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

e193a84fb2 Refactor message hashing into a utility function (Jeffrey Czyz)
f8f0d9893d Deduplicate the message signing code (Vasil Dimov)
2ce3447eb1 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 e193a84fb2
  achow101:
    ACK e193a84fb2
  instagibbs:
    utACK e193a84fb2
  meshcollider:
    utACK e193a84fb2

Tree-SHA512: b0e02a7d4623a98c8f8c77627af1725e6df07700de4630c2f75da6beacdf55414c38ba147bc6d2a757491ab07c827dddf93e8632fe600478760e255714ddab88

src/wallet/rpcwallet.cpp

@@ -19,11 +19,11 @@
 #include <script/sign.h>
 #include <util/bip32.h>
 #include <util/fees.h>
+#include <util/message.h> // For MessageSign()
 #include <util/moneystr.h>
 #include <util/string.h>
 #include <util/system.h>
 #include <util/url.h>
-#include <util/validation.h>
 #include <wallet/coincontrol.h>
 #include <wallet/feebumper.h>
 #include <wallet/psbtwallet.h>
@@ -576,15 +576,13 @@ static UniValue signmessage(const JSONRPCRequest& request)
         throw JSONRPCError(RPC_WALLET_ERROR, "Private key not available");
     }
 
-    CHashWriter ss(SER_GETHASH, 0);
-    ss << strMessageMagic;
-    ss << strMessage;
+    std::string signature;
 
-    std::vector<unsigned char> vchSig;
-    if (!key.SignCompact(ss.GetHash(), vchSig))
+    if (!MessageSign(key, strMessage, signature)) {
         throw JSONRPCError(RPC_INVALID_ADDRESS_OR_KEY, "Sign failed");
+    }
 
-    return EncodeBase64(vchSig.data(), vchSig.size());
+    return signature;
 }
 
 static UniValue getreceivedbyaddress(const JSONRPCRequest& request)