The Cryptanalysis of Feal with Twenty Chosen Plaintexts

The FEAL N cryptosystem has been developed by N T T as a highly programming e cient block cipher system as it does not use look up tables It was rst presented in It is essentially an N round Feistel block cipher operating on bit blocks and determined by a bit key FEAL is the standard block cipher but N T T intend that FEAL can be used in cipher block chaining mode when plaintexts are not revealed a cryptogram only environment or for data integrity usage The best published attack on FEAL was given by Den Boer who used chosen plaintexts to recover the key We shall give a method that uses at most twenty chosen plaintexts to recover the key Whereas it may be possible to ensure the absence of chosen plaintexts ensuring the absence of twenty plaintexts may well be too restrictive for most uses