sphinx: add route blinding helper methods to Router

In this commit, DecryptBlindHopData and NextEphemeral methods are added
to Router which makes use of the Router's onion key in order to unblind
blinded data and compute the next ephermal key respectively.

Author: ellemouton

sphinx.go

@@ -611,6 +611,22 @@ func (r *Router) ReconstructOnionPacket(onionPkt *OnionPacket, assocData []byte,
 	return processOnionPacket(onionPkt, &sharedSecret, assocData)
 }
 
+// DecryptBlindedHopData uses the router's private key to decrypt data encrypted
+// by the creator of the blinded route.
+func (r *Router) DecryptBlindedHopData(ephemPub *btcec.PublicKey,
+	encryptedData []byte) ([]byte, error) {
+
+	return decryptBlindedHopData(r.onionKey, ephemPub, encryptedData)
+}
+
+// NextEphemeral computes the next ephemeral key given the current ephemeral
+// key and the router's private key.
+func (r *Router) NextEphemeral(ephemPub *btcec.PublicKey) (*btcec.PublicKey,
+	error) {
+
+	return NextEphemeral(r.onionKey, ephemPub)
+}
+
 // unwrapPacket wraps a layer of the passed onion packet using the specified
 // shared secret and associated data. The associated data will be used to check
 // the HMAC at each hop to ensure the same data is passed along with the onion