createinvoice: make a minimal blinded "path" in bolt12 invoice if none presented.

The "path" is just a message to ourselves.  This meets the minimal
requirement for bolt12 invoices: that there be a blinded path (at
least so we can use the path_id inside in place of "payment_secret").

We expose the method to make this path_id to a common routine: offers
will need this for generating more sophisticated paths.

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

common/Makefile

@@ -47,6 +47,7 @@ COMMON_SRC_NOGEN :=				\
 	common/interactivetx.c			\
 	common/initial_channel.c		\
 	common/initial_commit_tx.c		\
+	common/invoice_path_id.c		\
 	common/iso4217.c			\
 	common/json_filter.c			\
 	common/json_param.c			\

common/invoice_path_id.c

@@ -0,0 +1,19 @@
+#include "config.h"
+#include <bitcoin/privkey.h>
+#include <ccan/crypto/sha256/sha256.h>
+#include <common/invoice_path_id.h>
+
+u8 *invoice_path_id(const tal_t *ctx,
+		    const struct secret *base_secret,
+		    const struct sha256 *payment_hash)
+{
+	struct sha256_ctx shactx;
+	struct sha256 secret;
+
+	sha256_init(&shactx);
+	sha256_update(&shactx, base_secret, sizeof(*base_secret));
+	sha256_update(&shactx, payment_hash, sizeof(*payment_hash));
+	sha256_done(&shactx, &secret);
+
+	return (u8 *)tal_dup(ctx, struct sha256, &secret);
+}

common/invoice_path_id.h

@@ -0,0 +1,29 @@
+#ifndef LIGHTNING_COMMON_INVOICE_PATH_ID_H
+#define LIGHTNING_COMMON_INVOICE_PATH_ID_H
+#include "config.h"
+#include <ccan/short_types/short_types.h>
+#include <ccan/tal/tal.h>
+
+struct secret;
+struct sha256;
+
+/* String to use with makesecret to get the invoice base secret */
+#define INVOICE_PATH_BASE_STRING "bolt12-invoice-base"
+
+/**
+ * invoice_path_id: generate the "path_id" field for the tlv_encrypted_data_tlv
+ * @ctx: tal context
+ * @payment_hash: the invoice payment hash
+ * @base_secret: the node-specific secret makesecret("bolt12-invoice-base")
+ *
+ * Receiving a blinded, encrypted tlv_encrypted_data_tlv containing
+ * the correct path_id is how we know this blinded path is the correct
+ * one for this invoice payment.
+ *
+ * It's exposed here as plugins may want to generate blinded paths.
+ */
+u8 *invoice_path_id(const tal_t *ctx,
+		    const struct secret *base_secret,
+		    const struct sha256 *payment_hash);
+
+#endif /* LIGHTNING_COMMON_INVOICE_PATH_ID_H */