something I've always wondered

[SINCLAIR DOUGLAS N]

> 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?
> 
> Since there are more combinations of key and plaintext than there are  
> possible cyphertexts outputs of length L, you know there must be some  
> combinations of key and plaintext that produce the same cyphertext.

Of course.  Take some random bytes.  Decrypt them with two different
keys.  You will end up with two plaintexts that when encrypted with
different keys make the same cyphertext.  The problem is finding
two plaintexts that make sense which encrypt to the same cyphertext.