Tuesday, March 21, 2017

Cryptography: AES - S-Box


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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