rpc: check and throw specific pubkey parsing errors in HexToPubKey

In the helper `HexToPubKey`, check for three different causes of legacy
public key parsing errors (in this order):

    - pubkey is not a hex string
    - pubkey doesn't have a valid length (33 or 65 bytes) [NEW]
    - pubkey is cryptographically invalid, i.e. not on curve
      (`IsFullyValid` check)

and throw a specific error message for each one. Note that the error
code is identical for all of them (-5), so this doesn't break RPC API
compatibility.

The helper is currently used for the RPCs `createmultisig` and
`addmultisigaddress`. The length checks can be removed from the
call-sites and error message checks in the functional tests are adapted.

src/rpc/output_script.cpp

@@ -124,11 +124,7 @@ static RPCHelpMan createmultisig()
             const UniValue& keys = request.params[1].get_array();
             std::vector<CPubKey> pubkeys;
             for (unsigned int i = 0; i < keys.size(); ++i) {
-                if (IsHex(keys[i].get_str()) && (keys[i].get_str().length() == 66 || keys[i].get_str().length() == 130)) {
-                    pubkeys.push_back(HexToPubKey(keys[i].get_str()));
-                } else {
-                    throw JSONRPCError(RPC_INVALID_ADDRESS_OR_KEY, strprintf("Invalid public key: %s\n.", keys[i].get_str()));
-                }
+                pubkeys.push_back(HexToPubKey(keys[i].get_str()));
             }
 
             // Get the output type

src/rpc/util.cpp

@@ -180,11 +180,14 @@ std::string HelpExampleRpcNamed(const std::string& methodname, const RPCArgList&
 CPubKey HexToPubKey(const std::string& hex_in)
 {
     if (!IsHex(hex_in)) {
-        throw JSONRPCError(RPC_INVALID_ADDRESS_OR_KEY, "Invalid public key: " + hex_in);
+        throw JSONRPCError(RPC_INVALID_ADDRESS_OR_KEY, "Pubkey \"" + hex_in + "\" must be a hex string");
+    }
+    if (hex_in.length() != 66 && hex_in.length() != 130) {
+        throw JSONRPCError(RPC_INVALID_ADDRESS_OR_KEY, "Pubkey \"" + hex_in + "\" must have a length of either 33 or 65 bytes");
     }
     CPubKey vchPubKey(ParseHex(hex_in));
     if (!vchPubKey.IsFullyValid()) {
-        throw JSONRPCError(RPC_INVALID_ADDRESS_OR_KEY, "Invalid public key: " + hex_in);
+        throw JSONRPCError(RPC_INVALID_ADDRESS_OR_KEY, "Pubkey \"" + hex_in + "\" must be cryptographically valid.");
     }
     return vchPubKey;
 }

test/functional/rpc_rawtransaction.py

@@ -491,11 +491,11 @@ def raw_multisig_transaction_legacy_tests(self):
         addr2Obj = self.nodes[2].getaddressinfo(addr2)
 
         # Tests for createmultisig and addmultisigaddress
-        assert_raises_rpc_error(-5, "Invalid public key", self.nodes[0].createmultisig, 1, ["01020304"])
+        assert_raises_rpc_error(-5, 'Pubkey "01020304" must have a length of either 33 or 65 bytes', self.nodes[0].createmultisig, 1, ["01020304"])
         # createmultisig can only take public keys
         self.nodes[0].createmultisig(2, [addr1Obj['pubkey'], addr2Obj['pubkey']])
         # addmultisigaddress can take both pubkeys and addresses so long as they are in the wallet, which is tested here
-        assert_raises_rpc_error(-5, "Invalid public key", self.nodes[0].createmultisig, 2, [addr1Obj['pubkey'], addr1])
+        assert_raises_rpc_error(-5, f'Pubkey "{addr1}" must be a hex string', self.nodes[0].createmultisig, 2, [addr1Obj['pubkey'], addr1])
 
         mSigObj = self.nodes[2].addmultisigaddress(2, [addr1Obj['pubkey'], addr1])['address']