Merge #17577: refactor: deduplicate the message sign/verify code

e193a84fb28068e38d5f54fbfd6208428c5bb655 Refactor message hashing into a utility function (Jeffrey Czyz)
f8f0d9893d7969bdaa870fadb94ec5d0dfa8334d Deduplicate the message signing code (Vasil Dimov)
2ce3447eb1e25ec7aec4b300dabf6c1e394f1906 Deduplicate the message verifying code (Vasil Dimov)

Pull request description:

  The message signing and verifying logic was replicated in a few places
  in the code. Consolidate in a newly introduced `MessageSign()` and
  `MessageVerify()` and add unit tests for them.

ACKs for top commit:
  Sjors:
    re-ACK e193a84fb28068e38d5f54fbfd6208428c5bb655
  achow101:
    ACK e193a84fb28068e38d5f54fbfd6208428c5bb655
  instagibbs:
    utACK https://github.com/bitcoin/bitcoin/pull/17577/commits/e193a84fb28068e38d5f54fbfd6208428c5bb655
  meshcollider:
    utACK e193a84fb28068e38d5f54fbfd6208428c5bb655

Tree-SHA512: b0e02a7d4623a98c8f8c77627af1725e6df07700de4630c2f75da6beacdf55414c38ba147bc6d2a757491ab07c827dddf93e8632fe600478760e255714ddab88

4 files changed, 146 insertions, 4 deletions

diff --git a/src/util/message.cpp b/src/util/message.cpp
new file mode 100644
index 0000000000..17603a43d2
--- /dev/null
+++ b/src/util/message.cpp
@@ -0,0 +1,78 @@
+// 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.h>             // For CKey
+#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>
+
+/**
+ * Text used to signify that a signed message follows and to prevent
+ * inadvertently signing a transaction.
+ */
+const std::string MESSAGE_MAGIC = "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;
+    }
+
+    CPubKey pubkey;
+    if (!pubkey.RecoverCompact(MessageHash(message), signature_bytes)) {
+        return MessageVerificationResult::ERR_PUBKEY_NOT_RECOVERED;
+    }
+
+    if (!(CTxDestination(PKHash(pubkey)) == destination)) {
+        return MessageVerificationResult::ERR_NOT_SIGNED;
+    }
+
+    return MessageVerificationResult::OK;
+}
+
+bool MessageSign(
+    const CKey& privkey,
+    const std::string& message,
+    std::string& signature)
+{
+    std::vector<unsigned char> signature_bytes;
+
+    if (!privkey.SignCompact(MessageHash(message), signature_bytes)) {
+        return false;
+    }
+
+    signature = EncodeBase64(signature_bytes.data(), signature_bytes.size());
+
+    return true;
+}
+
+uint256 MessageHash(const std::string& message)
+{
+    CHashWriter hasher(SER_GETHASH, 0);
+    hasher << MESSAGE_MAGIC << message;
+
+    return hasher.GetHash();
+}
diff --git a/src/util/message.h b/src/util/message.h
new file mode 100644
index 0000000000..01fd14ce2d
--- /dev/null
+++ b/src/util/message.h
@@ -0,0 +1,68 @@
+// 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 <key.h> // For CKey
+#include <uint256.h>
+
+#include <string>
+
+extern const std::string MESSAGE_MAGIC;
+
+/** 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);
+
+/** Sign a message.
+ * @param[in] privkey Private key to sign with.
+ * @param[in] message The message to sign.
+ * @param[out] signature Signature, base64 encoded, only set if true is returned.
+ * @return true if signing was successful. */
+bool MessageSign(
+    const CKey& privkey,
+    const std::string& message,
+    std::string& signature);
+
+/**
+ * Hashes a message for signing and verification in a manner that prevents
+ * inadvertently signing a transaction.
+ */
+uint256 MessageHash(const std::string& message);
+
+#endif // BITCOIN_UTIL_MESSAGE_H
diff --git a/src/util/validation.cpp b/src/util/validation.cpp
index 89bc6665a4..ffbee21aeb 100644
--- a/src/util/validation.cpp
+++ b/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";
diff --git a/src/util/validation.h b/src/util/validation.h
index da2cf9f102..5ee260a055 100644
--- a/src/util/validation.h
+++ b/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