Editing ICE (cipher) (section)

==Structure==
ICE is a 16-round [[Feistel network]].  Each round uses a 32→32 bit F function, which uses 60 bits of key material.

The structure of the F function is somewhat similar to DES: The input is expanded by taking overlapping fields, the expanded input is XORed with a key, and the result is fed to a number of reducing S-boxes which undo the expansion.

First, ICE divides the input into 4 overlapping 10-bit values.  They are bits 30, 31 and 0–7 of the input for the first 10-bit value and for the next values 6–15, 14–23, and 22–31.

Second is a keyed permutation, which is unique to ICE.  Using a 20-bit permutation subkey, bits are swapped between halves of the 40-bit expanded input.  (If subkey bit ''i'' is 1, then bits ''i'' and ''i''+20 are swapped.)

Third, the 40-bit value is XORed with 40 more subkey bits.

Fourth, the value is fed through 4 10-bit S-boxes, each of which produces 8 bits of output.  (These are much larger than DES's 8 6→4 bit S-boxes.)

Fifth, the S-box output bits are permuted so that each S-box's outputs are routed to each 4-bit field of 32-bit word, including 2 of the 8 "overlap" bits duplicated during the next round's expansion.

Like DES, a software implementation would typically store the S-boxes pre-permuted, in 4 1024×32 bit lookup tables.