Change to mitigate branch prediction attacks

Submitted by: Matthew D Wood
Reviewed by: Bodo Moeller

1 files changed, 38 insertions, 1 deletions

diff --git a/CHANGES b/CHANGES
index 2081bf68c9..21b90edcc7 100644
--- a/CHANGES
+++ b/CHANGES
@@ -483,6 +483,43 @@
 
  Changes between 0.9.8e and 0.9.8f  [xx XXX xxxx]
 
+  *) Mitigate branch prediction attacks, which can be practical if a
+     single processor is shared, allowing a spy process to extract
+     information.  For detailed background information, see
+     http://eprint.iacr.org/2007/039 (O. Aciicmez, S. Gueron,
+     J.-P. Seifert, "New Branch Prediction Vulnerabilities in OpenSSL
+     and Necessary Software Countermeasures").  The core of the change
+     are new versions BN_div_no_branch() and
+     BN_mod_inverse_no_branch() of BN_div() and BN_mod_inverse(),
+     respectively, which are slower, but avoid the security-relevant
+     conditional branches.  These are automatically called by BN_div()
+     and BN_mod_inverse() if the flag BN_FLG_CONSTTIME is set for the
+     modulus.  Also, BN_is_bit_set() has been changed to remove a
+     conditional branch.
+
+     BN_FLG_CONSTTIME is the new name for the previous
+     BN_FLG_EXP_CONSTTIME flag, since it now affects more than just
+     modular exponentiation.  (Since OpenSSL 0.9.7h, setting this flag
+     in the exponent causes BN_mod_exp_mont() to use the alternative
+     implementation in BN_mod_exp_mont_consttime().)  The old name
+     remains as a deprecated alias.
+
+     Similary, RSA_FLAG_NO_EXP_CONSTTIME is replaced by a more general
+     RSA_FLAG_NO_CONSTTIME flag since the RSA implementation now uses
+     constant-time implementations for more than just exponentiation.
+     Here too the old name is kept as a deprecated alias.
+
+     BN_BLINDING_new() will now use BN_dup() for the modulus so that
+     the BN_BLINDING structure gets an independent copy of the
+     modulus.  This means that the previous "BIGNUM *m" argument to
+     BN_BLINDING_new() and to BN_BLINDING_create_param() now
+     essentially becomes "const BIGNUM *m", although we can't actually
+     change this in the header file before 0.9.9.  It allows
+     RSA_setup_blinding() to use BN_with_flags() on the modulus to
+     enable BN_FLG_CONSTTIME.
+
+     [Matthew D Wood (Intel Corp)]
+
   *) In the SSL/TLS server implementation, be strict about session ID
      context matching (which matters if an application uses a single
      external cache for different purposes).  Previously,
@@ -1702,7 +1739,7 @@
 
      BN_mod_exp_mont_consttime() is the new exponentiation implementation,
      and this is automatically used by BN_mod_exp_mont() if the new flag
-     BN_FLG_EXP_CONSTTIME is set for the exponent.  RSA, DSA, and DH
+     BN_FLG_EXP_CONSTTIME is set for the exponent.  RSA, DSA, and DH 
      will use this BN flag for private exponents unless the flag
      RSA_FLAG_NO_EXP_CONSTTIME, DSA_FLAG_NO_EXP_CONSTTIME, or
      DH_FLAG_NO_EXP_CONSTTIME, respectively, is set.