yEnc

yEnc is a binary-to-text encoding scheme used principally to transfer binary data in Usenet postings and, to a lesser extent, email. Developed in the early 2000s, it was designed to be more space-efficient than older methods such as uuencode and Base64, while avoiding the common problems that occur when transmitting arbitrary binary data through text-based channels.

Encoding process: Each input byte is transformed by adding 128 modulo 256, so the resulting value lies in a high bit range. The resulting stream is then represented as bytes, with a simple escape mechanism to prevent a small set of problematic values (notably zero, line breaks, and the escape character) from appearing unescaped. When a value to be emitted would collide with these primitives, it is emitted as an escape sequence beginning with an equals sign and two hexadecimal digits representing the original value.

Format and metadata: A yEnc message consists of header blocks that describe metadata such as the filename, size, and part information, followed by the encoded data lines, and a footer marking the end. Line length is typically constrained for transport but can vary by implementation.

Usage and compatibility: yEnc is widely supported by Usenet clients and post-processing tools, with numerous libraries and plugins available for different platforms. It is not encryption and provides no authentication; decoding requires a corresponding decoder. In practice, it offers high efficiency for binary data but may be rejected by some mail systems or archives that do not recognize yEnc.