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binascii

binascii is a Common Lisp library for converting binary data to ASCII
text of some kind.  Such conversions are common in email protocols (for
encoding attachments to support old non-8-bit clean transports) or
encoding binary data in HTTP and XML applications.  binascii supports
the encodings described in RFC 4648: base64, base32, base16, and
variants.  It also supports base85, used in Adobe's PostScript and PDF
document formats, and a variant called ascii85, used by git for binary
diff files.

It is documented, flexible, and fast, and comes with a BSD license.