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bn/bn_exp.c: constant-time MOD_EXP_CTIME_COPY_FROM_PREBUF.

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Performance penalty varies from platform to platform, and even
key length. For rsa2048 sign it was observed to reach almost 10%.

CVE-2016-0702

Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>
This commit is contained in:
Andy Polyakov 2016-01-25 20:38:38 +01:00 committed by Matt Caswell
parent 2e0956ba6d
commit d6482a82bc
1 changed files with 57 additions and 18 deletions
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75
crypto/bn/bn_exp.c
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@ -109,6 +109,7 @@
*/
#include "internal/cryptlib.h"
#include "internal/constant_time_locl.h"
#include "bn_lcl.h"
#include <stdlib.h>
@ -605,15 +606,17 @@ static BN_ULONG bn_get_bits(const BIGNUM *a, int bitpos)
static int MOD_EXP_CTIME_COPY_TO_PREBUF(const BIGNUM *b, int top,
unsigned char *buf, int idx,
int width)
int window)
{
size_t i, j;
int i, j;
int width = 1 << window;
BN_ULONG *table = (BN_ULONG *)buf;
if (top > b->top)
top = b->top; /* this works because 'buf' is explicitly
* zeroed */
for (i = 0, j = idx; i < top * sizeof b->d[0]; i++, j += width) {
buf[j] = ((unsigned char *)b->d)[i];
for (i = 0, j = idx; i < top; i++, j += width) {
table[j] = b->d[i];
}
return 1;
@ -621,15 +624,51 @@ static int MOD_EXP_CTIME_COPY_TO_PREBUF(const BIGNUM *b, int top,
static int MOD_EXP_CTIME_COPY_FROM_PREBUF(BIGNUM *b, int top,
unsigned char *buf, int idx,
int width)
int window)
{
size_t i, j;
int i, j;
int width = 1 << window;
volatile BN_ULONG *table = (volatile BN_ULONG *)buf;
if (bn_wexpand(b, top) == NULL)
return 0;
for (i = 0, j = idx; i < top * sizeof b->d[0]; i++, j += width) {
((unsigned char *)b->d)[i] = buf[j];
if (window <= 3) {
for (i = 0; i < top; i++, table += width) {
BN_ULONG acc = 0;
for (j = 0; j < width; j++) {
acc |= table[j] &
((BN_ULONG)0 - (constant_time_eq_int(j,idx)&1));
}
b->d[i] = acc;
}
} else {
int xstride = 1 << (window - 2);
BN_ULONG y0, y1, y2, y3;
i = idx >> (window - 2); /* equivalent of idx / xstride */
idx &= xstride - 1; /* equivalent of idx % xstride */
y0 = (BN_ULONG)0 - (constant_time_eq_int(i,0)&1);
y1 = (BN_ULONG)0 - (constant_time_eq_int(i,1)&1);
y2 = (BN_ULONG)0 - (constant_time_eq_int(i,2)&1);
y3 = (BN_ULONG)0 - (constant_time_eq_int(i,3)&1);
for (i = 0; i < top; i++, table += width) {
BN_ULONG acc = 0;
for (j = 0; j < xstride; j++) {
acc |= ( (table[j + 0 * xstride] & y0) |
(table[j + 1 * xstride] & y1) |
(table[j + 2 * xstride] & y2) |
(table[j + 3 * xstride] & y3) )
& ((BN_ULONG)0 - (constant_time_eq_int(j,idx)&1));
}
b->d[i] = acc;
}
}
b->top = top;
@ -1063,9 +1102,9 @@ int BN_mod_exp_mont_consttime(BIGNUM *rr, const BIGNUM *a, const BIGNUM *p,
} else
#endif
{
if (!MOD_EXP_CTIME_COPY_TO_PREBUF(&tmp, top, powerbuf, 0, numPowers))
if (!MOD_EXP_CTIME_COPY_TO_PREBUF(&tmp, top, powerbuf, 0, window))
goto err;
if (!MOD_EXP_CTIME_COPY_TO_PREBUF(&am, top, powerbuf, 1, numPowers))
if (!MOD_EXP_CTIME_COPY_TO_PREBUF(&am, top, powerbuf, 1, window))
goto err;
/*
@ -1077,15 +1116,15 @@ int BN_mod_exp_mont_consttime(BIGNUM *rr, const BIGNUM *a, const BIGNUM *p,
if (window > 1) {
if (!BN_mod_mul_montgomery(&tmp, &am, &am, mont, ctx))
goto err;
if (!MOD_EXP_CTIME_COPY_TO_PREBUF
(&tmp, top, powerbuf, 2, numPowers))
if (!MOD_EXP_CTIME_COPY_TO_PREBUF(&tmp, top, powerbuf, 2,
window))
goto err;
for (i = 3; i < numPowers; i++) {
/* Calculate a^i = a^(i-1) * a */
if (!BN_mod_mul_montgomery(&tmp, &am, &tmp, mont, ctx))
goto err;
if (!MOD_EXP_CTIME_COPY_TO_PREBUF
(&tmp, top, powerbuf, i, numPowers))
if (!MOD_EXP_CTIME_COPY_TO_PREBUF(&tmp, top, powerbuf, i,
window))
goto err;
}
}
@ -1093,8 +1132,8 @@ int BN_mod_exp_mont_consttime(BIGNUM *rr, const BIGNUM *a, const BIGNUM *p,
bits--;
for (wvalue = 0, i = bits % window; i >= 0; i--, bits--)
wvalue = (wvalue << 1) + BN_is_bit_set(p, bits);
if (!MOD_EXP_CTIME_COPY_FROM_PREBUF
(&tmp, top, powerbuf, wvalue, numPowers))
if (!MOD_EXP_CTIME_COPY_FROM_PREBUF(&tmp, top, powerbuf, wvalue,
window))
goto err;
/*
@ -1114,8 +1153,8 @@ int BN_mod_exp_mont_consttime(BIGNUM *rr, const BIGNUM *a, const BIGNUM *p,
/*
* Fetch the appropriate pre-computed value from the pre-buf
*/
if (!MOD_EXP_CTIME_COPY_FROM_PREBUF
(&am, top, powerbuf, wvalue, numPowers))
if (!MOD_EXP_CTIME_COPY_FROM_PREBUF(&am, top, powerbuf, wvalue,
window))
goto err;
/* Multiply the result into the intermediate result */