signrpc: add DeriveSharedKey

lnrpc/signrpc/signer.proto

@@ -151,6 +151,16 @@ message VerifyMessageResp {
     bool valid = 1;
 }
 
+message SharedKeyRequest {
+    // The ephemeral public key to use for the DH key derivation.
+    bytes ephemeral_pubkey = 1;
+}
+
+message SharedKeyResponse {
+    // The shared public key, hashed with sha256.
+    bytes shared_key = 1;
+}
+
 service Signer {
     /**
     SignOutputRaw is a method that can be used to generated a signature for a
@@ -195,4 +205,14 @@ service Signer {
     used to sign the message does not have to be a node known to the network.
     */
     rpc VerifyMessage (VerifyMessageReq) returns (VerifyMessageResp);
+
+    /*
+    DeriveSharedKey returns a shared secret key by performing Diffie-Hellman key
+    derivation between the ephemeral public key in the request and the node's
+    identity private key:
+        P_shared = privKeyNodeID * ephemeralPubkey
+    The resulting shared public key is serialized in the compressed format and 
+    hashed with sha256, resulting in the final key length of 256bit.
+    */
+    rpc DeriveSharedKey (SharedKeyRequest) returns (SharedKeyResponse);
 }