Calculation of 0xe9
m(x) = 0x11b = 100011011 = x^8 + x^4 + x^3 + x + 1
a(x) = 0xe9 = 11101001 = x^7 + x^6 + x^5 + x^3 + 1
m(x) = (x + 1) * a(x) + (x^5)
Calculation of 0xe9-1 in the finite field GF(28)00100000 = 00000001 * 100011011 + 00000011 * 11101001
00001001 = 00000111 * 100011011 + 00001000 * 11101001
00000100 = 00011101 * 100011011 + 00100011 * 11101001
00000001 = 00111101 * 100011011 +
01001110 * 11101001
00000000 = 11101001 * 100011011 + 100011011 * 11101001
a
-1(x) = x^6 + x^3 + x^2 + x = 01001110 = 0x4e
The calculation of 0xe9
-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 1 1 1
1 1 1 0 0 0 1 1 1 0 1
1 1 1 1 0 0 0 1 1 0 1
SBOX(e9) = 1 1 1 1 1 0 0 0 * 0 + 0 = 1
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 0
SBOX(e9) = 00011110 = 1e
For more information see
FIPS 197.
Implemented by bachph [at] philba [dot] com