[opensslcommits] [openssl] master update
Andy Polyakov
appro at openssl.org
Thu Feb 1 20:46:11 UTC 2018
The branch master has been updated
via 39eeb64f59ff838f976ad305de7d15747d47a41c (commit)
via 972c87dfc7e765bd28a4964519c362f0d3a58ca4 (commit)
from 3c5a61dd0f9d9a9eac098419bcaf47d1c296ca81 (commit)
 Log 
commit 39eeb64f59ff838f976ad305de7d15747d47a41c
Author: David Benjamin <davidben at google.com>
Date: Tue Jan 23 13:57:10 2018 0500
Don't leak the exponent bit width in BN_mod_exp_mont_consttime.
The exponent here is one of d, dmp1, or dmq1 for RSA. This value and its
bit length are both secret. The only public upper bound is the bit width
of the corresponding modulus (RSA n, p, and q, respectively).
Although BN_num_bits is constanttime (sort of; see bn_correct_top notes
in preceding patch), this does not fix the root problem, which is that
the windows are based on the minimal bit width, not the upper bound. We
could use BN_num_bits(m), but BN_mod_exp_mont_consttime is public API
and may be called with larger exponents. Instead, use all top*BN_BITS2
bits in the BIGNUM. This is still sensitive to the longstanding
bn_correct_top leak, but we need to fix that regardless.
This may cause us to do a handful of extra multiplications for RSA keys
which are just above a whole number of words, but that is not a standard
RSA key size.
Reviewedby: Paul Dale <paul.dale at oracle.com>
Reviewedby: Rich Salz <rsalz at openssl.org>
(Merged from https://github.com/openssl/openssl/pull/5154)
commit 972c87dfc7e765bd28a4964519c362f0d3a58ca4
Author: David Benjamin <davidben at google.com>
Date: Tue Jan 23 13:46:53 2018 0500
Make BN_num_bits_word constanttime.
(This patch was written by Andy Polyakov. I only wrote the commit
message. Mistakes in the analysis are my fault.)
BN_num_bits, by way of BN_num_bits_word, currently leaks the
mostsignificant word of its argument via branching and memory access
pattern.
BN_num_bits is called on RSA prime factors in various places. These have
public bit lengths, but all bits beyond the high bit are secret. This
fully resolves those cases.
There are a few places where BN_num_bits is called on an input where the
bit length is also secret. This does *not* fully resolve those cases as
we still only look at the top word. Today, that is guaranteed to be
nonzero, but only because of the longstanding bn_correct_top timing
leak. Once that is fixed, a constanttime BN_num_bits on such inputs
must count bits on each word.
Instead, those cases should not call BN_num_bits at all. In particular,
BN_mod_exp_mont_consttime uses the exponent bit width to pick windows,
but it should be using the maximum bit width. The next patch will fix
this.
Thanks to Dinghao Wu, Danfeng Zhang, Shuai Wang, Pei Wang, and Xiao Liu
for reporting this issue.
Reviewedby: Paul Dale <paul.dale at oracle.com>
Reviewedby: Rich Salz <rsalz at openssl.org>
(Merged from https://github.com/openssl/openssl/pull/5154)

Summary of changes:
crypto/bn/bn_exp.c  6 ++
crypto/bn/bn_lib.c  107 ++++++++++++++++++++
2 files changed, 45 insertions(+), 68 deletions()
diff git a/crypto/bn/bn_exp.c b/crypto/bn/bn_exp.c
index 69970e6..b786bf0 100644
 a/crypto/bn/bn_exp.c
+++ b/crypto/bn/bn_exp.c
@@ 623,7 +623,11 @@ 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)) {
diff git a/crypto/bn/bn_lib.c b/crypto/bn/bn_lib.c
index b9f19d8..57fe452 100644
 a/crypto/bn/bn_lib.c
+++ b/crypto/bn/bn_lib.c
@@ 88,74 +88,47 @@ const BIGNUM *BN_value_one(void)
int BN_num_bits_word(BN_ULONG l)
{
 static const unsigned char bits[256] = {
 0, 1, 2, 2, 3, 3, 3, 3, 4, 4, 4, 4, 4, 4, 4, 4,
 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5,
 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6,
 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6,
 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7,
 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7,
 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7,
 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7,
 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8,
 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8,
 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8,
 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8,
 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8,
 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8,
 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8,
 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8,
 };

#if defined(SIXTY_FOUR_BIT_LONG)
 if (l & 0xffffffff00000000L) {
 if (l & 0xffff000000000000L) {
 if (l & 0xff00000000000000L) {
 return (bits[(int)(l >> 56)] + 56);
 } else
 return (bits[(int)(l >> 48)] + 48);
 } else {
 if (l & 0x0000ff0000000000L) {
 return (bits[(int)(l >> 40)] + 40);
 } else
 return (bits[(int)(l >> 32)] + 32);
 }
 } else
#else
# ifdef SIXTY_FOUR_BIT
 if (l & 0xffffffff00000000LL) {
 if (l & 0xffff000000000000LL) {
 if (l & 0xff00000000000000LL) {
 return (bits[(int)(l >> 56)] + 56);
 } else
 return (bits[(int)(l >> 48)] + 48);
 } else {
 if (l & 0x0000ff0000000000LL) {
 return (bits[(int)(l >> 40)] + 40);
 } else
 return (bits[(int)(l >> 32)] + 32);
 }
 } else
# endif
#endif
 {
#if defined(THIRTY_TWO_BIT)  defined(SIXTY_FOUR_BIT)  defined(SIXTY_FOUR_BIT_LONG)
 if (l & 0xffff0000L) {
 if (l & 0xff000000L)
 return (bits[(int)(l >> 24L)] + 24);
 else
 return (bits[(int)(l >> 16L)] + 16);
 } else
#endif
 {
#if defined(THIRTY_TWO_BIT)  defined(SIXTY_FOUR_BIT)  defined(SIXTY_FOUR_BIT_LONG)
 if (l & 0xff00L)
 return (bits[(int)(l >> 8)] + 8);
 else
+ BN_ULONG x, mask;
+ int bits = (l != 0);
+
+#if BN_BITS2 > 32
+ x = l >> 32;
+ mask = (0  x) & BN_MASK2;
+ mask = (0  (mask >> (BN_BITS2  1)));
+ bits += 32 & mask;
+ l ^= (x ^ l) & mask;
#endif
 return (bits[(int)(l)]);
 }
 }
+
+ x = l >> 16;
+ mask = (0  x) & BN_MASK2;
+ mask = (0  (mask >> (BN_BITS2  1)));
+ bits += 16 & mask;
+ l ^= (x ^ l) & mask;
+
+ x = l >> 8;
+ mask = (0  x) & BN_MASK2;
+ mask = (0  (mask >> (BN_BITS2  1)));
+ bits += 8 & mask;
+ l ^= (x ^ l) & mask;
+
+ x = l >> 4;
+ mask = (0  x) & BN_MASK2;
+ mask = (0  (mask >> (BN_BITS2  1)));
+ bits += 4 & mask;
+ l ^= (x ^ l) & mask;
+
+ x = l >> 2;
+ mask = (0  x) & BN_MASK2;
+ mask = (0  (mask >> (BN_BITS2  1)));
+ bits += 2 & mask;
+ l ^= (x ^ l) & mask;
+
+ x = l >> 1;
+ mask = (0  x) & BN_MASK2;
+ mask = (0  (mask >> (BN_BITS2  1)));
+ bits += 1 & mask;
+
+ return bits;
}
int BN_num_bits(const BIGNUM *a)
More information about the opensslcommits
mailing list