101 lines
1.6 KiB
ArmAsm
101 lines
1.6 KiB
ArmAsm
/* gf256mul.S */
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/*
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This file is part of the AVR-Crypto-Lib.
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Copyright (C) 2006-2015 Daniel Otte (bg@nerilex.org)
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This program is free software: you can redistribute it and/or modify
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it under the terms of the GNU General Public License as published by
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the Free Software Foundation, either version 3 of the License, or
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(at your option) any later version.
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This program is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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GNU General Public License for more details.
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You should have received a copy of the GNU General Public License
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along with this program. If not, see <http://www.gnu.org/licenses/>.
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*/
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/*
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* File: gf256mul.S
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* Author: Daniel Otte
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* Date: 2008-12-19
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* License: GPLv3 or later
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* Description: peasant's algorithm for multiplication in GF(2^8)
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*
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*/
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#include <avr/io.h>
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#define OPTIMIZE_SMALL_A
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/*
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* param a: r24
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* param b: r22
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* param reducer: r20
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*/
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A = 23
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B = 22
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P = 24
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.global gf256mul
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#ifdef OPTIMIZE_SMALL_A
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gf256mul:
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mov A, r24
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clr r24
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1:
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lsr A
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breq 4f
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brcc 2f
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eor P, B
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2:
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lsl B
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brcc 3f
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eor B, r20
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3:
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rjmp 1b
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4:
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brcc 2f
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eor P, B
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2:
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ret
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#else
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/*
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uint8_t gf256mul(uint8_t a, uint8_t b, uint8_t p) {
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uint8_t r = 0, c = 8;
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do {
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if (a & 1) {
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r ^= b;
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}
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a >>= 1;
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if (b & 0x80) {
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b ^= p;
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}
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b <<= 1;
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} while (--c);
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return r;
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}
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*/
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gf256mul:
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mov A, r24
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clr r24
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ldi r25, 8
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1:
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lsr A
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brcc 2f
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eor P, B
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2:
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lsl B
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brcc 3f
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eor B, r20
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3:
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dec r25
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brne 1b
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ret
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#endif
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