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Address a timing side channel whereby it is possible to determine some

information about the length of the scalar used in ECDSA operations
from a large number (2^32) of signatures.

Thanks to Neals Fournaise, Eliane Jaulmes and Jean-Rene Reinhard for
reporting this issue.

Refer to #4576 for further details.

Reviewed-by: Andy Polyakov <>
Reviewed-by: Matt Caswell <>
(Merged from #4623)
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paulidale authored and mattcaswell committed Oct 31, 2017
1 parent b96beba commit 23f7e974d59a576ad7d8cfd9f7ac957a883e361f
Showing with 19 additions and 5 deletions.
  1. +19 −5 crypto/ecdsa/ecs_ossl.c
@@ -95,6 +95,7 @@ static int ecdsa_sign_setup(EC_KEY *eckey, BN_CTX *ctx_in, BIGNUM **kinvp,
EC_POINT *tmp_point = NULL;
const EC_GROUP *group;
int ret = 0;
int order_bits;
if (eckey == NULL || (group = EC_KEY_get0_group(eckey)) == NULL) {
@@ -126,6 +127,13 @@ static int ecdsa_sign_setup(EC_KEY *eckey, BN_CTX *ctx_in, BIGNUM **kinvp,
goto err;
/* Preallocate space */
order_bits = BN_num_bits(order);
if (!BN_set_bit(k, order_bits)
|| !BN_set_bit(r, order_bits)
|| !BN_set_bit(X, order_bits))
goto err;
do {
/* get random k */
@@ -139,13 +147,19 @@ static int ecdsa_sign_setup(EC_KEY *eckey, BN_CTX *ctx_in, BIGNUM **kinvp,
* We do not want timing information to leak the length of k, so we
* compute G*k using an equivalent scalar of fixed bit-length.
* We unconditionally perform both of these additions to prevent a
* small timing information leakage. We then choose the sum that is
* one bit longer than the order. This guarantees the code
* path used in the constant time implementations elsewhere.
* TODO: revisit the BN_copy aiming for a memory access agnostic
* conditional copy.
if (!BN_add(k, k, order))
if (!BN_add(r, k, order)
|| !BN_add(X, r, order)
|| !BN_copy(k, BN_num_bits(r) > order_bits ? r : X))
goto err;
if (BN_num_bits(k) <= BN_num_bits(order))
if (!BN_add(k, k, order))
goto err;
/* compute r the x-coordinate of generator * k */
if (!EC_POINT_mul(group, tmp_point, k, NULL, NULL, ctx)) {

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