common/onion: split into decode and encode routines.

Some places (e.g. the pay plugin) only need to construct onions,
not decode them.

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

common/onion_encode.h

@@ -0,0 +1,58 @@
+#ifndef LIGHTNING_COMMON_ONION_ENCODE_H
+#define LIGHTNING_COMMON_ONION_ENCODE_H
+#include "config.h"
+#include <bitcoin/privkey.h>
+#include <common/amount.h>
+
+struct route_step;
+struct tlv_encrypted_data_tlv_payment_relay;
+
+enum onion_payload_type {
+	ONION_V0_PAYLOAD = 0,
+	ONION_TLV_PAYLOAD = 1,
+};
+
+struct onion_payload {
+	enum onion_payload_type type;
+
+	struct amount_msat amt_to_forward;
+	u32 outgoing_cltv;
+	struct amount_msat *total_msat;
+
+	/* One of these is set */
+	struct short_channel_id *forward_channel;
+	struct pubkey *forward_node_id;
+
+	struct secret *payment_secret;
+	u8 *payment_metadata;
+
+	/* If blinding is set, blinding_ss is the shared secret.*/
+	struct pubkey *blinding;
+	struct secret blinding_ss;
+
+	/* The raw TLVs contained in the payload. */
+	struct tlv_tlv_payload *tlv;
+};
+
+u8 *onion_nonfinal_hop(const tal_t *ctx,
+		       const struct short_channel_id *scid,
+		       struct amount_msat forward,
+		       u32 outgoing_cltv);
+
+/* Note that this can fail if we supply payment_secret or payment_metadata and !use_tlv! */
+u8 *onion_final_hop(const tal_t *ctx,
+		    struct amount_msat forward,
+		    u32 outgoing_cltv,
+		    struct amount_msat total_msat,
+		    const struct secret *payment_secret,
+		    const u8 *payment_metadata);
+
+/* Blinding has more complex rules on what fields are encoded: this is the
+ * generic interface, as used by blindedpay.h */
+u8 *onion_blinded_hop(const tal_t *ctx,
+		      const struct amount_msat *amt_to_forward,
+		      const u32 *outgoing_cltv_value,
+		      const u8 *enctlv,
+		      const struct pubkey *blinding)
+	NON_NULL_ARGS(4);
+#endif /* LIGHTNING_COMMON_ONION_ENCODE_H */