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punycode

Punycode is a standard that allows Unicode characters used in Internationalized Domain Names (IDNs) to be converted into ASCII characters, which are valid for use in domain names. This encoding and decoding technique ensures compatibility with existing Internet infrastructure.


Table of Contents

Overview

Unicode enables representation of various writing systems from around the world. IDNs facilitate the inclusion of non-ASCII characters in domain names, making it easier for users to access websites using their preferred language or script.

Punycode, also known as “ACE” (ASCII Compatible Encoding), was developed as a mechanism to convert non-ASCII Unicode characters into ASCII characters for use in domain names. This conversion ensures compatibility with the Domain Name System (DNS) which primarily supports ASCII characters.

Encoding and Decoding Punycode

The Punycode algorithm converts Unicode characters into ASCII characters by splitting the input into basic code points and encoded code points. The encoded code points are then prepended with a specific delimiter character (“%”). These characters are converted into ASCII and combined to form Punycode.

To encode a string of Unicode characters into Punycode, the following steps are followed:

  1. Convert each Unicode character into its decimal representation.
  2. Apply the Punycode algorithm to convert the decimal representations into ASCII characters.
  3. Prepend the ASCII characters with the delimiter character (“%”).
  4. The resulting Punycode is now a valid ASCII string.

To decode Punycode back into Unicode characters:

  1. Remove the delimiter character and separate the Punycode into basic and encoded code points.
  2. Convert the basic code points into ASCII.
  3. Apply the decoding algorithm to convert the ASCII characters back into the decimal representation.
  4. Convert the decimal representation back into Unicode characters.

Applications

Punycode has various applications, including:

  • Internationalized Domain Names (IDNs): Punycode enables non-ASCII characters to be used in domain names, allowing websites to have domain names in different languages or scripts.
  • Email Address Internationalization (EAI): Similar to IDNs, Punycode can be used to represent non-ASCII characters in email addresses, facilitating internationalization.
  • Character Set Transition: Punycode can be used to transition between different character sets, ensuring compatibility and interoperability between systems using different encoding schemes.

Limitations

While Punycode provides a solution for representing non-ASCII characters in ASCII domain names, there are some limitations to consider:

  • Punycode cannot be directly used for applications that require the preservation of the original Unicode characters.
  • There is a limited set of available code points to encode Unicode characters, which may result in conflicts for similar-looking characters.
  • Punycode does not support bidirectional domain names, which include both left-to-right and right-to-left scripts.

References

For more information on Punycode and related topics, please refer to the following resources: