Default POSIX/Windows RNG should be treated as user-set RNG.

The default RNG functions on POSIX and windows are secure, and
should be used as if the user had set an RNG. Also, don't set an
RNG at all on other platforms.

uECC.c

@@ -64,7 +64,11 @@ struct uECC_Curve_t {
     #include "asm_arm_small.inc"
 #endif
 
+#if default_RNG_defined
 static uECC_RNG_Function g_rng_function = &default_RNG;
+#else 
+static uECC_RNG_Function g_rng_function = 0;
+#endif
 
 void uECC_set_rng(uECC_RNG_Function rng_function) {
     g_rng_function = rng_function;
@@ -827,7 +831,7 @@ static void vli_bytesToNative(uint64_t *native, const uint8_t *bytes, uECC_Curve
 static cmpresult_t generate_random_int(uECC_word_t *random,
                                        wordcount_t num_words,
                                        const wordcount_t num_bits) {
-    if (!g_rng_function((uint8_t *)random, num_words * uECC_WORD_SIZE)) {
+    if (!g_rng_function || !g_rng_function((uint8_t *)random, num_words * uECC_WORD_SIZE)) {
         return 0;
     }
     if (num_words * uECC_WORD_SIZE * 8 > num_bits) {
@@ -887,7 +891,7 @@ int uECC_shared_secret(const uint8_t *public_key,
     
     /* If an RNG function was specified, try to get a random initial Z value to improve
        protection against side-channel attacks. */
-    if (g_rng_function != &default_RNG) {
+    if (g_rng_function) {
         for (tries = 0; tries < MAX_TRIES; ++tries) {
             if (!generate_random_int(p2[carry], curve->num_words, curve->num_bytes * 8)) {
                 return 0;
@@ -999,7 +1003,7 @@ static int uECC_sign_with_k(const uint8_t *private_key,
     }
     
     /* Attempt to get a random number to prevent side channel analysis of k. */
-    if (g_rng_function == &default_RNG) {
+    if (!g_rng_function) {
         vli_clear(tmp, curve->num_n_words);
         tmp[0] = 1;
     } else {