Skipjack is the secret key encryption algorithm developed by the NSA for the Clipper chip and Fortezza PC card. It uses an 80-bit key, 128 table lookup operations, and 320 XOR operations to map a 64- bit plaintext into a 64-bit ciphertext in 32 rounds. This paper describes an efficient attack on a variant, which we call Skipjack-3XOR (Skipjack minus 3 XORs). The only difference between Skipjack and Skipjack- 3XOR is the removal of 3 out of the 320 XOR operations. The attack uses the ciphertexts derived from about 500 plaintexts and its total running time is equivalent to about one million Skipjack encryptions, which can be carried out in seconds on a personal computer. We also present a new cryptographic tool, which we call the Yoyo game, and efficient attacks on Skipjack reduced to 16 rounds. We conclude that Skipjack does not have a conservative design with a large margin of safety.
['Eli Biham', 'Alex Biryukov', 'Orr Dunkelman', 'Eran Richardson', 'Adi Shamir']
Initial Observations on Skipjack: Cryptanalysis of Skipjack-3XOR