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openssl/crypto/bn/bn_mont.c
David Benjamin f345b1f39d Fix timing leak in BN_from_montgomery_word. 
BN_from_montgomery_word doesn't have a constant memory access pattern.
Replace the pointer trick with a constant-time select. There is, of
course, still the bn_correct_top leak pervasive in BIGNUM itself.

See also https://boringssl-review.googlesource.com/22904 from BoringSSL.

Reviewed-by: Andy Polyakov <appro@openssl.org>
Reviewed-by: Kurt Roeckx <kurt@roeckx.be>
(Merged from https://github.com/openssl/openssl/pull/5228)
2018-02-01 21:52:17 +01:00

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/*
* Copyright 1995-2018 The OpenSSL Project Authors. All Rights Reserved.
*
* Licensed under the OpenSSL license (the "License"). You may not use
* this file except in compliance with the License. You can obtain a copy
* in the file LICENSE in the source distribution or at
* https://www.openssl.org/source/license.html
*/
/*
* Details about Montgomery multiplication algorithms can be found at
* http://security.ece.orst.edu/publications.html, e.g.
* http://security.ece.orst.edu/koc/papers/j37acmon.pdf and
* sections 3.8 and 4.2 in http://security.ece.orst.edu/koc/papers/r01rsasw.pdf
*/
#include "internal/cryptlib.h"
#include "bn_lcl.h"
#define MONT_WORD /* use the faster word-based algorithm */
#ifdef MONT_WORD
static int BN_from_montgomery_word(BIGNUM *ret, BIGNUM *r, BN_MONT_CTX *mont);
#endif
int BN_mod_mul_montgomery(BIGNUM *r, const BIGNUM *a, const BIGNUM *b,
BN_MONT_CTX *mont, BN_CTX *ctx)
{
BIGNUM *tmp;
int ret = 0;
#if defined(OPENSSL_BN_ASM_MONT) && defined(MONT_WORD)
int num = mont->N.top;
if (num > 1 && a->top == num && b->top == num) {
if (bn_wexpand(r, num) == NULL)
return 0;
if (bn_mul_mont(r->d, a->d, b->d, mont->N.d, mont->n0, num)) {
r->neg = a->neg ^ b->neg;
r->top = num;
bn_correct_top(r);
return 1;
}
}
#endif
BN_CTX_start(ctx);
tmp = BN_CTX_get(ctx);
if (tmp == NULL)
goto err;
bn_check_top(tmp);
if (a == b) {
if (!BN_sqr(tmp, a, ctx))
goto err;
} else {
if (!BN_mul(tmp, a, b, ctx))
goto err;
}
/* reduce from aRR to aR */
#ifdef MONT_WORD
if (!BN_from_montgomery_word(r, tmp, mont))
goto err;
#else
if (!BN_from_montgomery(r, tmp, mont, ctx))
goto err;
#endif
bn_check_top(r);
ret = 1;
err:
BN_CTX_end(ctx);
return ret;
}
#ifdef MONT_WORD
static int BN_from_montgomery_word(BIGNUM *ret, BIGNUM *r, BN_MONT_CTX *mont)
{
BIGNUM *n;
BN_ULONG *ap, *np, *rp, n0, v, carry;
int nl, max, i;
n = &(mont->N);
nl = n->top;
if (nl == 0) {
ret->top = 0;
return 1;
}
max = (2 * nl); /* carry is stored separately */
if (bn_wexpand(r, max) == NULL)
return 0;
r->neg ^= n->neg;
np = n->d;
rp = r->d;
/* clear the top words of T */
i = max - r->top;
if (i)
memset(&rp[r->top], 0, sizeof(*rp) * i);
r->top = max;
n0 = mont->n0[0];
/*
* Add multiples of |n| to |r| until R = 2^(nl * BN_BITS2) divides it. On
* input, we had |r| < |n| * R, so now |r| < 2 * |n| * R. Note that |r|
* includes |carry| which is stored separately.
*/
for (carry = 0, i = 0; i < nl; i++, rp++) {
v = bn_mul_add_words(rp, np, nl, (rp[0] * n0) & BN_MASK2);
v = (v + carry + rp[nl]) & BN_MASK2;
carry |= (v != rp[nl]);
carry &= (v <= rp[nl]);
rp[nl] = v;
}
if (bn_wexpand(ret, nl) == NULL)
return 0;
ret->top = nl;
ret->neg = r->neg;
rp = ret->d;
/*
* Shift |nl| words to divide by R. We have |ap| < 2 * |n|. Note that |ap|
* includes |carry| which is stored separately.
*/
ap = &(r->d[nl]);
/*
* |v| is one if |ap| - |np| underflowed or zero if it did not. Note |v|
* cannot be -1. That would imply the subtraction did not fit in |nl| words,
* and we know at most one subtraction is needed.
