Use secp256k1_memclear() to clear stack memory instead of memset()

All of the invocations of secp256k1_memclear() operate on stack
memory and happen after the function is done with the memory object.
This commit replaces existing memset() invocations and also adds
secp256k1_memclear() to code locations where clearing was missing;
there is no guarantee that this commit covers all code locations
where clearing is necessary.

Co-Authored-By: isle2983 <isle2983@yahoo.com>

src/modules/ecdh/main_impl.h

@@ -61,9 +61,10 @@ int secp256k1_ecdh(const secp256k1_context* ctx, unsigned char *output, const se
 
     ret = hashfp(output, x, y, data);
 
-    memset(x, 0, 32);
-    memset(y, 0, 32);
+    secp256k1_memclear(x, sizeof(x));
+    secp256k1_memclear(y, sizeof(y));
     secp256k1_scalar_clear(&s);
+    secp256k1_ge_clear(&pt);
 
     return !!ret & !overflow;
 }

src/modules/ellswift/main_impl.h

@@ -580,7 +580,7 @@ int secp256k1_ellswift_xdh(const secp256k1_context *ctx, unsigned char *output,
     /* Invoke hasher */
     ret = hashfp(output, sx, ell_a64, ell_b64, data);
 
-    memset(sx, 0, 32);
+    secp256k1_memclear(sx, sizeof(sx));
     secp256k1_fe_clear(&px);
     secp256k1_scalar_clear(&s);
 

src/modules/musig/session_impl.h

@@ -385,11 +385,11 @@ static void secp256k1_nonce_function_musig(secp256k1_scalar *k, const unsigned c
         secp256k1_scalar_set_b32(&k[i], buf, NULL);
 
         /* Attempt to erase secret data */
-        memset(buf, 0, sizeof(buf));
-        memset(&sha_tmp, 0, sizeof(sha_tmp));
+        secp256k1_memclear(buf, sizeof(buf));
+        secp256k1_memclear(&sha_tmp, sizeof(sha_tmp));
     }
-    memset(rand, 0, sizeof(rand));
-    memset(&sha, 0, sizeof(sha));
+    secp256k1_memclear(rand, sizeof(rand));
+    secp256k1_memclear(&sha, sizeof(sha));
 }
 
 int secp256k1_musig_nonce_gen_internal(const secp256k1_context* ctx, secp256k1_musig_secnonce *secnonce, secp256k1_musig_pubnonce *pubnonce, const unsigned char *input_nonce, const unsigned char *seckey, const secp256k1_pubkey *pubkey, const unsigned char *msg32, const secp256k1_musig_keyagg_cache *keyagg_cache, const unsigned char *extra_input32) {
@@ -509,7 +509,7 @@ int secp256k1_musig_nonce_gen_counter(const secp256k1_context* ctx, secp256k1_mu
     if (!secp256k1_musig_nonce_gen_internal(ctx, secnonce, pubnonce, buf, seckey, &pubkey, msg32, keyagg_cache, extra_input32)) {
         return 0;
     }
-    memset(seckey, 0, sizeof(seckey));
+    secp256k1_memclear(seckey, sizeof(seckey));
     return 1;
 }
 

src/modules/schnorrsig/main_impl.h

@@ -187,7 +187,7 @@ static int secp256k1_schnorrsig_sign_internal(const secp256k1_context* ctx, unsi
     secp256k1_memczero(sig64, 64, !ret);
     secp256k1_scalar_clear(&k);
     secp256k1_scalar_clear(&sk);
-    memset(seckey, 0, sizeof(seckey));
+    secp256k1_memclear(seckey, sizeof(seckey));
 
     return ret;
 }