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Calculation of 0xd5

m(x) = 0x11b = 100011011 = x^8 + x^4 + x^3 + x + 1
a(x) =   0xd5 =  11010101 = x^7 + x^6 + x^4 + x^2 + 1
m(x) = (x + 1) * a(x) + (x^6 + x^5 + x^2)

Calculation of 0xd5-1 in the finite field GF(28)

01100100 = 00000001 * 100011011 + 00000011 * 11010101
00011101 = 00000010 * 100011011 + 00000111 * 11010101
00001101 = 00001011 * 100011011 + 00011000 * 11010101
00000111 = 00010100 * 100011011 + 00110111 * 11010101
00000011 = 00100011 * 100011011 + 01110110 * 11010101
00000001 = 01010010 * 100011011 + 11011011 * 11010101
00000000 = 11010101 * 100011011 + 100011011 * 11010101

a-1(x) = x^7 + x^6 + x^4 + x^3 + x + 1 = 11011011 = 0xdb

The calculation of 0xd5-1 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     1
           1 1 0 0 0 1 1 1      1     1     1
           1 1 1 0 0 0 1 1      0     0     0
           1 1 1 1 0 0 0 1      1     0     0
SBOX(d5) = 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      1     0     0


SBOX(d5) = 00000011 = 3

For more information see FIPS 197.



Implemented by bachph [at] philba [dot] com