Calculation of 0x2a

m(x) = 0x11b = 100011011 = x^8 + x^4 + x^3 + x + 1
a(x) =   0x2a =  00101010 = x^5 + x^3 + x

m(x) = (x^3 + x) * a(x) + (x^4 + x^3 + x^2 + x + 1)

Calculation of 0x2a^-1 in the finite field GF(2⁸)

00011111 = 00000001 * 100011011 + 00001010 * 101010
00001011 = 00000011 * 100011011 + 00011111 * 101010
00000010 = 00000100 * 100011011 + 00101011 * 101010
00000001 = 00010111 * 100011011 + 10011000 * 101010
00000000 = 00101010 * 100011011 + 100011011 * 101010

a^-1(x) = x^7 + x^4 + x^3 = 10011000 = 0x98

The calculation of 0x2a^-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      0     1     1
           1 1 0 0 0 1 1 1      0     1     0
           1 1 1 0 0 0 1 1      0     0     1
           1 1 1 1 0 0 0 1      1     0     0
SBOX(2a) = 1 1 1 1 1 0 0 0   *  1  +  0  =  0
           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     1

SBOX(2a) = 11100101 = e5

For more information see FIPS 197.

Implemented by bachph [at] philba [dot] com