invoice: output payment_secret in JSON.

This makes it easier to access (rather than decoding bolt11).

Changelog-Added: JSON-RPC: `invoice` now outputs explicit `payment_secret` it its own field.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

doc/lightning-invoice.7

@@ -85,6 +85,8 @@ On success, an object is returned, containing:
 .IP \[bu]
 \fBpayment_hash\fR (hex): the hash of the \fIpayment_preimage\fR which will prove payment (always 64 characters)
 .IP \[bu]
+\fBpayment_secret\fR (hex): the \fIpayment_secret\fR to place in the onion (always 64 characters)
+.IP \[bu]
 \fBexpires_at\fR (u64): UNIX timestamp of when invoice expires
 
 .RE
@@ -136,4 +138,4 @@ Rusty Russell \fI<rusty@rustcorp.com.au\fR> is mainly responsible\.
 
 Main web site: \fIhttps://github.com/ElementsProject/lightning\fR
 
-\" SHA256STAMP:f27f015d8d612d280a8abca6930dbd42ef37fdac00bff1b114d1fd9138b15097
+\" SHA256STAMP:1617a52d2c9c5adbee4b410d15889a5f1c32e0a2fb2982dac82c5e74fb028ff3

doc/lightning-invoice.7.md

@@ -75,6 +75,7 @@ RETURN VALUE
 On success, an object is returned, containing:
 - **bolt11** (string): the bolt11 string
 - **payment_hash** (hex): the hash of the *payment_preimage* which will prove payment (always 64 characters)
+- **payment_secret** (hex): the *payment_secret* to place in the onion (always 64 characters)
 - **expires_at** (u64): UNIX timestamp of when invoice expires
 
 The following warnings may also be returned:
@@ -111,4 +112,4 @@ RESOURCES
 
 Main web site: <https://github.com/ElementsProject/lightning>
 
-[comment]: # ( SHA256STAMP:e63e87b91a14b8ae823ae67fc25315bc31b15a14378c05f4c972830bf3515af1)
+[comment]: # ( SHA256STAMP:cba838098a7f95ab74906f387b2d67777b33ccc40349cb9a8ef1f7c9cc28c6ef)

doc/schemas/invoice.schema.json

@@ -2,7 +2,7 @@
     "$schema": "http://json-schema.org/draft-07/schema#",
     "type": "object",
     "additionalProperties": false,
-    "required": [ "payment_hash", "expires_at", "bolt11" ],
+    "required": [ "payment_hash", "expires_at", "bolt11", "payment_secret" ],
     "properties": {
 	"bolt11": {
 	    "type": "string",
@@ -14,6 +14,12 @@
 	    "maxLength": 64,
 	    "minLength": 64
 	},
+	"payment_secret": {
+	    "type": "hex",
+	    "description": "the *payment_secret* to place in the onion",
+	    "maxLength": 64,
+	    "minLength": 64
+	},
 	"expires_at": {
 	    "type": "u64",
 	    "description": "UNIX timestamp of when invoice expires"

lightningd/invoice.c

@@ -855,6 +855,7 @@ invoice_complete(struct invoice_info *info,
 	struct invoice invoice;
 	char *b11enc;
 	const struct invoice_details *details;
+	struct secret payment_secret;
 	struct wallet *wallet = info->cmd->ld->wallet;
 
 	b11enc = bolt11_encode(info, info->b11, false,
@@ -904,6 +905,8 @@ invoice_complete(struct invoice_info *info,
 	json_add_sha256(response, "payment_hash", &details->rhash);
 	json_add_u64(response, "expires_at", details->expiry_time);
 	json_add_string(response, "bolt11", details->invstring);
+	invoice_secret(&details->r, &payment_secret);
+	json_add_secret(response, "payment_secret", &payment_secret);
 
 	notify_invoice_creation(info->cmd->ld, info->b11->msat,
 				info->payment_preimage, info->label);

lightningd/test/run-invoice-select-inchan.c

@@ -334,6 +334,11 @@ void json_add_node_id(struct json_stream *response UNNEEDED,
 void json_add_preimage(struct json_stream *result UNNEEDED, const char *fieldname UNNEEDED,
 		     const struct preimage *preimage UNNEEDED)
 { fprintf(stderr, "json_add_preimage called!\n"); abort(); }
+/* Generated stub for json_add_secret */
+void json_add_secret(struct json_stream *response UNNEEDED,
+		     const char *fieldname UNNEEDED,
+		     const struct secret *secret UNNEEDED)
+{ fprintf(stderr, "json_add_secret called!\n"); abort(); }
 /* Generated stub for json_add_sha256 */
 void json_add_sha256(struct json_stream *result UNNEEDED, const char *fieldname UNNEEDED,
 		     const struct sha256 *hash UNNEEDED)