*/
v = bn_sub_words(rp, ap, np, nl) - carry;
v = 0 - v;
for (i = 0; i < nl; i++) {
rp[i] = (v & ap[i]) | (~v & rp[i]);
ap[i] = 0;
}
bn_correct_top(r);
bn_correct_top(ret);
bn_check_top(ret);
return 1;
}
#endif /* MONT_WORD */
int BN_from_montgomery(BIGNUM *ret, const BIGNUM *a, BN_MONT_CTX *mont,
BN_CTX *ctx)
{
int retn = 0;
#ifdef MONT_WORD
BIGNUM *t;
BN_CTX_start(ctx);
if ((t = BN_CTX_get(ctx)) && BN_copy(t, a))
retn = BN_from_montgomery_word(ret, t, mont);
BN_CTX_end(ctx);
#else /* !MONT_WORD */
BIGNUM *t1, *t2;
BN_CTX_start(ctx);
t1 = BN_CTX_get(ctx);
t2 = BN_CTX_get(ctx);
if (t2 == NULL)
goto err;
if (!BN_copy(t1, a))
goto err;
BN_mask_bits(t1, mont->ri);
if (!BN_mul(t2, t1, &mont->Ni, ctx))
goto err;
BN_mask_bits(t2, mont->ri);
if (!BN_mul(t1, t2, &mont->N, ctx))
goto err;
if (!BN_add(t2, a, t1))
goto err;
if (!BN_rshift(ret, t2, mont->ri))
goto err;
if (BN_ucmp(ret, &(mont->N)) >= 0) {
if (!BN_usub(ret, ret, &(mont->N)))
goto err;
}
retn = 1;
bn_check_top(ret);
err:
BN_CTX_end(ctx);
#endif /* MONT_WORD */
return retn;
}
BN_MONT_CTX *BN_MONT_CTX_new(void)
{
BN_MONT_CTX *ret;
if ((ret = OPENSSL_malloc(sizeof(*ret))) == NULL)
return NULL;
BN_MONT_CTX_init(ret);
ret->flags = BN_FLG_MALLOCED;
return ret;
}
void BN_MONT_CTX_init(BN_MONT_CTX *ctx)
{
ctx->ri = 0;
bn_init(&(ctx->RR));
bn_init(&(ctx->N));
bn_init(&(ctx->Ni));
ctx->n0[0] = ctx->n0[1] = 0;
ctx->flags = 0;
}
void BN_MONT_CTX_free(BN_MONT_CTX *mont)
{
if (mont == NULL)
return;
BN_clear_free(&(mont->RR));
BN_clear_free(&(mont->N));
BN_clear_free(&(mont->Ni));
if (mont->flags & BN_FLG_MALLOCED)
OPENSSL_free(mont);
}
int BN_MONT_CTX_set(BN_MONT_CTX *mont, const BIGNUM *mod, BN_CTX *ctx)
{
int ret = 0;
BIGNUM *Ri, *R;
if (BN_is_zero(mod))
return 0;
BN_CTX_start(ctx);
if ((Ri = BN_CTX_get(ctx)) == NULL)
goto err;
R = &(mont->RR); /* grab RR as a temp */
if (!BN_copy(&(mont->N), mod))
goto err; /* Set N */
if (BN_get_flags(mod, BN_FLG_CONSTTIME) != 0)
BN_set_flags(&(mont->N), BN_FLG_CONSTTIME);
mont->N.neg = 0;
#ifdef MONT_WORD
{
BIGNUM tmod;
BN_ULONG buf[2];
bn_init(&tmod);
tmod.d = buf;
tmod.dmax = 2;
tmod.neg = 0;
if (BN_get_flags(mod, BN_FLG_CONSTTIME) != 0)
BN_set_flags(&tmod, BN_FLG_CONSTTIME);
mont->ri = (BN_num_bits(mod) + (BN_BITS2 - 1)) / BN_BITS2 * BN_BITS2;
# if defined(OPENSSL_BN_ASM_MONT) && (BN_BITS2<=32)
/*
* Only certain BN_BITS2<=32 platforms actually make use of n0[1],
* and we could use the #else case (with a shorter R value) for the
* others. However, currently only the assembler files do know which
* is which.
