 In addition to RSA, Shamir's other numerous inventions and contributions to cryptography include the Shamir secret sharing scheme, the breaking of the Merkle-Hellman knapsack cryptosystem, visual cryptography, and the TWIRL and TWINKLE factoring devices.Together with Eli Biham, he discovered differential cryptanalysis in the late 1980s, a general method for attacking block ciphers.It later emerged that differential cryptanalysis was already known — and kept a secret — by both IBM and the National Security Agency (NSA).