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>     Given the source to a program using DES, is it possible to 
> write a "decrypter" program?
 
>     Benjamin Chen
>     arpa: bchen@wolf.berkeley.edu
>     uucp: ucbvax!wolf!bchen

Based on the discussion that's gone on in the last few weeks, I'd say that
first of all, it's not the method that's a mystery. The DES algorithm is
well known. Given a small message that is not known to the cryptanalyst,
I think everyone agrees the solution is to try all possible keys (not what
you're looking for). Finally, there has been some discussion of possible ways
of reverse engineering a DES key based on known plaintext. This last has yet
to be proved possible though.

I believe your question indicated some concept that knowing the method of
encryption would allow decryption. In general, this is not true, as generally
the only thing that can reliably be kept secret is the key, not the method.
Cryptographers assume knowledge of the method when constructing crypto-systems.
If you want the full documentation on DES, it is available as a National
Bureau of Standards document (FIPS PUB 46 and 81) and is also described in
Numerical Recipes

dov

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