Cryptanalysis of the CFB mode of the DES witha reduced number

Three attacks on the DES with a reduced number of rounds in the Cipher Feedback Mode (CFB) are studied, namely a meet in the middle attack, a diierential attack, and a linear attack. These attacks are based on the same principles as the corresponding attacks on the ECB mode. They are compared to the three basic attacks on the CFB mode, namely an exhaustive key search, a comparison attack, and a tabulation attack. In 8-bit CFB and with 8 rounds in stead of 16, a diierential attack with 2 39:4 chosen ciphertexts can nd 3 key bits, and a linear attack with 2 31 known plaintexts can nd 7 key bits. This suggests that it is not safe to reduce the number of rounds in order to improve the performance. Moreover, it is shown that the nal permutation has some cryptographic signiicance in the CFB mode.