XXencode

xxencode is a historical text encoding used to convert binary data into ASCII text for transmission over channels that are not guaranteed to be 8-bit clean. It belongs to the family of 6-bit encodings that includes uuencode and similar schemes, but it uses its own alphabet and line-format rules. The practical goal is to preserve data integrity on systems and protocols that may misinterpret or mis-handle arbitrary bytes.

In typical implementations, xxencode operates on 3-byte blocks, producing 4 ASCII characters. Each 6-bit group is mapped to a printable character by applying a fixed offset to the 6-bit value, yielding characters within a restricted ASCII range. Output is arranged in lines; the first character of a line often encodes the length of the data on that line, with the remaining characters representing the encoded data. Short final lines are padded as required to complete a quartet.

There is no single universal standard for xxencode; different programs may use slightly different alphabets or line lengths. It was used in several early networks and mail- or Usenet-posting tools as a way to embed binary content in text-only streams. Over time, it largely gave way to more widely adopted schemes such as base64, which provides similar functionality with broader standardization and tooling.

See also UUencode, base64, yEnc.