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Calculation of 0xb1

m(x) = 0x11b = 100011011 = x^8 + x^4 + x^3 + x + 1
a(x) =   0xb1 =  10110001 = x^7 + x^5 + x^4 + 1
m(x) = (x) * a(x) + (x^6 + x^5 + x^4 + x^3 + 1)

Calculation of 0xb1-1 in the finite field GF(28)

01111001 = 00000001 * 100011011 + 00000010 * 10110001
00111010 = 00000011 * 100011011 + 00000111 * 10110001
00001101 = 00000111 * 100011011 + 00001100 * 10110001
00000011 = 00011000 * 100011011 + 00111011 * 10110001
00000001 = 01100111 * 100011011 + 11100000 * 10110001
00000000 = 10110001 * 100011011 + 100011011 * 10110001

a-1(x) = x^7 + x^6 + x^5 = 11100000 = 0xe0

The calculation of 0xb1-1 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     0
           1 1 1 0 0 0 1 1      0     0     0
           1 1 1 1 0 0 0 1      0     0     1
SBOX(b1) = 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      1     0     1


SBOX(b1) = 11001000 = c8

For more information see FIPS 197.



Implemented by bachph [at] philba [dot] com