Calculation of 0xb8
m(x) = 0x11b = 100011011 = x^8 + x^4 + x^3 + x + 1
a(x) = 0xb8 = 10111000 = x^7 + x^5 + x^4 + x^3
m(x) = (x) * a(x) + (x^6 + x^5 + x^3 + x + 1)
Calculation of 0xb8-1 in the finite field GF(28)01101011 = 00000001 * 100011011 + 00000010 * 10111000
00000101 = 00000011 * 100011011 + 00000111 * 10111000
00000010 = 00100110 * 100011011 + 01010001 * 10111000
00000001 = 01001111 * 100011011 +
10100101 * 10111000
00000000 = 10111000 * 100011011 + 100011011 * 10111000
a
-1(x) = x^7 + x^5 + x^2 + 1 = 10100101 = 0xa5
The calculation of 0xb8
-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 1
1 1 1 1 0 0 0 1 0 0 1
SBOX(b8) = 1 1 1 1 1 0 0 0 * 0 + 0 = 0
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(b8) = 01101100 = 6c
For more information see
FIPS 197.
Implemented by bachph [at] philba [dot] com