@@ -105,6 +105,166 @@ void EC_ec_pre_comp_free(EC_PRE_COMP *pre)
OPENSSL_free (pre);
}
#define EC_POINT_set_flags (P, flags ) do { \
BN_set_flags ((P)->X , (flags)); \
BN_set_flags ((P)->Y , (flags)); \
BN_set_flags ((P)->Z , (flags)); \
} while (0 )
/*
* This functions computes (in constant time) a point multiplication over the
* EC group.
*
* It performs either a fixed scalar point multiplication
* (scalar * generator)
* when point is NULL, or a generic scalar point multiplication
* (scalar * point)
* when point is not NULL.
*
* scalar should be in the range [0,n) otherwise all constant time bets are off.
*
* NB: This says nothing about EC_POINT_add and EC_POINT_dbl,
* which of course are not constant time themselves.
*
* The product is stored in r.
*
* Returns 1 on success, 0 otherwise.
*/
static int ec_mul_consttime (const EC_GROUP *group, EC_POINT *r, const BIGNUM *scalar,
const EC_POINT *point, BN_CTX *ctx)
{
int i, order_bits, group_top, kbit, pbit, Z_is_one, ret;
ret = 0 ;
EC_POINT *s = NULL ;
BIGNUM *k = NULL ;
BIGNUM *lambda = NULL ;
BN_CTX *new_ctx = NULL ;
if (ctx == NULL )
if ((ctx = new_ctx = BN_CTX_secure_new ()) == NULL )
return 0 ;
if ((group->order == NULL ) || (group->field == NULL ))
goto err;
order_bits = BN_num_bits (group->order );
s = EC_POINT_new (group);
if (s == NULL )
goto err;
if (point == NULL ) {
if (group->generator == NULL )
goto err;
if (!EC_POINT_copy (s, group->generator ))
goto err;
} else {
if (!EC_POINT_copy (s, point))
goto err;
}
EC_POINT_set_flags (s, BN_FLG_CONSTTIME);
BN_CTX_start (ctx);
lambda = BN_CTX_get (ctx);
k = BN_CTX_get (ctx);
if (k == NULL )
goto err;
/*
* Group orders are often on a word boundary.
* So when we pad the scalar, some timing diff might
* pop if it needs to be expanded due to carries.
* So expand ahead of time.
*/
group_top = bn_get_top (group->order );
if ((bn_wexpand (k, group_top + 1 ) == NULL )
|| (bn_wexpand (lambda, group_top + 1 ) == NULL ))
goto err;
if (!BN_copy (k, scalar))
goto err;
BN_set_flags (k, BN_FLG_CONSTTIME);
if ((BN_num_bits (k) > order_bits) || (BN_is_negative (k))) {
/*
* this is an unusual input, and we don't guarantee
* constant-timeness
*/
if (!BN_nnmod (k, k, group->order , ctx))
goto err;
}
if (!BN_add (lambda, k, group->order ))
goto err;
BN_set_flags (lambda, BN_FLG_CONSTTIME);
if (!BN_add (k, lambda, group->order ))
goto err;
/*
* lambda := scalar + order
* k := scalar + 2*order
*/
kbit = BN_is_bit_set (lambda, order_bits);
BN_consttime_swap (kbit, k, lambda, group_top + 1 );
group_top = bn_get_top (group->field );
if ((bn_wexpand (s->X , group_top) == NULL )
|| (bn_wexpand (s->Y , group_top) == NULL )
|| (bn_wexpand (s->Z , group_top) == NULL )
|| (bn_wexpand (r->X , group_top) == NULL )
|| (bn_wexpand (r->Y , group_top) == NULL )
|| (bn_wexpand (r->Z , group_top) == NULL ))
goto err;
/* top bit is a 1, in a fixed pos */
if (!EC_POINT_copy (r, s))
goto err;
EC_POINT_set_flags (r, BN_FLG_CONSTTIME);
if (!EC_POINT_dbl (group, s, s, ctx))
goto err;
pbit = 0 ;
#define EC_POINT_CSWAP (c, a, b, w, t ) do { \
BN_consttime_swap (c, (a)->X , (b)->X , w); \
BN_consttime_swap (c, (a)->Y , (b)->Y , w); \
BN_consttime_swap (c, (a)->Z , (b)->Z , w); \
t = ((a)->Z_is_one ^ (b)->Z_is_one ) & (c); \
(a)->Z_is_one ^= (t); \
(b)->Z_is_one ^= (t); \
} while (0 )
for (i = order_bits - 1 ; i >= 0 ; i--) {
kbit = BN_is_bit_set (k, i) ^ pbit;
EC_POINT_CSWAP (kbit, r, s, group_top, Z_is_one);
if (!EC_POINT_add (group, s, r, s, ctx))
goto err;
if (!EC_POINT_dbl (group, r, r, ctx))
goto err;
/*
* pbit logic merges this cswap with that of the
* next iteration
*/
pbit ^= kbit;
}
/* one final cswap to move the right value into r */
EC_POINT_CSWAP (pbit, r, s, group_top, Z_is_one);
#undef EC_POINT_CSWAP
ret = 1 ;
err:
EC_POINT_free (s);
BN_CTX_end (ctx);
BN_CTX_free (new_ctx);
return ret;
}
#undef EC_POINT_set_flags
/*
* TODO: table should be optimised for the wNAF-based implementation,
* sometimes smaller windows will give better performance (thus the
@@ -130,6 +290,28 @@ int ec_wNAF_mul(const EC_GROUP *group, EC_POINT *r, const BIGNUM *scalar,
size_t num, const EC_POINT *points[], const BIGNUM *scalars[],
BN_CTX *ctx)
{
if ((scalar != NULL ) && (num == 0 )) {
/* In this case we want to compute scalar * GeneratorPoint:
* this codepath is reached most prominently by (ephemeral) key
* generation of EC cryptosystems (i.e. ECDSA keygen and sign setup,
* ECDH keygen/first half), where the scalar is always secret.
* This is why we ignore if BN_FLG_CONSTTIME is actually set and we
* always call the constant time version.
*/
return ec_mul_consttime (group, r, scalar, NULL , ctx);
}
if ((scalar == NULL ) && (num == 1 )) {
/* In this case we want to compute scalar * GenericPoint:
* this codepath is reached most prominently by the second half of
* ECDH, where the secret scalar is multiplied by the peer's public
* point.
* To protect the secret scalar, we ignore if BN_FLG_CONSTTIME is
* actually set and we always call the constant time version.
*/
return ec_mul_consttime (group, r, scalars[0 ], points[0 ], ctx);
}
BN_CTX *new_ctx = NULL ;
const EC_POINT *generator = NULL ;
EC_POINT *tmp = NULL ;