UUencode

UUencode, short for Unix-to-Unix Encoding, is a binary-to-text encoding designed to transmit binary data over channels that are primarily text-oriented, such as email and Usenet. Developed in the early days of UUCP and Unix software, it was widely used on Unix systems to send programs, archives, and other binary files. Over time, it was largely supplanted by MIME Base64 in email, but remains part of historical and compatibility contexts.

How it works: UUencode converts binary data into printable ASCII characters. The input is processed in 3-byte groups, which are expanded into four 6-bit groups. Each 6-bit group is then mapped to a printable ASCII character by adding 32 to yield a value in the range 32 to 95. This produces lines of encoded text, with a line length indicator at the start of each line that encodes how many input bytes are on that line (up to 45 bytes). A typical line begins with a character such as 'M', which denotes 45 bytes of data on that line. The encoded block is usually preceded by a header line like "begin 644 filename" that records the file permissions, followed by the encoded data lines, and concluded with a line that reads "end." Decoding is performed by a uudecode tool, which reverses the process to recover the original bytes and file metadata.

Usage and limitations: UUencode is simple and widely supported by older mail and Usenet tooling, but it offers no integrity or confidentiality guarantees and relies on proper line handling in transmission. It is susceptible to line-wrapping and filtering in some mail systems, and modern email standards favor MIME with Base64. Despite its decline, UUencode remains a reference point in the history of binary-to-text encodings.