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

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

This doesn't rate as a CVE because:

* For the non-constant time code, there are easier ways to extract
  more information.

* For the constant time code, it requires a significant number of signatures
  to leak a small amount of information.

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

Reviewed-by: Andy Polyakov <appro@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/4576)
This commit is contained in:
Pauli 2017-11-01 06:58:39 +10:00 committed by Matt Caswell
parent c0caa945f6
commit 4a089bbdf1
1 changed files with 20 additions and 6 deletions
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26
crypto/ec/ecdsa_ossl.c
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@ -1,5 +1,5 @@
/*
* Copyright 2002-2016 The OpenSSL Project Authors. All Rights Reserved.
* Copyright 2002-2017 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
@ -41,6 +41,7 @@ static int ecdsa_sign_setup(EC_KEY *eckey, BN_CTX *ctx_in,
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) {
ECerr(EC_F_ECDSA_SIGN_SETUP, ERR_R_PASSED_NULL_PARAMETER);
@ -77,6 +78,13 @@ static int ecdsa_sign_setup(EC_KEY *eckey, BN_CTX *ctx_in,
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 */
do
@ -100,13 +108,19 @@ static int ecdsa_sign_setup(EC_KEY *eckey, BN_CTX *ctx_in,
/*
* 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)) {