Calculation of 0x05

m(x) = 0x11b = 100011011 = x^8 + x^4 + x^3 + x + 1
a(x) =   0x5 =  00000101 = x^2 + 1

m(x) = (x^6 + x^4 + x) * a(x) + (1)

Calculation of 0x05^-1 in the finite field GF(2⁸)

00000001 = 00000001 * 100011011 + 01010010 * 101
00000000 = 00000101 * 100011011 + 100011011 * 101

a^-1(x) = x^6 + x^4 + x = 01010010 = 0x52

The calculation of 0x05^-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      1     1     1
           1 1 1 0 0 0 1 1      0     0     0
           1 1 1 1 0 0 0 1      0     0     1
SBOX(05) = 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      1     1     1
           0 0 0 1 1 1 1 1      0     0     0

SBOX(05) = 01101011 = 6b

For more information see FIPS 197.

Implemented by bachph [at] philba [dot] com