A technique in cryptanalysis
where the analyst
tries to break a cipher
by observing its effects on the mutual differences of a sequence of plaintext
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.