The branch OpenSSL_1_0_2-stable has been updated via 4981e6fc1da4aec6775fc248643c91dd1e87e0b7 (commit) via 66509ddbd00179e8be58d54cf5576fb6b74d0131 (commit) from d498e526832bd6c50238f3dc0bcac9375179926e (commit)
- Log ----------------------------------------------------------------- commit 4981e6fc1da4aec6775fc248643c91dd1e87e0b7 Author: David Benjamin <david...@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 constant-time (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 long-standing 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. Reviewed-by: Paul Dale <paul.d...@oracle.com> Reviewed-by: Rich Salz <rs...@openssl.org> (Merged from https://github.com/openssl/openssl/pull/5154) (cherry picked from commit 39eeb64f59ff838f976ad305de7d15747d47a41c) commit 66509ddbd00179e8be58d54cf5576fb6b74d0131 Author: David Benjamin <david...@google.com> Date: Tue Jan 23 13:46:53 2018 -0500 Make BN_num_bits_word constant-time. (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 most-significant 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 non-zero, but only because of the long-standing bn_correct_top timing leak. Once that is fixed, a constant-time 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. Reviewed-by: Paul Dale <paul.d...@oracle.com> Reviewed-by: Rich Salz <rs...@openssl.org> (Merged from https://github.com/openssl/openssl/pull/5154) (cherry picked from commit 972c87dfc7e765bd28a4964519c362f0d3a58ca4) ----------------------------------------------------------------------- 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 c4b63e4..9fc545a 100644 --- a/crypto/bn/bn_exp.c +++ b/crypto/bn/bn_exp.c @@ -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)) { diff --git a/crypto/bn/bn_lib.c b/crypto/bn/bn_lib.c index 10b78f5..27b9bdb 100644 --- a/crypto/bn/bn_lib.c +++ b/crypto/bn/bn_lib.c @@ -144,74 +144,47 @@ 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 + 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 - { -#if defined(THIRTY_TWO_BIT) || defined(SIXTY_FOUR_BIT) || defined(SIXTY_FOUR_BIT_LONG) - if (l & 0xff00L) - return (bits[(int)(l >> 8)] + 8); - else -#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) _____ openssl-commits mailing list To unsubscribe: https://mta.openssl.org/mailman/listinfo/openssl-commits