Calculation of 0xaa
m(x) = 0x11b = 100011011 = x^8 + x^4 + x^3 + x + 1
a(x) = 0xaa = 10101010 = x^7 + x^5 + x^3 + x
m(x) = (x) * a(x) + (x^6 + x^3 + x^2 + x + 1)
Calculation of 0xaa-1 in the finite field GF(28)01001111 = 00000001 * 100011011 + 00000010 * 10101010
00110100 = 00000010 * 100011011 + 00000101 * 10101010
00010011 = 00000111 * 100011011 + 00001101 * 10101010
00000001 = 00001011 * 100011011 +
00010010 * 10101010
00000000 = 10101010 * 100011011 + 100011011 * 10101010
a
-1(x) = x^4 + x = 00010010 = 0x12
The calculation of 0xaa
-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 0
1 1 1 0 0 0 1 1 0 0 1
1 1 1 1 0 0 0 1 0 0 1
SBOX(aa) = 1 1 1 1 1 0 0 0 * 1 + 0 = 0
0 1 1 1 1 1 0 0 0 1 1
0 0 1 1 1 1 1 0 0 1 0
0 0 0 1 1 1 1 1 0 0 1
SBOX(aa) = 10101100 = ac
For more information see
FIPS 197.
Implemented by bachph [at] philba [dot] com