misc: Use SecureZeroMemory if available for explicit_bzero

Useful on Windows platforms where SecureZeroMemory is available.

Signed-off-by: Alberto Aguirre <albaguirre@gmail.com>
Reviewed-by: Andreas Schneider <asn@cryptomilk.org>

ConfigureChecks.cmake

@@ -180,6 +180,8 @@ if (WIN32)
     check_function_exists(_strtoui64 HAVE__STRTOUI64)
 
     set(HAVE_SELECT TRUE)
+
+    check_symbol_exists(SecureZeroMemory "windows.h" HAVE_SECURE_ZERO_MEMORY)
 else (WIN32)
     check_function_exists(poll HAVE_POLL)
     check_function_exists(select HAVE_SELECT)

config.h.cmake

@@ -166,6 +166,9 @@
 /* Define to 1 if you have the `memset_s' function. */
 #cmakedefine HAVE_MEMSET_S 1
 
+/* Define to 1 if you have the `SecureZeroMemory' function. */
+#cmakedefine HAVE_SECURE_ZERO_MEMORY 1
+
 /*************************** LIBRARIES ***************************/
 
 /* Define to 1 if you have the `crypto' library (-lcrypto). */

src/misc.c

@@ -1057,6 +1057,8 @@ void explicit_bzero(void *s, size_t n)
 {
 #if defined(HAVE_MEMSET_S)
     memset_s(s, n, '\0', n);
+#elif defined(HAVE_SECURE_ZERO_MEMORY)
+    SecureZeroMemory(s, n);
 #else
     memset(s, '\0', n);
 #if defined(HAVE_GCC_VOLATILE_MEMORY_PROTECTION)