Update the function with good practices

Signed-off-by: Harshil Jani <[email protected]>

examples/ecdh.c

@@ -16,14 +16,6 @@
 
 #include "random.h"
 
-void secure_erase(void *buf, size_t size) {
-   volatile char *vbuf = (volatile char *)buf;
-   size_t i;
-    for (i = 0; i < size; ++i) {
-        vbuf[i] = 0;
-    }
-}
-
 int main(void) {
     unsigned char seckey1[32];
     unsigned char seckey2[32];

examples/ecdsa.c

@@ -15,14 +15,6 @@
 
 #include "random.h"
 
-void secure_erase(void *buf, size_t size) {
-    volatile char *vbuf = (volatile char *)buf;
-    size_t i;
-    for (i = 0; i < size; ++i) {
-        vbuf[i] = 0;
-    }
-}
-
 int main(void) {
     /* Instead of signing the message directly, we must sign a 32-byte hash.
      * Here the message is "Hello, world!" and the hash function was SHA-256.

examples/random.h

@@ -71,3 +71,32 @@ static void print_hex(unsigned char* data, size_t size) {
     }
     printf("\n");
 }
+
+#if defined(_MSC_VER)
+// For SecureZeroMemory
+#include <Windows.h>
+#endif
+/* Cleanses memory to prevent leaking sensitive info. Won't be optimized out. */
+static SECP256K1_INLINE void secure_erase(void *ptr, size_t len) {
+#if defined(_MSC_VER)
+    /* SecureZeroMemory is guaranteed not to be optimized out by MSVC. */
+    SecureZeroMemory(ptr, len);
+#elif defined(__GNUC__)
+    /* We use a memory barrier that scares the compiler away from optimizing out the memset.
+     *
+     * Quoting Adam Langley <[email protected]> in commit ad1907fe73334d6c696c8539646c21b11178f20f
+     * in BoringSSL (ISC License):
+     *    As best as we can tell, this is sufficient to break any optimisations that
+     *    might try to eliminate "superfluous" memsets.
+     * This method used in memzero_explicit() the Linux kernel, too. Its advantage is that it is
+     * pretty efficient, because the compiler can still implement the memset() efficently,
+     * just not remove it entirely. See "Dead Store Elimination (Still) Considered Harmful" by
+     * Yang et al. (USENIX Security 2017) for more background.
+     */
+    memset(ptr, 0, len);
+    __asm__ __volatile__("" : : "r"(ptr) : "memory");
+#else
+    void *(*volatile const volatile_memset)(void *, int, size_t) = memset;
+    volatile_memset(ptr, 0, len);
+#endif
+}

examples/schnorr.c

@@ -17,14 +17,6 @@
 
 #include "random.h"
 
-void secure_erase(void *buf, size_t size) {
-    volatile char *vbuf = (volatile char *)buf;
-    size_t i;
-    for (i = 0; i < size; ++i) {
-        vbuf[i] = 0;
-    }
-}
-
 int main(void) {
     unsigned char msg[12] = "Hello World!";
     unsigned char msg_hash[32];