lightningd: Add signinvoice to sign a BOLT11 invoice.

Though there's already a `createinvoice` command, there are usecases where a
user may want to sign an invoice that they don't yet have the preimage to. For
example, they may have an htlc_accepted plugin that pays to obtain the preimage
from someone else and returns a `{ "result": "resolve", ... }`.

This RPC command addresses this usecase without overly complicating the
semantics of the existing `createinvoice` command.

Changlog-Added: JSON-RPC: `signinvoice` new command to sign BOLT11
invoices.

doc/index.rst

@@ -112,6 +112,7 @@ Core Lightning Documentation
    lightning-sendpsbt <lightning-sendpsbt.7.md>
    lightning-setchannel <lightning-setchannel.7.md>
    lightning-setchannelfee <lightning-setchannelfee.7.md>
+   lightning-signinvoice <lightning-signinvoice.7.md>
    lightning-signmessage <lightning-signmessage.7.md>
    lightning-signpsbt <lightning-signpsbt.7.md>
    lightning-staticbackup <lightning-staticbackup.7.md>

doc/lightning-signinvoice.7.md

@@ -0,0 +1,47 @@
+lightning-signinvoice -- Low-level invoice signing
+=====================================================
+
+SYNOPSIS
+--------
+
+**signinvoice** *invstring*
+
+DESCRIPTION
+-----------
+
+The **signinvoice** RPC command signs an invoice.  Unlike
+**createinvoice** it does not save the invoice into the database and
+thus does not require the preimage.
+
+The *invstring* parameter is of bolt11 form, but the final signature
+is ignored.  Minimal sanity checks are done.
+
+RETURN VALUE
+------------
+
+[comment]: # (GENERATE-FROM-SCHEMA-START)
+[comment]: # (GENERATE-FROM-SCHEMA-END)
+
+On failure, an error is returned.
+
+The following error codes may occur:
+- -1: Catchall nonspecific error.
+
+AUTHOR
+------
+
+Carl Dong <<contact@carldong.me>> is mainly responsible.
+
+SEE ALSO
+--------
+
+lightning-createinvoice(7), lightning-invoice(7), lightning-listinvoices(7),
+lightning-delinvoice(7), lightning-getroute(7), lightning-sendpay(7),
+lightning-offer(7).
+
+RESOURCES
+---------
+
+Main web site: <https://github.com/ElementsProject/lightning>
+
+[comment]: # ( SHA256STAMP:9fc2c7cb6e5980774a768dd9ddfe81e254c084554c159e6b07e92e703dc10595)

doc/schemas/signinvoice.request.json

@@ -0,0 +1,14 @@
+{
+  "$schema": "http://json-schema.org/draft-07/schema#",
+  "type": "object",
+  "additionalProperties": false,
+  "required": [
+    "invstring"
+  ],
+  "properties": {
+    "invstring": {
+      "type": "string",
+      "description": ""
+    }
+  }
+}

doc/schemas/signinvoice.schema.json

@@ -0,0 +1,14 @@
+{
+  "$schema": "http://json-schema.org/draft-07/schema#",
+  "type": "object",
+  "additionalProperties": false,
+  "required": [
+    "bolt11"
+  ],
+  "properties": {
+    "bolt11": {
+      "type": "string",
+      "description": "the bolt11 string"
+    }
+  }
+}

lightningd/invoice.c

@@ -1788,3 +1788,56 @@ static const struct json_command createinvoice_command = {
 };
 
 AUTODATA(json_command, &createinvoice_command);
+
+static struct command_result *json_signinvoice(struct command *cmd,
+						 const char *buffer,
+						 const jsmntok_t *obj UNNEEDED,
+						 const jsmntok_t *params)
+{
+	const char *invstring;
+	struct json_stream *response;
+	struct bolt11 *b11;
+	struct sha256 hash;
+	u5 *sig;
+	bool have_n;
+	char *fail;
+
+	if (!param(cmd, buffer, params,
+		   p_req("invstring", param_string, &invstring),
+		   NULL))
+		return command_param_failed();
+
+	b11 = bolt11_decode_nosig(cmd, invstring, cmd->ld->our_features,
+				  NULL, chainparams, &hash, &sig, &have_n,
+				  &fail);
+
+	if (!b11)
+		return command_fail(cmd, JSONRPC2_INVALID_PARAMS,
+				    "Unparsable invoice '%s': %s",
+				    invstring, fail);
+
+	/* This adds the signature */
+	char *b11enc = bolt11_encode(cmd, b11, have_n,
+				     hsm_sign_b11, cmd->ld);
+
+	if (!b11->description)
+		return command_fail(cmd, JSONRPC2_INVALID_PARAMS,
+				    "Missing description in invoice");
+
+	if (!b11->expiry)
+		return command_fail(cmd, JSONRPC2_INVALID_PARAMS,
+				    "Missing expiry in invoice");
+
+	response = json_stream_success(cmd);
+	json_add_invstring(response, b11enc);
+	return command_success(cmd, response);
+}
+
+static const struct json_command signinvoice_command = {
+	"signinvoice",
+	"payment",
+	json_signinvoice,
+	"Lowlevel command to sign invoice {invstring}."
+};
+
+AUTODATA(json_command, &signinvoice_command);

tests/test_invoices.py

@@ -533,6 +533,19 @@ def test_waitanyinvoice(node_factory, executor):
         l2.rpc.waitanyinvoice('non-number')
 
 
+def test_signinvoice(node_factory, executor):
+    # Setup
+    l1, l2 = node_factory.line_graph(2)
+
+    # Create an invoice for l1
+    inv1 = l1.rpc.invoice(1000, 'inv1', 'inv1')['bolt11']
+    assert l1.rpc.decodepay(inv1)['payee'] == l1.info['id']
+
+    # Have l2 re-sign the invoice
+    inv2 = l2.rpc.signinvoice(inv1)['bolt11']
+    assert l1.rpc.decodepay(inv2)['payee'] == l2.info['id']
+
+
 def test_waitanyinvoice_reversed(node_factory, executor):
     """Test waiting for invoices, where they are paid in reverse order
     to when they are created.