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.c

@@ -0,0 +1,122 @@
+#include "config.h"
+#include <assert.h>
+#include <ccan/array_size/array_size.h>
+#include <ccan/cast/cast.h>
+#include <ccan/mem/mem.h>
+#include <common/blindedpath.h>
+#include <common/ecdh.h>
+#include <common/onion_encode.h>
+#include <common/sphinx.h>
+#include <sodium/crypto_aead_chacha20poly1305.h>
+
+/* BOLT #4:
+ *
+ * ### `tlv_payload` format
+ *
+ * This is a more flexible format, which avoids the redundant
+ * `short_channel_id` field for the final node.  It is formatted
+ * according to the Type-Length-Value format defined in [BOLT
+ * #1](01-messaging.md#type-length-value-format).
+ */
+static u8 *make_tlv_hop(const tal_t *ctx,
+			const struct tlv_tlv_payload *tlv)
+{
+	/* We can't have over 64k anyway */
+	u8 *tlvs = tal_arr(ctx, u8, 3);
+
+	towire_tlv_tlv_payload(&tlvs, tlv);
+
+	switch (bigsize_put(tlvs, tal_bytelen(tlvs) - 3)) {
+	case 1:
+		/* Move over two unused bytes */
+		memmove(tlvs + 1, tlvs + 3, tal_bytelen(tlvs) - 3);
+		tal_resize(&tlvs, tal_bytelen(tlvs) - 2);
+		return tlvs;
+	case 3:
+		return tlvs;
+	}
+	abort();
+}
+
+u8 *onion_nonfinal_hop(const tal_t *ctx,
+		       const struct short_channel_id *scid,
+		       struct amount_msat forward,
+		       u32 outgoing_cltv)
+{
+	struct tlv_tlv_payload *tlv = tlv_tlv_payload_new(tmpctx);
+
+	/* BOLT #4:
+	 *
+	 * The writer:
+	 *...
+	 *  - For every node:
+	 *    - MUST include `amt_to_forward` and `outgoing_cltv_value`.
+	 *  - For every non-final node:
+	 *    - MUST include `short_channel_id`
+	 *    - MUST NOT include `payment_data`
+	 */
+	tlv->amt_to_forward = &forward.millisatoshis; /* Raw: TLV convert */
+	tlv->outgoing_cltv_value = &outgoing_cltv;
+	tlv->short_channel_id = cast_const(struct short_channel_id *, scid);
+	return make_tlv_hop(ctx, 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)
+{
+	struct tlv_tlv_payload *tlv = tlv_tlv_payload_new(tmpctx);
+	struct tlv_tlv_payload_payment_data tlv_pdata;
+
+	/* These go together! */
+	if (!payment_secret)
+		assert(amount_msat_eq(total_msat, forward));
+
+	/* BOLT #4:
+	 *
+	 * The writer:
+	 *...
+	 *  - For every node:
+	 *    - MUST include `amt_to_forward` and `outgoing_cltv_value`.
+	 *...
+	 *  - For the final node:
+	 *    - MUST NOT include `short_channel_id`
+	 *    - if the recipient provided `payment_secret`:
+	 *      - MUST include `payment_data`
+	 *      - MUST set `payment_secret` to the one provided
+	 *      - MUST set `total_msat` to the total amount it will send
+	 */
+	tlv->amt_to_forward = &forward.millisatoshis; /* Raw: TLV convert */
+	tlv->outgoing_cltv_value = &outgoing_cltv;
+
+	if (payment_secret) {
+		tlv_pdata.payment_secret = *payment_secret;
+		tlv_pdata.total_msat = total_msat.millisatoshis; /* Raw: TLV convert */
+		tlv->payment_data = &tlv_pdata;
+	}
+	tlv->payment_metadata = cast_const(u8 *, payment_metadata);
+	return make_tlv_hop(ctx, tlv);
+}
+
+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)
+{
+	struct tlv_tlv_payload *tlv = tlv_tlv_payload_new(tmpctx);
+
+	if (amt_to_forward) {
+		tlv->amt_to_forward
+			= cast_const(u64 *,
+				     &amt_to_forward->millisatoshis); /* Raw: TLV convert */
+	}
+	tlv->outgoing_cltv_value = cast_const(u32 *, outgoing_cltv_value);
+	tlv->encrypted_recipient_data = cast_const(u8 *, enctlv);
+	tlv->blinding_point = cast_const(struct pubkey *, blinding);
+
+	return make_tlv_hop(ctx, tlv);
+}