signrpc: sign and verify messages with custom key

To allow signing of messages with any key in the key chain
we add two new methods to the signer RPC. These behave differently
to the methods with the same name in the main RPC as described
in the documentation comment.

lnrpc/signrpc/signer.proto

@@ -13,17 +13,17 @@ message KeyLocator {
 }
 
 message KeyDescriptor {
-     /**
-     The raw bytes of the key being identified. Either this or the KeyLocator
-     must be specified.
-     */
-     bytes raw_key_bytes = 1;
+    /**
+    The raw bytes of the key being identified. Either this or the KeyLocator
+    must be specified.
+    */
+    bytes raw_key_bytes = 1;
 
-     /** 
-     The key locator that identifies which key to use for signing. Either this
-     or the raw bytes of the target key must be specified.
-     */
-     KeyLocator key_loc = 2;
+    /** 
+    The key locator that identifies which key to use for signing. Either this
+    or the raw bytes of the target key must be specified.
+    */
+    KeyLocator key_loc = 2;
 }
 
 message TxOut {
@@ -119,6 +119,38 @@ message InputScriptResp {
     repeated InputScript input_scripts = 1;
 }
 
+message SignMessageReq {
+    /// The message to be signed.
+    bytes msg = 1;
+
+    /// The key locator that identifies which key to use for signing.
+    KeyLocator key_loc = 2;
+}
+message SignMessageResp {
+    /**
+    The signature for the given message in the fixed-size LN wire format.
+    */
+    bytes signature = 1;
+}
+
+message VerifyMessageReq {
+    /// The message over which the signature is to be verified.
+    bytes msg = 1;
+
+    /**
+    The fixed-size LN wire encoded signature to be verified over the given
+    message.
+    */
+    bytes signature = 2;
+
+    /// The public key the signature has to be valid for.
+    bytes pubkey = 3;
+}
+message VerifyMessageResp {
+    /// Whether the signature was valid over the given message.
+    bool valid = 1;
+}
+
 service Signer {
     /**
     SignOutputRaw is a method that can be used to generated a signature for a
@@ -130,7 +162,7 @@ service Signer {
     If we are  unable to sign using the specified keys, then an error will be
     returned.
     */
-    rpc SignOutputRaw(SignReq) returns (SignResp);
+    rpc SignOutputRaw (SignReq) returns (SignResp);
 
     /**
     ComputeInputScript generates a complete InputIndex for the passed
@@ -144,5 +176,23 @@ service Signer {
     in the TxOut field, the value in that same field, and finally the input
     index.
     */
-    rpc ComputeInputScript(SignReq) returns (InputScriptResp); 
+    rpc ComputeInputScript (SignReq) returns (InputScriptResp);
+
+    /**
+    SignMessage signs a message with the key specified in the key locator. The
+    returned signature is fixed-size LN wire format encoded.
+    
+    The main difference to SignMessage in the main RPC is that a specific key is
+    used to sign the message instead of the node identity private key.
+    */
+    rpc SignMessage (SignMessageReq) returns (SignMessageResp);
+
+    /**
+    VerifyMessage verifies a signature over a message using the public key
+    provided. The signature must be fixed-size LN wire format encoded.
+    
+    The main difference to VerifyMessage in the main RPC is that the public key
+    used to sign the message does not have to be a node known to the network.
+    */
+    rpc VerifyMessage (VerifyMessageReq) returns (VerifyMessageResp);
 }