Calculation of 0x02

m(x) = 0x11b = 100011011 = x^8 + x^4 + x^3 + x + 1
a(x) =   0x2 =  00000010 = x

m(x) = (x^7 + x^3 + x^2 + 1) * a(x) + (1)

Calculation of 0x02^-1 in the finite field GF(2⁸)

00000001 = 00000001 * 100011011 + 10001101 * 10
00000000 = 00000010 * 100011011 + 100011011 * 10

a^-1(x) = x^7 + x^3 + x^2 + 1 = 10001101 = 0x8d

The calculation of 0x02^-1 is made with the Extended Euclidean algorithm. Instead of normal division and multiplication you need to use Polynomialdivision and Polynomialmultiplication.

Affine transformation over GF(2)

           1 0 0 0 1 1 1 1      1     1     1
           1 1 0 0 0 1 1 1      0     1     1
           1 1 1 0 0 0 1 1      1     0     1
           1 1 1 1 0 0 0 1      1     0     0
SBOX(02) = 1 1 1 1 1 0 0 0   *  0  +  0  =  1
           0 1 1 1 1 1 0 0      0     1     1
           0 0 1 1 1 1 1 0      0     1     1
           0 0 0 1 1 1 1 1      1     0     0

SBOX(02) = 01110111 = 77

For more information see FIPS 197.

Implemented by bachph [at] philba [dot] com