Calculation of 0xbd

m(x) = 0x11b = 100011011 = x^8 + x^4 + x^3 + x + 1
a(x) =   0xbd =  10111101 = x^7 + x^5 + x^4 + x^3 + x^2 + 1

m(x) = (x) * a(x) + (x^6 + x^5 + 1)

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

01100001 = 00000001 * 100011011 + 00000010 * 10111101
00011110 = 00000011 * 100011011 + 00000111 * 10111101
00000111 = 00001110 * 100011011 + 00011001 * 10111101
00000010 = 00111011 * 100011011 + 01100011 * 10111101
00000001 = 01000011 * 100011011 + 10111100 * 10111101
00000000 = 10111101 * 100011011 + 100011011 * 10111101

a^-1(x) = x^7 + x^5 + x^4 + x^3 + x^2 = 10111100 = 0xbc

The calculation of 0xbd^-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     0
           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     1
SBOX(bd) = 1 1 1 1 1 0 0 0   *  1  +  0  =  1
           0 1 1 1 1 1 0 0      1     1     1
           0 0 1 1 1 1 1 0      0     1     1
           0 0 0 1 1 1 1 1      1     0     0

SBOX(bd) = 01111010 = 7a

For more information see FIPS 197.

Implemented by bachph [at] philba [dot] com