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The Fast Data Encipherment Algorithm (FEAL) was presented by Shimizu and Miyaguchi as an alternative to DES. The original cipher (called FEAL-4) was a four-round cryptosystem with a 64-bit block size and a 64-bit key size and it was designed to give high performance in software. Soon a variety of attacks against FEAL-4 were announced including one attack that required only 20 chosen plaintexts. Several results in the cryptanalysis of FEAL-8 (eight-round version) led the designers to introduce a revised version, FEAL-N, where N denoted the number of rounds. Biham and Shamir developed differential cryptanalytic attacks against FEAL-N for up to 31 rounds. In 1994, Ohta and Aoki presented a linear cryptanalytic attack against FEAL-8 that required 225 known plaintexts, and other improvements followed. In the wake of these numerous attacks, FEAL and its derivatives should be considered insecure.