plugin: Implement the ability to receive keysend payments

This still uses the experimental TLV-type, but once the type is standardized
we can add detection for the new type quite easily.

Changelog-Added: pay: The `keysend` plugin implements the ability to receive spontaneous payments (keysend)

plugins/Makefile

@@ -59,7 +59,7 @@ plugins/fundchannel: common/addr.o $(PLUGIN_FUNDCHANNEL_OBJS) $(PLUGIN_LIB_OBJS)
 
 plugins/bcli: bitcoin/chainparams.o $(PLUGIN_BCLI_OBJS) $(PLUGIN_LIB_OBJS) $(PLUGIN_COMMON_OBJS) $(JSMN_OBJS) $(CCAN_OBJS)
 
-plugins/keysend: bitcoin/chainparams.o $(PLUGIN_KEYSEND_OBJS) $(PLUGIN_LIB_OBJS) $(PLUGIN_COMMON_OBJS) $(JSMN_OBJS) $(CCAN_OBJS)
+plugins/keysend: bitcoin/chainparams.o wire/gen_onion_wire.o $(PLUGIN_KEYSEND_OBJS) $(PLUGIN_LIB_OBJS) $(PLUGIN_COMMON_OBJS) $(JSMN_OBJS) $(CCAN_OBJS)
 
 $(PLUGIN_PAY_OBJS) $(PLUGIN_AUTOCLEAN_OBJS) $(PLUGIN_FUNDCHANNEL_OBJS) $(PLUGIN_BCLI_OBJS) $(PLUGIN_LIB_OBJS): $(PLUGIN_LIB_HEADER)
 

plugins/keysend.c

@@ -1,5 +1,8 @@
 #include <ccan/array_size/array_size.h>
 #include <plugins/libplugin.h>
+#include <wire/gen_onion_wire.h>
+
+#define PREIMAGE_TLV_TYPE 5482373484
 
 static void init(struct plugin *p, const char *buf UNUSED,
 		 const jsmntok_t *config UNUSED)
@@ -9,10 +12,108 @@ static void init(struct plugin *p, const char *buf UNUSED,
 static const struct plugin_command commands[] = {
 };
 
+static struct command_result *htlc_accepted_continue(struct command *cmd)
+{
+	struct json_stream *response;
+	response = jsonrpc_stream_success(cmd);
+	json_add_string(response, "result", "continue");
+	return command_finished(cmd, response);
+}
+
+static struct command_result *htlc_accepted_resolve(struct command *cmd,
+						    char *hexpreimage)
+{
+	struct json_stream *response;
+	response = jsonrpc_stream_success(cmd);
+	json_add_string(response, "result", "resolve");
+	json_add_string(response, "payment_key", hexpreimage);
+	return command_finished(cmd, response);
+}
+
+static struct command_result *htlc_accepted_call(struct command *cmd, const char *buf,
+					  const jsmntok_t *params)
+{
+	const jsmntok_t *payloadt = json_delve(buf, params, ".onion.payload");
+	const jsmntok_t *payment_hash_tok = json_delve(buf, params, ".htlc.payment_hash");
+	const u8 *rawpayload;
+	size_t max;
+	struct tlv_tlv_payload *payload;
+	struct tlv_field *preimage_field = NULL;
+	char *hexpreimage, *hexpaymenthash;
+	struct sha256 payment_hash;
+	bigsize_t s;
+
+	if (!payloadt)
+		return htlc_accepted_continue(cmd);
+
+	rawpayload = json_tok_bin_from_hex(cmd, buf, payloadt);
+	max = tal_bytelen(rawpayload);
+	payload = tlv_tlv_payload_new(cmd);
+
+	s = fromwire_varint(&rawpayload, &max);
+	if (s != max) {
+		return htlc_accepted_continue(cmd);
+	}
+	fromwire_tlv_payload(&rawpayload, &max, payload);
+
+	/* Try looking for the field that contains the preimage */
+	for (int i=0; i<tal_count(payload->fields); i++) {
+		if (payload->fields[i].numtype == PREIMAGE_TLV_TYPE) {
+			preimage_field = &payload->fields[i];
+			break;
+		}
+	}
+
+	/* If we don't have a preimage field then this is not a keysend, let
+	 * someone else take care of it. */
+	if (preimage_field == NULL)
+		return htlc_accepted_continue(cmd);
+
+	/* If the preimage is not 32 bytes long then we can't accept the
+	 * payment. */
+	if (preimage_field->length != 32) {
+		plugin_log(cmd->plugin, LOG_UNUSUAL,
+			   "Sender specified a preimage that is %zu bytes long, "
+			   "we expected 32 bytes. Ignoring this HTLC.",
+			   preimage_field->length);
+		return htlc_accepted_continue(cmd);
+	}
+
+	hexpreimage = tal_hex(cmd, preimage_field->value);
+
+	/* If the preimage doesn't hash to the payment_hash we must continue,
+	 * maybe someone else knows how to handle these. */
+	sha256(&payment_hash, preimage_field->value, preimage_field->length);
+	hexpaymenthash = tal_hexstr(cmd, &payment_hash, sizeof(payment_hash));
+	if (!json_tok_streq(buf, payment_hash_tok, hexpaymenthash)) {
+		plugin_log(
+		    cmd->plugin, LOG_UNUSUAL,
+		    "Preimage provided by the sender does not match the "
+		    "payment_hash: SHA256(%s)=%s != %.*s. Ignoring keysend.",
+		    hexpreimage, hexpaymenthash,
+		    payment_hash_tok->end - payment_hash_tok->start,
+		    buf + payment_hash_tok->start);
+		return htlc_accepted_continue(cmd);
+	}
+
+	/* Finally we can resolve the payment with the preimage. */
+	plugin_log(cmd->plugin, LOG_INFORM,
+		   "Resolving incoming HTLC with preimage for payment_hash %s "
+		   "provided in the onion payload.",
+		   hexpaymenthash);
+	return htlc_accepted_resolve(cmd, hexpreimage);
+}
+
+static const struct plugin_hook hooks[] = {
+	{
+		"htlc_accepted",
+		htlc_accepted_call
+	},
+};
 
 int main(int argc, char *argv[])
 {
 	setup_locale();
 	plugin_main(argv, init, PLUGIN_RESTARTABLE, commands, ARRAY_SIZE(commands),
-	            NULL, 0, NULL, 0, NULL);
+	            NULL, 0, hooks, ARRAY_SIZE(hooks), NULL);
 }