README.md

Inverse Mix Column with pre-calculated finite field example.

I've found plenty examples of forward Mix Column examples with pre-calculated Galois field, but I couldn't find one for inverse Mix Column, so I created this repo. The heart of this example lies in the excerpt below:

void inv_mix_column (uint8_t data [16])
{
	for (int i = 0 ; i < 4 ; i ++)
	{
		uint8_t a [4], b [4], c [4], d [4], h;

		for (int j = 0 ; j < 4 ; j ++)
		{
			a [j] = data [4 * i + j];
			
			h = (uint8_t) ((signed char) a [j] >> 7);
			b [j] = (a [j] << 1) ^ (0x1b & h);

			h = (uint8_t) ((signed char) b [j] >> 7);
			c [j] = (b [j] << 1) ^ (0x1b & h);

			h = (uint8_t) ((signed char) c [j] >> 7);
			d [j] = (c [j] << 1) ^ (0x1b & h) ^ a [j];
		}
		
		data [4 * i + 0] = (d [0] ^ c [0] ^ b [0] ^ a [0]) ^ (d [1] ^ b [1]) ^ (d [2] ^ c [2]) ^ d [3];
		data [4 * i + 1] = d [0] ^ (d [1] ^ c [1] ^ b [1] ^ a [1]) ^ (d [2] ^ b [2]) ^ (d [3] ^ c [3]);
		data [4 * i + 2] = (d [0] ^ c [0]) ^ d [1] ^ (d [2] ^ c [2] ^ b [2] ^ a [2]) ^ (d [3] ^ b [3]);
		data [4 * i + 3] = (d [0] ^ b [0]) ^ (d [1] ^ c [1]) ^ d [2] ^ (d [3] ^ c [3] ^ b [3] ^ a [3]);
	}
}

Essentially, d[n] is 9*a[n], c[n] is 4*a[n], and b[n] is 2*a[n]. It simply adds and/or substracts (using xor) elements of these arrays to get 14*a[n], 13*a[n], 11*a[n], and 9*a[n].

signed char is useful because we can broadcast an entire 8-bit variable with 1s when right shift is applied.

The decryption algorithm implemented is vulnerable. To make things secure, the bitslice technique needs to be used.

Sample result

Hardware Spec.

CPU: AMD Ryzen 9 7900X (24)
RAM: 32 GB
OS : Linux 6.7.2 (zen, x86-64)

Command Lines

nasm -f elf64 -o invmc.o invmc.asm
clang++ -std=c++20 -O3 -o invmc invmc.cpp invmc.o
./invmc

Result:

The results are for just the decryption algorithm only.

Block size    : 16
Block count   : 8192
Vector size   : 131072
AES-NI        : 13.305μs
Soft Precalc  : 1631.12μs
Soft GMul     : 2762.82μs

Just use AES-NI.