A technique in
cryptanalysis where the
analyst tries to break a
cipher by observing its effects on the mutual differences of a sequence of
plaintexts.
Cognoscenti have proved this to be a much stronger attack than other traditional methods: rumor has it that the NSA knew about differential cryptoanalysis when they - supposedly- influenced DES's design - many years before any publication about differential cryptanalysis was ever released to the public.
This belief was born when DES was found to be surprisingly resistant to the newly found technique.