something I've always wondered

[bill.stewart@pleasantonca.ncr.com +1-510-484-6204]

> Jim Miller says:
> > Does DES (or name your favorite encryption algorithm) produce as output  
> > all possible cyphertexts of length L, given all possible conbinations of  
> > keys and plaintexts of length L?

As Perry points out, for any key, cycling through the 2**64 inputs
will produce all 2**64 outputs.  On the other hand, there are
(2**64)! mappings of the set of inputs to the set of outputs,
and DES only uses 2**56 of those mappings, so there are probably
values of P and C for which there is no key K such that C = DES(P,K).