Description of The Sorkin Variant
The variant described in (Sorkin, 1984) has 16 Feistel rounds, like DES, but no initial or final permutations. The key and block sizes are both 128 bits. The Feistel function operates on a 64-bit half-block of data, together with a 64-bit subkey and 8 "interchange control bits" (ICBs). The ICBs control a swapping operation. The 64-bit data block is considered as a series of eight 8-bit bytes, and if the ICB corresponding to a particular byte is zero, the left and right 4-bit halves (nibbles) are swapped. If the ICB is one, the byte is left unchanged. Each byte is then operated on by two 4×4-bit S-boxes, denoted S0 and S1 — S0 operates on the left 4-bit nibble and S1 operates on the right. The resultant outputs are concatenated and then combined with the subkey using exclusive or (XOR); this is termed "key interruption". This is followed by a permutation operation in two stages; the first permutes each byte under a fixed permutation. The second stage mixes bits between the bytes.
The key-scheduling algorithm is relatively simple. Initially, the 128 key bits are loaded into a shift register. Each round, the left 64 bits of the register form the subkey, and right eight bits form the ICB bits. After each round, the register is rotated 56 bits to the left.
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