bitcoin/pubkey: add pubkey_from_secret.

Really, we should have a 'struct point' since we don't use all points
as pubkeys.  But this is the minimal fix to avoid type cast nastiness.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

bitcoin/pubkey.c

@@ -28,14 +28,18 @@ void pubkey_to_der(u8 der[PUBKEY_DER_LEN], const struct pubkey *key)
 	assert(outlen == PUBKEY_DER_LEN);
 }
 
-/* Pubkey from privkey */
+bool pubkey_from_secret(const struct secret *secret, struct pubkey *key)
+{
+	if (!secp256k1_ec_pubkey_create(secp256k1_ctx,
+					&key->pubkey, secret->data))
+		return false;
+	return true;
+}
+
 bool pubkey_from_privkey(const struct privkey *privkey,
 			 struct pubkey *key)
 {
-	if (!secp256k1_ec_pubkey_create(secp256k1_ctx,
+	return pubkey_from_secret(&privkey->secret, key);
-					&key->pubkey, privkey->secret.data))
-		return false;
-	return true;
 }
 
 bool pubkey_from_hexstr(const char *derstr, size_t slen, struct pubkey *key)

bitcoin/pubkey.h

@@ -9,6 +9,7 @@
 #include <secp256k1.h>
 
 struct privkey;
+struct secret;
 
 #define PUBKEY_DER_LEN 33
 
@@ -28,6 +29,9 @@ char *pubkey_to_hexstr(const tal_t *ctx, const struct pubkey *key);
 /* Convenience wrapper for a raw secp256k1_pubkey */
 char *secp256k1_pubkey_to_hexstr(const tal_t *ctx, const secp256k1_pubkey *key);
 
+/* Point from secret */
+bool pubkey_from_secret(const struct secret *secret, struct pubkey *key);
+
 /* Pubkey from privkey */
 bool pubkey_from_privkey(const struct privkey *privkey,
 			 struct pubkey *key);

lightningd/test/run-commit_tx.c

@@ -56,14 +56,6 @@ static struct secret secret_from_hex(const char *hex)
 	return s;
 }
 
-static bool pubkey_from_secret(const struct secret *secret,
-			       struct pubkey *key)
-{
-	return secp256k1_ec_pubkey_create(secp256k1_ctx,
-					  &key->pubkey,
-					  secret->data);
-}
-
 static void tx_must_be_eq(const struct bitcoin_tx *a,
 			  const struct bitcoin_tx *b)
 {