Calculation of 0xa1
m(x) = 0x11b = 100011011 = x^8 + x^4 + x^3 + x + 1
a(x) = 0xa1 = 10100001 = x^7 + x^5 + 1
m(x) = (x) * a(x) + (x^6 + x^4 + x^3 + 1)
Calculation of 0xa1-1 in the finite field GF(28)01011001 = 00000001 * 100011011 + 00000010 * 10100001
00010011 = 00000010 * 100011011 + 00000101 * 10100001
00000110 = 00001011 * 100011011 + 00010011 * 10100001
00000001 = 00110011 * 100011011 +
01111100 * 10100001
00000000 = 10100001 * 100011011 + 100011011 * 10100001
a
-1(x) = x^6 + x^5 + x^4 + x^3 + x^2 = 01111100 = 0x7c
The calculation of 0xa1
-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 0
SBOX(a1) = 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 1 1 0
0 0 0 1 1 1 1 1 0 0 0
SBOX(a1) = 00110010 = 32
For more information see
FIPS 197.
Implemented by bachph [at] philba [dot] com