta += (m - n) * (h + 1); + n = m; + + for (j = 0; j < input_length; ++j) { + /* Punycode does not need to check whether input[j] is basic: */ + if (input[j] < n /* || basic(input[j]) */ ) { + if (++delta == 0) return punycode_overflow; + } + + if (input[j] == n) { + /* Represent delta as a generalized variable-length integer: */ + + for (q = delta, k = base; ; k += base) { + if (out >= max_out) return punycode_big_output; + + + +Costello Standards Track [Page 27] + +RFC 3492 IDNA Punycode March 2003 + + + t = k <= bias /* + tmin */ ? tmin : /* +tmin not needed */ + k >= bias + tmax ? tmax : k - bias; + if (q < t) break; + output[out++] = encode_digit(t + (q - t) % (base - t), 0); + q = (q - t) / (base - t); + } + + output[out++] = encode_digit(q, case_flags && case_flags[j]); + bias = adapt(delta, h + 1, h == b); + delta = 0; + ++h; + } + } + + ++delta, ++n; + } + + *output_length = out; + return punycode_success; +} + +/*** Main decode function ***/ + +enum punycode_status punycode_decode( + punycode_uint input_length, + const char input[], + punycode_uint *output_length, + punycode_uint output[], + unsigned char case_flags[] ) +{ + punycode_uint n, out, i, max_out, bias, + b, j, in, oldi, w, k, digit, t; + + /* Initialize the state: */ + + n = initial_n; + out = i = 0; + max_out = *output_length; + bias = initial_bias; + + /* Handle the basic code points: Let b be the number of input code */ + /* points before the last delimiter, or 0 if there is none, then */ + /* copy the first b code points to the output. */ + + for (b = j = 0; j < input_length; ++j) if (delim(input[j])) b = j; + if (b > max_out) return punycode_big_output; + + for (j = 0; j < b; ++j) { + + + +Costello Standards Track [Page 28] + +RFC 3492 IDNA Punycode March 2003 + + + if (case_flags) case_flags[out] = flagged(input[j]); + if (!basic(input[j])) return punycode_bad_input; + output[out++] = input[j]; + } + + /* Main decoding loop: Start just after the last delimiter if any */ + /* basic code points were copied; start at the beginning otherwise. */ + + for (in = b > 0 ? b + 1 : 0; in < input_length; ++out) { + + /* in is the index of the next character to be consumed, and */ + /* out is the number of code points in the output array. */ + + /* Decode a generalized variable-length integer into delta, */ + /* which gets added to i. The overflow checking is easier */ + /* if we increase i as we go, then subtract off its starting */ + /* value at the end to obtain delta. */ + + for (oldi = i, w = 1, k = base; ; k += base) { + if (in >= input_length) return punycode_bad_input; + digit = decode_digit(input[in++]); + if (digit >= base) return punycode_bad_input; + if (digit > (maxint - i) / w) return punycode_overflow; + i += digit * w; + t = k <= bias /* + tmin */ ? tmin : /* +tmin not needed */ + k >= bias + tmax ? tmax : k - bias; + if (digit < t) break; + if (w > maxint / (base - t)) return punycode_overflow; + w *= (base - t); + } + + bias = adapt(i - oldi, out + 1, oldi == 0); + + /* i was supposed to wrap around from out+1 to 0, */ + /* incrementing n each time, so we'll fix that now: */ + + if (i / (out + 1) > maxint - n) return punycode_overflow; + n += i / (out + 1); + i %= (out + 1); + + /* Insert n at position i of the output: */ + + /* not needed for Punycode: */ + /* if (decode_digit(n) <= base) return punycode_invalid_input; */ + if (out >= max_out) return punycode_big_output; + + if (case_flags) { + memmove(case_flags + i + 1, case_flags + i, out - i); + + + +Costello Standards Track [Page 29] + +RFC 3492 IDNA Punycode March 2003 + + + /* Case of last character determines uppercase flag: */ + case_flags[i] = flagged(input[in - 1]); + } + + memmove(output + i + 1, output + i, (out - i) * sizeof *output); + output[i++] = n; + } + + *output_length = out; + return punycode_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_length = 256 +}; + +static void usage(char **argv) +{ + fprintf(stderr, + "\n" + "%s -e reads code points and writes a Punycode string.\n" + "%s -d reads a Punycode string and writes code points.\n" + "\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" + "Although the specification allows Punycode strings to contain\n" + "any characters from the ASCII repertoire, this test code\n" + "supports only the printable characters, and needs the Punycode\n" + "string to be followed by a newline.\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) + + + +Costello Standards Track [Page 30] + +RFC 3492 IDNA Punycode March 2003 + + +{ + 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 overflow[] = "arithmetic overflow\n"; +static const char io_error[] = "I/O error\n"; + +/* The following string is used to convert printable */ +/* characters between ASCII and the native charset: */ + +static const char print_ascii[] = + "\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n" + "\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n" + " !