Compilers like GCC optimize away code like memset(var, 0, sizeof(var)) if the code occurs at the end of a function and var is not used anymore [1]. But security relevant code like hash and encryption use this to overwrite sensitive data with zeros.

The code in _sha3module.c uses memset() to clear its internal state. The other hash modules don't clear their internal states yet.


There exists a couple of solutions for the problem:

 * C11 [ISO/IEC 9899:2011] has a memset_s() function
 * MSVC has SecureZeroMemory()
 * GCC can disable the optimization with #pragma GCC optimize ("O0") since GCC 4.4
 * [2] contains an example for a custom implementation of memset_s() with volatile.

[1] http://gcc.gnu.org/bugzilla/show_bug.cgi?id=8537

[2] https://www.securecoding.cert.org/confluence/display/seccode/MSC06-C.+Be+aware+of+compiler+optimization+when+dealing+with+sensitive+data