Deduplicate the message verifying code

The logic of verifying a message was duplicated in 2 places:

src/qt/signverifymessagedialog.cpp
  SignVerifyMessageDialog::on_verifyMessageButton_VM_clicked()

src/rpc/misc.cpp
  verifymessage()

with the only difference being the result handling. Move the logic into
a dedicated

src/util/message.cpp
  MessageVerify()

which returns a set of result codes, call it from the 2 places and just
handle the results differently in the callers.

src/util/message.cpp

@@ -0,0 +1,53 @@
+// Copyright (c) 2009-2010 Satoshi Nakamoto
+// Copyright (c) 2009-2020 The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#include <hash.h>            // For CHashWriter
+#include <key_io.h>          // For DecodeDestination()
+#include <pubkey.h>          // For CPubKey
+#include <script/standard.h> // For CTxDestination, IsValidDestination(), PKHash
+#include <serialize.h>       // For SER_GETHASH
+#include <util/message.h>
+#include <util/strencodings.h> // For DecodeBase64()
+
+#include <string>
+#include <vector>
+
+const std::string strMessageMagic = "Bitcoin Signed Message:\n";
+
+MessageVerificationResult MessageVerify(
+    const std::string& address,
+    const std::string& signature,
+    const std::string& message)
+{
+    CTxDestination destination = DecodeDestination(address);
+    if (!IsValidDestination(destination)) {
+        return MessageVerificationResult::ERR_INVALID_ADDRESS;
+    }
+
+    if (boost::get<PKHash>(&destination) == nullptr) {
+        return MessageVerificationResult::ERR_ADDRESS_NO_KEY;
+    }
+
+    bool invalid = false;
+    std::vector<unsigned char> signature_bytes = DecodeBase64(signature.c_str(), &invalid);
+    if (invalid) {
+        return MessageVerificationResult::ERR_MALFORMED_SIGNATURE;
+    }
+
+    CHashWriter ss(SER_GETHASH, 0);
+    ss << strMessageMagic;
+    ss << message;
+
+    CPubKey pubkey;
+    if (!pubkey.RecoverCompact(ss.GetHash(), signature_bytes)) {
+        return MessageVerificationResult::ERR_PUBKEY_NOT_RECOVERED;
+    }
+
+    if (!(CTxDestination(PKHash(pubkey)) == destination)) {
+        return MessageVerificationResult::ERR_NOT_SIGNED;
+    }
+
+    return MessageVerificationResult::OK;
+}

src/util/message.h

@@ -0,0 +1,49 @@
+// Copyright (c) 2009-2010 Satoshi Nakamoto
+// Copyright (c) 2009-2020 The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#ifndef BITCOIN_UTIL_MESSAGE_H
+#define BITCOIN_UTIL_MESSAGE_H
+
+#include <string>
+
+extern const std::string strMessageMagic;
+
+/** The result of a signed message verification.
+ * Message verification takes as an input:
+ * - address (with whose private key the message is supposed to have been signed)
+ * - signature
+ * - message
+ */
+enum class MessageVerificationResult {
+    //! The provided address is invalid.
+    ERR_INVALID_ADDRESS,
+
+    //! The provided address is valid but does not refer to a public key.
+    ERR_ADDRESS_NO_KEY,
+
+    //! The provided signature couldn't be parsed (maybe invalid base64).
+    ERR_MALFORMED_SIGNATURE,
+
+    //! A public key could not be recovered from the provided signature and message.
+    ERR_PUBKEY_NOT_RECOVERED,
+
+    //! The message was not signed with the private key of the provided address.
+    ERR_NOT_SIGNED,
+
+    //! The message verification was successful.
+    OK
+};
+
+/** Verify a signed message.
+ * @param[in] address Signer's bitcoin address, it must refer to a public key.
+ * @param[in] signature The signature in base64 format.
+ * @param[in] message The message that was signed.
+ * @return result code */
+MessageVerificationResult MessageVerify(
+    const std::string& address,
+    const std::string& signature,
+    const std::string& message);
+
+#endif // BITCOIN_UTIL_MESSAGE_H

src/util/validation.cpp

@@ -21,5 +21,3 @@ std::string FormatStateMessage(const ValidationState &state)
 
     return state.GetRejectReason();
 }
-
-const std::string strMessageMagic = "Bitcoin Signed Message:\n";

src/util/validation.h

@@ -13,6 +13,4 @@ class ValidationState;
 /** Convert ValidationState to a human-readable message for logging */
 std::string FormatStateMessage(const ValidationState &state);
 
-extern const std::string strMessageMagic;
-
 #endif // BITCOIN_UTIL_VALIDATION_H