Calculation of 0xf0

m(x) = 0x11b = 100011011 = x^8 + x^4 + x^3 + x + 1
a(x) =   0xf0 =  11110000 = x^7 + x^6 + x^5 + x^4

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

Calculation of 0xf0^-1 in the finite field GF(2⁸)

00001011 = 00000001 * 100011011 + 00000011 * 11110000
00000101 = 00011011 * 100011011 + 00101100 * 11110000
00000001 = 00110111 * 100011011 + 01011011 * 11110000
00000000 = 11110000 * 100011011 + 100011011 * 11110000

a^-1(x) = x^6 + x^4 + x^3 + x + 1 = 01011011 = 0x5b

The calculation of 0xf0^-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      1     1     0
           1 1 1 0 0 0 1 1      0     0     1
           1 1 1 1 0 0 0 1      1     0     1
SBOX(f0) = 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      1     1     0
           0 0 0 1 1 1 1 1      0     0     1

SBOX(f0) = 10001100 = 8c

For more information see FIPS 197.

Implemented by bachph [at] philba [dot] com