common/onion_message_parse: generic routine for parsing onion messages.

Instead of open coding in connectd/onion_message, we move it to common
with a nice API.

This lets us process the BOLT test vectors.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

common/onion_message_parse.h

@@ -0,0 +1,37 @@
+#ifndef LIGHTNING_COMMON_ONION_MESSAGE_PARSE_H
+#define LIGHTNING_COMMON_ONION_MESSAGE_PARSE_H
+#include "config.h"
+#include <bitcoin/privkey.h>
+#include <common/amount.h>
+
+struct tlv_onionmsg_tlv;
+struct node_id;
+struct pubkey;
+
+/**
+ * onion_message_parse: core routine to check onion_message
+ * @ctx: context to allocate @next_onion_msg or @final_om/@path_id off
+ * @onion_message_packet: Sphinx-encrypted onion
+ * @blinding: Blinding we were given for @onion_message_packet
+ * @peer: node_id of peer (for status_peer_debug msgs)
+ * @me: my pubkey
+ * @next_onion_msg (out): set if we should forward, otherwise NULL.
+ * @next_node_id (out): set to node id to fwd to, iff *@next_onion_msg.
+ * @final_om (out): set if we're the final hop, otherwise NULL.
+ * @final_alias (out): our alias (if *@final_om), or our own ID
+ * @final_path_id (out): secret enclosed, if any (iff *@final_om).
+ *
+ * Returns false if it wasn't valid.
+ */
+bool onion_message_parse(const tal_t *ctx,
+			 const u8 *onion_message_packet,
+			 const struct pubkey *blinding,
+			 const struct node_id *peer,
+			 const struct pubkey *me,
+			 u8 **next_onion_msg,
+			 struct pubkey *next_node_id,
+			 struct tlv_onionmsg_tlv **final_om,
+			 struct pubkey *final_alias,
+			 struct secret **final_path_id);
+
+#endif /* LIGHTNING_COMMON_ONION_MESSAGE_PARSE_H */