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-rw-r--r--crypto/bn/bn_exp.c8
1 files changed, 6 insertions, 2 deletions
diff --git a/crypto/bn/bn_exp.c b/crypto/bn/bn_exp.c
index c4b63e44ba36..40115fc72052 100644
--- a/crypto/bn/bn_exp.c
+++ b/crypto/bn/bn_exp.c
@@ -56,7 +56,7 @@
* [including the GNU Public Licence.]
*/
/* ====================================================================
- * Copyright (c) 1998-2005 The OpenSSL Project. All rights reserved.
+ * Copyright (c) 1998-2018 The OpenSSL Project. All rights reserved.
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions
@@ -727,7 +727,11 @@ int BN_mod_exp_mont_consttime(BIGNUM *rr, const BIGNUM *a, const BIGNUM *p,
top = m->top;
- bits = BN_num_bits(p);
+ /*
+ * Use all bits stored in |p|, rather than |BN_num_bits|, so we do not leak
+ * whether the top bits are zero.
+ */
+ bits = p->top * BN_BITS2;
if (bits == 0) {
/* x**0 mod 1 is still zero. */
if (BN_is_one(m)) {