Calculation of 0x7c
m(x) = 0x11b = 100011011 = x^8 + x^4 + x^3 + x + 1
a(x) = 0x7c = 01111100 = x^6 + x^5 + x^4 + x^3 + x^2
m(x) = (x^2 + x) * a(x) + (x^4 + x + 1)
Calculation of 0x7c-1 in the finite field GF(28)00010011 = 00000001 * 100011011 + 00000110 * 1111100
00000101 = 00000111 * 100011011 + 00010011 * 1111100
00000010 = 00011010 * 100011011 + 01011001 * 1111100
00000001 = 00110011 * 100011011 +
10100001 * 1111100
00000000 = 01111100 * 100011011 + 100011011 * 1111100
a
-1(x) = x^7 + x^5 + 1 = 10100001 = 0xa1
The calculation of 0x7c
-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 0 0 0
1 1 1 1 0 0 0 1 0 0 0
SBOX(7c) = 1 1 1 1 1 0 0 0 * 0 + 0 = 1
0 1 1 1 1 1 0 0 1 1 0
0 0 1 1 1 1 1 0 0 1 0
0 0 0 1 1 1 1 1 1 0 0
SBOX(7c) = 00010000 = 10
For more information see
FIPS 197.
Implemented by bachph [at] philba [dot] com