How is the S-box constructed?
1. Initialize the S-box with the byte values in
ascending sequence row by row. The first row contains {00}, {01}, {02}, etc.,
the second row contains {10}, {11}, etc., and so on. Thus, the value of the
byte at row x, column y is {xy}.
2. Map each byte in the S-box to its
multiplicative inverse in the finite field GF(28); the value {00} is mapped to itself.
3. Consider that each byte in the S-box
consists of 8 bits labeled (b7, b6, b5, b4, b3, b2, b1, b0). Apply the following transformation to each bit
of each byte in the S-box:
where ci
is the ith bit of byte c with the value {63}; that is, (c7c6c5c4c3c2c1c0) =
(01100011). The prime (') indicates that the variable is to be updated by the
value on the right.
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