walletrpc: add sign/verify methods

Adding the grpc functionality to sign and verify messages with
single addresses

lnrpc/walletrpc/walletkit.proto

@@ -79,6 +79,47 @@ service WalletKit {
     */
     rpc ListAddresses (ListAddressesRequest) returns (ListAddressesResponse);
 
+    /*
+    SignMessageWithAddr returns the compact signature (base64 encoded) created
+    with the private key of the provided address. This requires the address
+    to be solely based on a public key lock (no scripts). Obviously the internal
+    lnd wallet has to possess the private key of the address otherwise
+    an error is returned.
+
+    This method aims to provide full compatibility with the bitcoin-core and
+    btcd implementation. Bitcoin-core's algorithm is not specified in a
+    BIP and only applicable for legacy addresses. This method enhances the
+    signing for additional address types: P2WKH, NP2WKH, P2TR.
+    For P2TR addresses this represents a special case. ECDSA is used to create
+    a compact signature which makes the public key of the signature recoverable.
+    */
+    rpc SignMessageWithAddr (SignMessageWithAddrRequest)
+        returns (SignMessageWithAddrResponse);
+
+    /*
+    VerifyMessageWithAddr returns the validity and the recovered public key of
+    the provided compact signature (base64 encoded). The verification is
+    twofold. First the validity of the signature itself is checked and then
+    it is verified that the recovered public key of the signature equals
+    the public key of the provided address. There is no dependence on the
+    private key of the address therefore also external addresses are allowed
+    to verify signatures.
+    Supported address types are P2PKH, P2WKH, NP2WKH, P2TR.
+
+    This method is the counterpart of the related signing method
+    (SignMessageWithAddr) and aims to provide full compatibility to
+    bitcoin-core's implementation. Although bitcoin-core/btcd only provide
+    this functionality for legacy addresses this function enhances it to
+    the address types: P2PKH, P2WKH, NP2WKH, P2TR.
+
+    The verification for P2TR addresses is a special case and requires the
+    ECDSA compact signature to compare the reovered public key to the internal
+    taproot key. The compact ECDSA signature format was used because there
+    are still no known compact signature schemes for schnorr signatures.
+    */
+    rpc VerifyMessageWithAddr (VerifyMessageWithAddrRequest)
+        returns (VerifyMessageWithAddrResponse);
+
     /*
     ImportAccount imports an account backed by an account extended public key.
     The master key fingerprint denotes the fingerprint of the root key
@@ -497,6 +538,43 @@ message ListAddressesResponse {
     repeated AccountWithAddresses account_with_addresses = 1;
 }
 
+message SignMessageWithAddrRequest {
+    // The message to be signed. When using REST, this field must be encoded as
+    // base64.
+    bytes msg = 1;
+
+    // The address which will be used to look up the private key and sign the
+    // corresponding message.
+    string addr = 2;
+}
+
+message SignMessageWithAddrResponse {
+    // The compact ECDSA signature for the given message encoded in base64.
+    string signature = 1;
+}
+
+message VerifyMessageWithAddrRequest {
+    // The message to be signed. When using REST, this field must be encoded as
+    // base64.
+    bytes msg = 1;
+
+    // The compact ECDSA signature to be verified over the given message
+    // ecoded in base64.
+    string signature = 2;
+
+    // The address which will be used to look up the public key and verify the
+    // the signature.
+    string addr = 3;
+}
+
+message VerifyMessageWithAddrResponse {
+    // Whether the signature was valid over the given message.
+    bool valid = 1;
+
+    // The pubkey recovered from the signature.
+    bytes pubkey = 2;
+}
+
 message ImportAccountRequest {
     // A name to identify the account with.
     string name = 1;