From 20529b77df678f63008ae4eb3d561943a9fe13f9 Mon Sep 17 00:00:00 2001
From: Matthias Schiffer <mschiffer@universe-factory.net>
Date: Thu, 15 Mar 2012 07:24:00 +0100
Subject: [PATCH] Improve modular multiplication performance
---
src/ec25519_secret.c | 26 +++++++++++---------------
1 file changed, 11 insertions(+), 15 deletions(-)
diff --git a/src/ec25519_secret.c b/src/ec25519_secret.c
index b79b46c..fd703d6 100644
--- a/src/ec25519_secret.c
+++ b/src/ec25519_secret.c
@@ -25,8 +25,8 @@
*/
/*
- Simple finite field operations on the prime field F_p for
- p = 2^252 + 27742317777372353535851937790883648493, which
+ Simple finite field operations on the prime field F_q for
+ q = 2^252 + 27742317777372353535851937790883648493, which
is the order of the base point used for ec25519
*/
@@ -152,35 +152,31 @@ void ecc_25519_secret_reduce(ecc_secret_key_256 *out, const ecc_secret_key_256 *
/* Montgomery modular multiplication algorithm */
static void montgomery(unsigned char out[32], const unsigned char a[32], const unsigned char b[32]) {
- unsigned int a_i;
unsigned int i, j;
- unsigned int r0;
+ unsigned int nq;
unsigned int u;
for (i = 0; i < 32; i++)
out[i] = 0;
- for (i = 0; i < 256; i++) {
- a_i = a[i / 8] >> (i & 7);
- a_i &= 1;
-
- u = out[0] + a_i*b[0];
- r0 = u & 1;
- u += r0 * q[0];
+ for (i = 0; i < 32; i++) {
+ u = out[0] + a[i]*b[0];
+ nq = (u*27) & 255;
+ u += nq*q[0];
for (j = 1; j < 32; ++j) {
- u += (out[j] + a_i*b[j] + r0*q[j]) << 8;
- out[j-1] = (u >> 1) & 255;
+ u += (out[j] + a[i]*b[j] + nq*q[j]) << 8;
u >>= 8;
+ out[j-1] = u;
}
- out[31] = u >> 1;
+ out[31] = u >> 8;
}
}
void ecc_25519_secret_mult(ecc_secret_key_256 *out, const ecc_secret_key_256 *in1, const ecc_secret_key_256 *in2) {
- /* 2^512 mod p */
+ /* 2^512 mod q */
static const unsigned char C[32] = {
0x01, 0x0f, 0x9c, 0x44, 0xe3, 0x11, 0x06, 0xa4,
0x47, 0x93, 0x85, 0x68, 0xa7, 0x1b, 0x0e, 0xd0,