Results Announcement: D. Wagner, N. Ferguson, and B. Schneier, "Cryptanalysis of Frog," Counterpane Systems Report, Aug 1998. Abstract: We 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 which 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. Taken together, these observations suggest that FROG is not a very strong candidate for the AES. This paper is available at http://www.counterpane.com/publish.html, and will be made available at the AES Workshop next week. Cheers, Bruce ********************************************************************** Bruce Schneier, President, Counterpane Systems Phone: 612-823-1098 101 E Minnehaha Parkway, Minneapolis, MN 55419 Fax: 612-823-1590 Free crypto newsletter. See: http://www.counterpane.com