punycode

Punycode is an encoding algorithm that enables Unicode strings to be represented using only ASCII characters, allowing internationalized text to be used in contexts that require ASCII. It is the encoding component of ACE (ASCII Compatible Encoding) in the Internationalized Domain Names in Applications (IDNA) framework. Punycode is defined in RFC 3492 and is widely implemented in browsers, mail systems, and software libraries to support non-ASCII domain labels.

Punycode works by separating ASCII (basic) code points from non-ASCII (non-basic) code points. The basic code points are copied to the output, while the non-basic code points are encoded using a delta-based scheme that uses ASCII letters a–z and digits 0–9. When used in a domain name, the encoded label is typically prefixed with xn--, forming an ACE label that DNS can resolve. Decoding reverses the process to recover the original Unicode string.

The encoding uses a base-36 numeral system and includes a bias adaptation mechanism that keeps the length of the encoding reasonable for characters near the ASCII range. The process is designed to be reversible and deterministic, ensuring that each Unicode string maps to a unique ASCII representation and can be restored.

Standards and adoption: Punycode is specified in RFC 3492, and its use within domain names is described by the IDNA specifications (RFC 5890–5894). It is widely supported by major web browsers, operating systems, and DNS resolvers, enabling users to input and access domain names across diverse writing systems.