Unformatted text preview: en-key chosen-plaintexts in 248 steps . While this attack is time-consuming and largely theoretical, it shows that NewDES is weaker than DES. 13.4 FEAL
FEAL was designed by Akihiro Shimizu and Shoji Miyaguchi from NTT Japan . It uses a 64-bit block and a 64-bit key. The idea was to make a DES-like algorithm with a stronger round function. Needing fewer rounds, the algorithm would run faster. Unfortunately, reality fell far short of the design goals. Description of FEAL
Figure 13.3 is a block diagram of one round of FEAL. The encryption process starts with a 64-bit block of plaintext. First, the data block is XORed with 64 key bits. The data block is then split into a left half and a right half. The left half is XORed with the right half to form a new right half. The left and new right halves go through n rounds (four, initially). In each round the right half is combined with 16 bits of key material (using function f) and XORed with the left half to form the new right half. The original right half (before the round) forms the new left half. After n rounds (remember not to switch the left and right halves after the nth round) the left half is again XORed with the right half to form a new right half, and then the left and right halves are concatenated together to form a 64-bit whole. The data block is XORed with another 64 bits of key material, and the algorithm terminates. Figure 13.3 One round of FEAL. Function f takes the 32 bits of data and 16 bits of key material and mixes them together. First the data block is broken up into 8-bit chunks, then the chunks are XORed and substituted with each other. Figure 13.4 is a block diagram of function f. The two functions S0 and S1, are defined as: S0(a,b) = rotate left two bits ((a + b) mod 256) S1(a,b) = rotate left two bits ((a + b + 1) mod 256) The same algorithm can be used for decryption. The only difference is: When decrypting, the key material must be used in the reverse order. Figure 13.5 is a block diagram of the key-generating function. First the 64-bit key is divided into two ha...
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