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$NetBSD: patch-crypto_ecdsa_ecs__ossl.c,v 1.1.2.2 2011/06/04 01:42:24 sbd Exp $
Add protection against ECDSA timing attacks as mentioned in the paper
by Billy Bob Brumley and Nicola Tuveri, see:
http://eprint.iacr.org/2011/232.pdf
[Billy Bob Brumley and Nicola Tuveri]
--- crypto/ecdsa/ecs_ossl.c.orig 2009-12-01 11:32:16.000000000 -0600
+++ crypto/ecdsa/ecs_ossl.c 2011-05-31 11:17:14.168169900 -0500
@@ -144,6 +144,16 @@
}
while (BN_is_zero(k));
+#ifdef ECDSA_POINT_MUL_NO_CONSTTIME
+ /* 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. */
+
+ if (!BN_add(k, k, order)) goto err;
+ if (BN_num_bits(k) <= BN_num_bits(order))
+ if (!BN_add(k, k, order)) goto err;
+#endif /* def(ECDSA_POINT_MUL_NO_CONSTTIME) */
+
/* compute r the x-coordinate of generator * k */
if (!EC_POINT_mul(group, tmp_point, k, NULL, NULL, ctx))
{
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