*/
BN_zero(R);
if (!(BN_set_bit(R, 2 * BN_BITS2)))
goto err;
tmod.top = 0;
if ((buf[0] = mod->d[0]))
tmod.top = 1;
if ((buf[1] = mod->top > 1 ? mod->d[1] : 0))
tmod.top = 2;
if ((BN_mod_inverse(Ri, R, &tmod, ctx)) == NULL)
goto err;
if (!BN_lshift(Ri, Ri, 2 * BN_BITS2))
goto err; /* R*Ri */
if (!BN_is_zero(Ri)) {
if (!BN_sub_word(Ri, 1))
goto err;
} else { /* if N mod word size == 1 */
if (bn_expand(Ri, (int)sizeof(BN_ULONG) * 2) == NULL)
goto err;
/* Ri-- (mod double word size) */
Ri->neg = 0;
Ri->d[0] = BN_MASK2;
Ri->d[1] = BN_MASK2;
Ri->top = 2;
}
if (!BN_div(Ri, NULL, Ri, &tmod, ctx))
goto err;
/*
* Ni = (R*Ri-1)/N, keep only couple of least significant words:
*/
mont->n0[0] = (Ri->top > 0) ? Ri->d[0] : 0;
mont->n0[1] = (Ri->top > 1) ? Ri->d[1] : 0;
# else
BN_zero(R);
if (!(BN_set_bit(R, BN_BITS2)))
goto err; /* R */
buf[0] = mod->d[0]; /* tmod = N mod word size */
buf[1] = 0;
tmod.top = buf[0] != 0 ? 1 : 0;
/* Ri = R^-1 mod N */
if ((BN_mod_inverse(Ri, R, &tmod, ctx)) == NULL)
goto err;
if (!BN_lshift(Ri, Ri, BN_BITS2))
goto err; /* R*Ri */
if (!BN_is_zero(Ri)) {
if (!BN_sub_word(Ri, 1))
goto err;
} else { /* if N mod word size == 1 */
if (!BN_set_word(Ri, BN_MASK2))
goto err; /* Ri-- (mod word size) */
}
if (!BN_div(Ri, NULL, Ri, &tmod, ctx))
goto err;
/*
* Ni = (R*Ri-1)/N, keep only least significant word:
*/
mont->n0[0] = (Ri->top > 0) ? Ri->d[0] : 0;
mont->n0[1] = 0;
# endif
}
#else /* !MONT_WORD */
{ /* bignum version */
mont->ri = BN_num_bits(&mont->N);
BN_zero(R);
if (!BN_set_bit(R, mont->ri))
goto err; /* R = 2^ri */
/* Ri = R^-1 mod N */
if ((BN_mod_inverse(Ri, R, &mont->N, ctx)) == NULL)
goto err;
if (!BN_lshift(Ri, Ri, mont->ri))
goto err; /* R*Ri */
if (!BN_sub_word(Ri, 1))
goto err;
/*
* Ni = (R*Ri-1) / N
*/
if (!BN_div(&(mont->Ni), NULL, Ri, &mont->N, ctx))
goto err;
}
#endif
/* setup RR for conversions */
BN_zero(&(mont->RR));
if (!BN_set_bit(&(mont->RR), mont->ri * 2))
goto err;
if (!BN_mod(&(mont->RR), &(mont->RR), &(mont->N), ctx))
goto err;
ret = 1;
err:
BN_CTX_end(ctx);
return ret;
}
BN_MONT_CTX *BN_MONT_CTX_copy(BN_MONT_CTX *to, BN_MONT_CTX *from)
{
if (to == from)
return to;
if (!BN_copy(&(to->RR), &(from->RR)))
return NULL;
if (!BN_copy(&(to->N), &(from->N)))
return NULL;
if (!BN_copy(&(to->Ni), &(from->Ni)))
return NULL;
to->ri = from->ri;
to->n0[0] = from->n0[0];
to->n0[1] = from->n0[1];
return to;
}
BN_MONT_CTX *BN_MONT_CTX_set_locked(BN_MONT_CTX **pmont, CRYPTO_RWLOCK *lock,
const BIGNUM *mod, BN_CTX *ctx)
{
BN_MONT_CTX *ret;
CRYPTO_THREAD_read_lock(lock);
ret = *pmont;
CRYPTO_THREAD_unlock(lock);
if (ret)
return ret;
/*
* We don't want to serialise globally while doing our lazy-init math in
* BN_MONT_CTX_set. That punishes threads that are doing independent
* things. Instead, punish the case where more than one thread tries to
* lazy-init the same 'pmont', by having each do the lazy-init math work
* independently and only use the one from the thread that wins the race
* (the losers throw away the work they've done).
*/
ret = BN_MONT_CTX_new();
if (ret == NULL)
return NULL;
if (!BN_MONT_CTX_set(ret, mod, ctx)) {
BN_MONT_CTX_free(ret);
return NULL;
}
/* The locked compare-and-set, after the local work is done. */
CRYPTO_THREAD_write_lock(lock);
if (*pmont) {
BN_MONT_CTX_free(ret);
ret = *pmont;
} else
*pmont = ret;
CRYPTO_THREAD_unlock(lock);
return ret;
}
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