steganography

[TO1SITTLER@APSICC.APS.EDU (Kragen Sittler)]

In a previous message, which my VMS newsreader is too stupid to remember and
can just barely quote, an Evil Genius For A Better Tomorrow, mike@egfabt.org,

says he thinks that xor'ing 'x' over a message would help to hide the fact that
it is cyphertext.
Problem: If someone is looking for encrypted information, IDEALLY we would like
steganographically for it to be unconditionally impossible to determine that a
file is cyphertext w/o the key.  (It is of course only computationally
infeasible to guess the key, if we pick our cryptosystem right.)

The suggestions that the Evil Genius makes could easily be defeated with an hour
or two of programming using pieces of PGP's source, or ripem, or a little piece
of software designed to tell whether a file is Dolphin Encrypted or Mailsafed.
The resulting program could check hundreds of database files easily, probably
in almost exactly the time required for an open, a single disk access, and a
close on each file.

For a steganographically strong code, the cyphertext must not be recognizable
to its home program.  This means, among other things, no CRC's, no MD5's, etc,
intended to assure data integrity.  This means no delimiter-structured files.
(I think--I'm not absolutely sure on this one.)  Every field in the file must
be either fixed-length or have a number somewhere in it
(in a format indistinguishable from the rest of the cyphertext) which tells its
length.  And every field, individually, must not be recognizable as something
unusual. (For instance, no sending of large prime numbers in the clear, as they
are very unlikely to appear in a random file.)

The foregoing "field" stuff has to do with things like PGP, which have a
message cyphertext, encrypted with a session key, and a session key, encrypted
with a PKC, in the same message. (Possible several cyphertexts of the session
key.)

Oh, and it would be nice, though not essential, if a corruption in the file
made it completely, rather than partially, nonsense when decrypted.  This way
you can't have a file with no use to the recipient used as evidence against
them.

The important qualifications seem to rule out any crypto package in current
widespread use, don't they?

Kragen, an ignorant crypto-wannabe