xxencode is a binary-to-text encoding similar to uuencode which uses only the alphanumeric characters, and the plus and minus signs.
It was invented as a means to transfer files in a format which would survive character set translation, particularly that between ASCII and the EBCDIC encoding used on IBM mainframes.
[1] xxencoded data starts with a line of the form: Where
If the input is not evenly divisible by 45, the last line will contain the remaining N output characters, preceded by the number of remaining input bytes encoded as above.
This assumes at least one full-length line (45 encoded bytes/60 characters) in the output.