netann: add MsgHash helper

This commit adds the MsgHash helper function which can be used to
calculate the digest of a message to be signed using schnorr signatures.

netann/msg_hash.go

@@ -0,0 +1,30 @@
+package netann
+
+import (
+	"github.com/btcsuite/btcd/chaincfg/chainhash"
+)
+
+// MsgHashTag will prefix the message name and the field name in order to
+// construct the message tag.
+const MsgHashTag = "lightning"
+
+// MsgTag computes the full tag that will be used to prefix a message before
+// calculating the tagged hash. The tag is constructed as follows:
+//
+//	tag = "lightning"||"msg_name"||"field_name"
+func MsgTag(msgName, fieldName string) []byte {
+	tag := []byte(MsgHashTag)
+	tag = append(tag, []byte(msgName)...)
+
+	return append(tag, []byte(fieldName)...)
+}
+
+// MsgHash computes the tagged hash of the given message as follows:
+//
+//	tag = "lightning"||"msg_name"||"field_name"
+//	hash = sha256(sha246(tag) || sha256(tag) || msg)
+func MsgHash(msgName, fieldName string, msg []byte) *chainhash.Hash {
+	tag := MsgTag(msgName, fieldName)
+
+	return chainhash.TaggedHash(tag, msg)
+}