examine some attacks on the FROG cipher. First we give a differential attack which uses about 2^58 chosen plaintexts and very little time for the analysis; it works for about 2^-33.0 of the keyspace. Then we describe a linear attack that uses 2^56 known texts and works for 2^-31.8 of the keyspace. The linear attack can also be converted to a ciphertext-only attack using 2^64 known ciphertexts. Also, the decryption function of FROG is a lot weaker than the encryption function. We show a differential attack on the decryption function that requires 2^36 chosen ciphertexts and works on 2^-29.3 of the keyspace. Using our best attack, an attacker with a sufficient number of cryptanalytical targets can expect to recover his first key after 2^56.7 work.

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