FROG is another block cipher which is based on whole-byte operations. In structure, it has a slight resemblance to NewDES. The original description of it is available at http://www.tecapro.com/aesfrog.htm.A weakness has been uncovered in it, but a minor modification would correct that: of its eight rounds, the last four could be performed in deciphering mode. (An appropriate rearrangement of bytes between the first four rounds and the last four would ensure against the possibility of a key existing that would cause the last four rounds to undo the work of the first four.)
It does, however, require a considerable amount of RAM to store an unusually large amount of subkey material.
Each round consists of sixteen steps, one for each byte of the block. Each round has the following subkey material:
In each step, one does the following:
The sequence of numbers from 0 to 15 is generated so that it is a permutation of the numbers from 0 to 15 with a single cycle. This ensures that no number is its own substitute. Then, if any number has itself plus 1 as its substitute, that number is incremented (modulo 16) to avoid cancelling the fixed XOR.
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