Calculation of 0x40

m(x) = 0x11b = 100011011 = x^8 + x^4 + x^3 + x + 1
a(x) =   0x40 =  01000000 = x^6

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

Calculation of 0x40^-1 in the finite field GF(2⁸)

00011011 = 00000001 * 100011011 + 00000100 * 1000000
00000001 = 00000111 * 100011011 + 00011101 * 1000000
00000000 = 01000000 * 100011011 + 100011011 * 1000000

a^-1(x) = x^4 + x^3 + x^2 + 1 = 00011101 = 0x1d

The calculation of 0x40^-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     0
           1 1 1 0 0 0 1 1      1     0     0
           1 1 1 1 0 0 0 1      1     0     1
SBOX(40) = 1 1 1 1 1 0 0 0   *  1  +  0  =  0
           0 1 1 1 1 1 0 0      0     1     0
           0 0 1 1 1 1 1 0      0     1     0
           0 0 0 1 1 1 1 1      0     0     0

SBOX(40) = 00001001 = 9

For more information see FIPS 197.

Implemented by bachph [at] philba [dot] com