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Timing vulnerability in ECDSA signature generation (CVE-2018-0735)

Preallocate an extra limb for some of the big numbers to avoid a reallocation
that can potentially provide a side channel.

Reviewed-by: Bernd Edlinger <bernd.edlinger@hotmail.de>
(Merged from #7486)

(cherry picked from commit 99540ec)
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paulidale committed Oct 26, 2018
1 parent 8abfe72 commit b1d6d55ece1c26fa2829e2b819b038d7b6d692b4
Showing with 3 additions and 3 deletions.
  1. +3 −3 crypto/ec/ec_mult.c
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@@ -206,8 +206,8 @@ int ec_scalar_mul_ladder(const EC_GROUP *group, EC_POINT *r,
*/
cardinality_bits = BN_num_bits(cardinality);
group_top = bn_get_top(cardinality);
if ((bn_wexpand(k, group_top + 1) == NULL)
|| (bn_wexpand(lambda, group_top + 1) == NULL)) {
if ((bn_wexpand(k, group_top + 2) == NULL)
|| (bn_wexpand(lambda, group_top + 2) == NULL)) {
ECerr(EC_F_EC_SCALAR_MUL_LADDER, ERR_R_BN_LIB);
goto err;
}
@@ -244,7 +244,7 @@ int ec_scalar_mul_ladder(const EC_GROUP *group, EC_POINT *r,
* k := scalar + 2*cardinality
*/
kbit = BN_is_bit_set(lambda, cardinality_bits);
BN_consttime_swap(kbit, k, lambda, group_top + 1);
BN_consttime_swap(kbit, k, lambda, group_top + 2);
group_top = bn_get_top(group->field);
if ((bn_wexpand(s->X, group_top) == NULL)

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