\"#$%&'()*+,-./" + "0123456789:;<=>?" + "@ABCDEFGHIJKLMNO" + "PQRSTUVWXYZ[\\]^_" + "`abcdefghijklmno" + "pqrstuvwxyz{|}~\n"; + +int main(int argc, char **argv) +{ + enum punycode_status status; + int r; + unsigned int input_length, output_length, j; + unsigned char case_flags[unicode_max_length]; + + if (argc != 2) usage(argv); + if (argv[1][0] != '-') usage(argv); + if (argv[1][2] != 0) usage(argv); + + if (argv[1][1] == 'e') { + punycode_uint input[unicode_max_length]; + unsigned long codept; + char output[ace_max_length+1], uplus[3]; + int c; + + /* Read the input code points: */ + + input_length = 0; + + for (;;) { + r = scanf("%2s%lx", uplus, &codept); + if (ferror(stdin)) fail(io_error); + + + +Costello Standards Track [Page 31] + +RFC 3492 IDNA Punycode March 2003 + + + if (r == EOF || r == 0) break; + + if (r != 2 || uplus[1] != '+' || codept > (punycode_uint)-1) { + fail(invalid_input); + } + + if (input_length == unicode_max_length) fail(too_big); + + if (uplus[0] == 'u') case_flags[input_length] = 0; + else if (uplus[0] == 'U') case_flags[input_length] = 1; + else fail(invalid_input); + + input[input_length++] = codept; + } + + /* Encode: */ + + output_length = ace_max_length; + status = punycode_encode(input_length, input, case_flags, + &output_length, output); + if (status == punycode_bad_input) fail(invalid_input); + if (status == punycode_big_output) fail(too_big); + if (status == punycode_overflow) fail(overflow); + assert(status == punycode_success); + + /* Convert to native charset and output: */ + + for (j = 0; j < output_length; ++j) { + c = output[j]; + assert(c >= 0 && c <= 127); + if (print_ascii[c] == 0) fail(invalid_input); + output[j] = print_ascii[c]; + } + + output[j] = 0; + r = puts(output); + if (r == EOF) fail(io_error); + return EXIT_SUCCESS; + } + + if (argv[1][1] == 'd') { + char input[ace_max_length+2], *p, *pp; + punycode_uint output[unicode_max_length]; + + /* Read the Punycode input string and convert to ASCII: */ + + fgets(input, ace_max_length+2, stdin); + if (ferror(stdin)) fail(io_error); + + + +Costello Standards Track [Page 32] + +RFC 3492 IDNA Punycode March 2003 + + + if (feof(stdin)) fail(invalid_input); + input_length = strlen(input) - 1; + if (input[input_length] != '\n') fail(too_big); + input[input_length] = 0; + + for (p = input; *p != 0; ++p) { + pp = strchr(print_ascii, *p); + if (pp == 0) fail(invalid_input); + *p = pp - print_ascii; + } + + /* Decode: */ + + output_length = unicode_max_length; + status = punycode_decode(input_length, input, &output_length, + output, case_flags); + if (status == punycode_bad_input) fail(invalid_input); + if (status == punycode_big_output) fail(too_big); + if (status == punycode_overflow) fail(overflow); + assert(status == punycode_success); + + /* Output the result: */ + + for (j = 0; j < output_length; ++j) { + r = printf("%s+%04lX\n", + case_flags[j] ? "U" : "u", + (unsigned long) output[j] ); + if (r < 0) fail(io_error); + } + + return EXIT_SUCCESS; + } + + usage(argv); + return EXIT_SUCCESS; /* not reached, but quiets compiler warning */ +} + + + + + + + + + + + + + + + +Costello Standards Track [Page 33] + +RFC 3492 IDNA Punycode March 2003 + + +Author's Address + + Adam M. Costello + University of California, Berkeley + http://www.nicemice.net/amc/ + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +Costello Standards Track [Page 34] + +RFC 3492 IDNA Punycode March 2003 + + +Full Copyright Statement + + Copyright (C) The Internet Society (2003). All Rights Reserved. + + This document and translations of it may be copied and furnished to + others, and derivative works that comment on or otherwise explain it + or assist in its implementation may be prepared, copied, published + and distributed, in whole or in part, without restriction of any + kind, provided that the above copyright notice and this paragraph are + included on all such copies and derivative works. However, this + document itself may not be modified in any way, such as by removing + the copyright notice or references to the Internet Society or other + Internet organizations, except as needed for the purpose of + developing Internet standards in which case the procedures for + copyrights defined in the Internet Standards process must be + followed, or as required to translate it into languages other than + English. + + The limited permissions granted above are perpetual and will not be + revoked by the Internet Society or its successors or assigns. + + This document and the information contained herein is provided on an + "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING + TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING + BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION + HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF + MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. + +Acknowledgement + + Funding for the RFC Editor function is currently provided by the + Internet Society. + + + + + + + + + + + + + + + + + + + +Costello Standards Track [Page 35] + |