Punycode is a representation ofUnicode with the limitedASCII character subset used for Internethostnames. Using Punycode, host names containing Unicode characters are transcoded to a subset of ASCII consisting of letters, digits, and hyphens, which is called the letter–digit–hyphen (LDH) subset. For example, theGermanMünchen (English:Munich) is encoded asMnchen-3ya.

While theDomain Name System (DNS) technically supports arbitrary sequences of octets in domain name labels, the DNS standards recommend the use of the LDH subset of ASCII conventionally used for host names, and require that string comparisons between DNS domain names should be case-insensitive. The Punycode syntax is a method of encoding strings containing Unicode characters, such asinternationalized domain names (IDNA), into the LDH subset of ASCII favored by DNS. It is specified inIETFRequest for Comments 3492.^[1]

The RFC author, Adam Costello, is reported to have written:

Why “Punycode”? It rhymes with Unicode and is intended to encode Unicode strings. It is “puny” in three senses: The repertoire of characters used in the encoded strings is small, the encoded strings are short, and the implementation is small.^[2]

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As stated in RFC 3492, "Punycode is an instance of a more general algorithm calledBootstring, 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 defines parameters for the general Bootstring algorithm to match the characteristics of Unicode text. This section demonstrates the procedure for Punycode encoding, using as an example theGerman string "bücher" (English:books), which is translated into the label "bcher-kva".

To make the encoding and decoding algorithms simple, no attempt has been made to prevent some encoded values from encoding inadmissible Unicode values: however, these should be checked for and detected during decoding.

Punycode is designed to work across all scripts, and to be self-optimizing by attempting to adapt to the character set ranges within the string as it operates. It is optimized for the case where the string is composed of zero or more ASCII characters and in addition characters from only one other script system, but will cope with any arbitrary Unicode string. Note that for DNS use, the domain name string is assumed to have been normalized usingnameprep and (fortop-level domains) filtered against an officially registered language table before being punycoded, and that the DNS protocol sets limits on the acceptable lengths of the output Punycode string.

First, allASCII characters in the string are copied from input to output, skipping over any other characters. For example, "bücher" is copied to "bcher". If any characters were copied, i.e. if there was at least one ASCII character in the input, an ASCII hyphen is appended to the output (e.g., "bücher" → "bcher-", but "ü" → "").

Note that hyphens are themselves ASCII characters. Thus, they can be present in the input and, if so, they will be copied to the output. This causes no ambiguity: if the output contains hyphens, the one that got added is always the last one. It marks the end of the ASCII characters.

The non-ASCII characters are sorted by Unicode value, lowest first. (If a character occurs more than once they are sorted by position.) Each is then encoded as a single number. This single number defines both the location to insert the character at and which character to insert.

• i is an index into the result to insert the code at, starting at 0 (for insertion at the start).^{[citation needed]}
• n is the number of possible insertion points (current length of the result plus one).
• j is the Unicode code point to insert minus 127.^{[citation needed]}

The encoded number isn ×j +i. By dividing byn and also getting the remainder, a decoder can determinej andi.

There are 6 possible places to insert a character in the string "bcher" (including before the first character and after the last one).ü is Unicode code point0xFC or 252 (seeLatin-1 Supplement), and252 − 127 is 124. Theü is inserted at position 1, after theb. Thus the encoder will add the number(6 × 124) + 1 = 745, and the decoder can retrieve these by⌊745 ÷ 6⌋ = 124 and745 mod 6 = 1.

These numbers are strictly increasing. For the second and subsequent inserted characters, the difference between the number and the previous one is written.

The number is encoded using the lettersa throughz and the digits0 through9. It is not base-36 but a more complex scheme described below, which allows the numbers to be concatenated, with nothing separating them.

Punycode usesgeneralized variable-length integers to represent these values. For example, this is how "kva" is used to represent the code number 745:

A number system withlittle-endian ordering is used which allows variable-length codes without separate delimiters: a digit lower than a threshold value marks that it is the most-significant digit, hence the end of the number. The threshold value depends on the position in the number and also on previous insertions, to increase efficiency. Correspondingly the weights of the digits vary.

In this case a number system with 36 symbols is used, with thecase-insensitive 'a' through 'z' equal to the decimal numbers 0 through 25, and '0' through '9' equal to the decimal numbers 26 through 35. Thus "kva", corresponds to the decimal number string "10 21 0".

To decode this string of symbols, a sequence of thresholds will be needed, in this case it's (1, 1, 26, 26, ...).^[3] The weight (orplace value) of the least-significant digit is always 1: 'k' (=10) with a weight of 1 equals 10. After this, the weight of the next digit depends on the first threshold: generally, for anyn, the weight of the (n+1)-th digit isw × (36 −t), wherew is the previous weight andt is the threshold of then-th digit. So in this case, the second symbol has a place value of 36 minus the previous threshold value of 1, which equals 35. Therefore, the sum of the first two symbols 'k' (=10) and 'v' (=21) is 10 × 1 + 21 × 35. Since the second symbol is not less than its threshold value of 1, there is more to come. However, since the third symbol in this example is 'a' (=0), we may ignore calculating its weight. Therefore, "kva" represents the decimal number (10 × 1) + (21 × 35) = 745.

Number 745 will be encoded as 10 + 21 × 35 + 0 (base 35 used for second digit, the most significant digit 0 needed as terminator), 10 → 'k', 21 → 'v', 0 → 'a', so "bücher" → "bcher-kva".

The thresholds themselves are determined for each successive encoded character by an algorithm keeping them between 1 and 26 inclusive.^[4] The case can then be used to provide information about the original case of the string.^[5]

Because special characters are sorted by their code points by encoding algorithm, for the insertion of a second special character in "bücher", the first possibility is "büücher" with code "bcher-kvaa", the second "bücüher" with code "bcher-kvab", etc. After "bücherü" with code "bcher-kvae" comes codes representing insertion of ý, the Unicode character following ü, starting with "ýbücher" with code "bcher-kvaf" (different from "übücher" coded "bcher-jvab"), etc.

To prevent hyphens in non-international domain names from triggering a Punycode decoding, the stringxn-- is prepended to Punycode sequences in internationalized domain names. This is called ACE (ASCII Compatible Encoding).^[6]

Thus the domain name "bücher.tld" would be represented in a URL as "xn--bcher-kva.tld".

The following table shows examples of Punycode encodings for different types of input.^[7]

• Emoji domain
• UTF-5
• UTF-6
• Website spoofing

• IETF Punycode standard
• ICU IDNA Demonstration An online demonstration of howICU performs IDN operations
• List of TLDs considered by the Mozilla developers to have an effective anti-spoofing policy for name registration
• IDN and Punycode in IE7
• Simple Punycode converter

• Unicode Transformation Formats
• Internationalized domain names

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