/* gf256mul.S */
/*
This file is part of the AVR-Crypto-Lib.
Copyright (C) 2008 Daniel Otte (daniel.otte@rub.de)
This program is free software: you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation, either version 3 of the License, or
(at your option) any later version.
This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License for more details.
You should have received a copy of the GNU General Public License
along with this program. If not, see .
*/
/*
* File: gf256mul.S
* Author: Daniel Otte
* Date: 2008-12-19
* License: GPLv3 or later
* Description: peasant's algorithm for multiplication in GF(2^8)
*
*/
#include
#define OPTIMIZE_SMALL_A
/*
* param a: r24
* param b: r22
* param reducer: r20
*/
A = 23
B = 22
P = 24
.global gf256mul
#ifdef OPTIMIZE_SMALL_A
gf256mul:
mov A, r24
clr r24
1:
lsr A
breq 4f
brcc 2f
eor P, B
2:
lsl B
brcc 3f
eor B, r20
3:
rjmp 1b
4:
brcc 2f
eor P, B
2:
ret
#else
/*
uint8_t gf256mul(uint8_t a, uint8_t b, uint8_t p) {
uint8_t r = 0, c = 8;
do {
if (a & 1) {
r ^= b;
}
a >>= 1;
if (b & 0x80) {
b ^= p;
}
b <<= 1;
} while (--c);
return r;
}
*/
gf256mul:
mov A, r24
clr r24
ldi r25, 8
1:
lsr A
brcc 2f
eor P, B
2:
lsl B
brcc 3f
eor B, r20
3:
dec r25
brne 1b
ret
#endif