Calculation of 0x33

m(x) = 0x11b = 100011011 = x^8 + x^4 + x^3 + x + 1
a(x) =   0x33 =  00110011 = x^5 + x^4 + x + 1

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

Calculation of 0x33^-1 in the finite field GF(2⁸)

00011010 = 00000001 * 100011011 + 00001111 * 110011
00000111 = 00000010 * 100011011 + 00011111 * 110011
00000001 = 00001011 * 100011011 + 01101100 * 110011
00000000 = 00110011 * 100011011 + 100011011 * 110011

a^-1(x) = x^6 + x^5 + x^3 + x^2 = 01101100 = 0x6c

The calculation of 0x33^-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     1
           1 1 1 0 0 0 1 1      1     0     0
           1 1 1 1 0 0 0 1      1     0     0
SBOX(33) = 1 1 1 1 1 0 0 0   *  0  +  0  =  0
           0 1 1 1 1 1 0 0      1     1     0
           0 0 1 1 1 1 1 0      1     1     1
           0 0 0 1 1 1 1 1      0     0     1

SBOX(33) = 11000011 = c3

For more information see FIPS 197.

Implemented by bachph [at] philba [dot] com