Fix nits in SecureHash.

crypto/secure_hash_default.cc

Index: crypto/secure_hash_default.cc
===================================================================
--- crypto/secure_hash_default.cc	(revision 168842)
+++ crypto/secure_hash_default.cc	(working copy)
@@ -24,6 +24,7 @@
   }
 
   virtual ~SecureHashSHA256NSS() {
+    memset(&ctx_, 0, sizeof(ctx_));

[wtc, 2012/11/28 20:49:30]

This matches what the SecureHashSHA256OpenSSL destructor does:

  virtual ~SecureHashSHA256OpenSSL() {
    OPENSSL_cleanse(&ctx_, sizeof(ctx_));
  }

[agl, 2012/11/28 21:25:30]

I believe that there is a significant chance that compilers will remove this:
http://www.viva64.com/en/b/0178/

[Ryan Sleevi, 2012/11/28 22:17:08]

Agreed. It's gonna be a nop on -O2 and higher.

See
http://cwe.mitre.org/data/definitions/14.html

[wtc, 2012/11/28 22:39:36]

Thanks for pointing this out. We should add a function to
src/base or NSS to do this.

OPENSSL_cleanse seems to rely on the use of a global variable
(cleanse_ctr in crypto/mem_clr.c) to make the compiler think
the function cannot be optimized away. I wonder if there is
another simple way to prevent compiler from removing the
function call. (On Windows we can just call the Windows
SecureZeroMemory function.)

   }
 
   // SecureHash implementation: