Bicom is a data compressor in the PPM family.  It is freely available and
Open Source.  Its most unique characteristic is that compression with
bicom is completely bijective -- any file is a possible bicom output that
can be decompressed, and then recompressed back to its original form.  Of
course, any file is also a possible bicom input that can be compressed,
and then decompressed back to its original form.  To support encryption
applications, bicom also includes a passphrase-protection option that
will automatically encrypt after compressing, or decrypt before
decompressing.

WWW: http://www3.sympatico.ca/mt0000/bicom/