Calculation of 0xbf
m(x) = 0x11b = 100011011 = x^8 + x^4 + x^3 + x + 1
a(x) = 0xbf = 10111111 = x^7 + x^5 + x^4 + x^3 + x^2 + x + 1
m(x) = (x) * a(x) + (x^6 + x^5 + x^2 + 1)
Calculation of 0xbf-1 in the finite field GF(28)01100101 = 00000001 * 100011011 + 00000010 * 10111111
00010000 = 00000011 * 100011011 + 00000111 * 10111111
00000101 = 00001011 * 100011011 + 00010000 * 10111111
00000001 = 00100100 * 100011011 +
01010111 * 10111111
00000000 = 10111111 * 100011011 + 100011011 * 10111111
a
-1(x) = x^6 + x^4 + x^2 + x + 1 = 01010111 = 0x57
The calculation of 0xbf
-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 1 1 0
1 1 1 0 0 0 1 1 1 0 0
1 1 1 1 0 0 0 1 0 0 1
SBOX(bf) = 1 1 1 1 1 0 0 0 * 1 + 0 = 0
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(bf) = 00001000 = 8
For more information see
FIPS 197.
Implemented by bachph [at] philba [dot] com