Calculation of 0xfc

m(x) = 0x11b = 100011011 = x^8 + x^4 + x^3 + x + 1
a(x) =   0xfc =  11111100 = x^7 + x^6 + x^5 + x^4 + x^3 + x^2

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

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

00011111 = 00000001 * 100011011 + 00000011 * 11111100
00000100 = 00001000 * 100011011 + 00011001 * 11111100
00000011 = 00111001 * 100011011 + 01001100 * 11111100
00000001 = 01000011 * 100011011 + 11001101 * 11111100
00000000 = 11111100 * 100011011 + 100011011 * 11111100

a^-1(x) = x^7 + x^6 + x^3 + x^2 + 1 = 11001101 = 0xcd

The calculation of 0xfc^-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     0
           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     0
SBOX(fc) = 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      1     1     0
           0 0 0 1 1 1 1 1      1     0     1

SBOX(fc) = 10110000 = b0

For more information see FIPS 197.

Implemented by bachph [at] philba [dot] com