lightningd: new internal JSONRPC "decryptencrypteddata"

I'm not sure about interface yet, so don't document.  It's ugly.

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

common/blindedpath.c

@@ -156,7 +156,7 @@ bool unblind_onion(const struct pubkey *blinding,
 					     hmac.data) == 1;
 }
 
-static u8 *decrypt_encmsg_raw(const tal_t *ctx,
+u8 *decrypt_encmsg_raw(const tal_t *ctx,
 		       const struct pubkey *blinding,
 		       const struct secret *ss,
 		       const u8 *enctlv)
@@ -253,7 +253,9 @@ void blindedpath_next_blinding(const struct tlv_encrypted_data_tlv *enc,
 	if (enc->next_blinding_override)
 		*next_blinding = *enc->next_blinding_override;
 	else {
-		/* E_{i-1} = H(E_i || ss_i) * E_i */
+		/* BOLT #4:
+		 * $`E_{i+1} = SHA256(E_i || ss_i) * E_i`$
+		 */
 		struct sha256 h;
 		blinding_hash_e_and_ss(blinding, ss, &h);
 		blinding_next_pubkey(blinding, &h, next_blinding);

common/blindedpath.h

@@ -76,6 +76,12 @@ struct tlv_encrypted_data_tlv *decrypt_encrypted_data(const tal_t *ctx,
 						      const u8 *enctlv)
 	NON_NULL_ARGS(2, 3);
 
+/* Low-level accessor */
+u8 *decrypt_encmsg_raw(const tal_t *ctx,
+		       const struct pubkey *blinding,
+		       const struct secret *ss,
+		       const u8 *enctlv);
+
 /**
  * blindedpath_next_blinding - Calculate or extract next blinding pubkey
  */

lightningd/onion_message.c

@@ -1,7 +1,9 @@
 #include "config.h"
 #include <ccan/mem/mem.h>
 #include <common/blindedpath.h>
+#include <common/blinding.h>
 #include <common/configdir.h>
+#include <common/ecdh.h>
 #include <common/json_command.h>
 #include <common/json_param.h>
 #include <connectd/connectd_wiregen.h>
@@ -328,3 +330,60 @@ static const struct json_command injectonionmessage_command = {
 	"Unwrap using {blinding}, encoded over {hops} (id, tlv)"
 };
 AUTODATA(json_command, &injectonionmessage_command);
+
+static struct command_result *json_decryptencrypteddata(struct command *cmd,
+							const char *buffer,
+							const jsmntok_t *obj UNNEEDED,
+							const jsmntok_t *params)
+{
+	u8 *encdata, *decrypted;
+	struct pubkey *blinding, next_blinding;
+	struct secret ss;
+	struct sha256 h;
+	struct json_stream *response;
+
+	if (!param_check(cmd, buffer, params,
+			 p_req("encrypted_data", param_bin_from_hex, &encdata),
+			 p_req("blinding", param_pubkey, &blinding),
+			 NULL))
+		return command_param_failed();
+
+	/* BOLT #4:
+	 *
+	 * - MUST compute:
+	 *   - $`ss_i = SHA256(k_i * E_i)`$ (standard ECDH)
+	 *...
+	 * - MUST decrypt the `encrypted_data` field using $`rho_i`$
+	 */
+	ecdh(blinding, &ss);
+
+	decrypted = decrypt_encmsg_raw(cmd, blinding, &ss, encdata);
+	if (!decrypted)
+		return command_fail(cmd, JSONRPC2_INVALID_PARAMS,
+				    "Decryption failed!");
+
+	if (command_check_only(cmd))
+		return command_check_done(cmd);
+
+	/* BOLT #4:
+	 *
+	 *   - $`E_{i+1} = SHA256(E_i || ss_i) * E_i`$
+	 */
+	blinding_hash_e_and_ss(blinding, &ss, &h);
+	blinding_next_pubkey(blinding, &h, &next_blinding);
+
+	response = json_stream_success(cmd);
+	json_object_start(response, "decryptencrypteddata");
+	json_add_hex_talarr(response, "decrypted", decrypted);
+	json_add_pubkey(response, "next_blinding", &next_blinding);
+	json_object_end(response);
+	return command_success(cmd, response);
+}
+
+static const struct json_command decryptencrypteddata_command = {
+	"decryptencrypteddata",
+	"utility",
+	json_decryptencrypteddata,
+	"Decrypt {encrypted_data} using {blinding}, return decryption and next blinding"
+};
+AUTODATA(json_command, &decryptencrypteddata_command);

tests/test_pay.py

@@ -5782,6 +5782,36 @@ def test_onionmessage_ratelimit(node_factory):
     l1.rpc.fetchinvoice(offer['bolt12'])
 
 
+def test_decryptencrypteddata(node_factory):
+    l1, l2, l3 = node_factory.line_graph(3, fundchannel=False,
+                                         opts={'experimental-offers': None})
+
+    # Private channel from l2->l3, makes l3 add a blinded path to invoice
+    # (l1's existence makes sure l3 doesn't see l2 as a dead end!)
+    node_factory.join_nodes([l1, l2], wait_for_announce=True)
+    node_factory.join_nodes([l2, l3], announce_channels=False)
+    wait_for(lambda: ['alias' in n for n in l3.rpc.listnodes()['nodes']] == [True, True])
+
+    offer = l3.rpc.offer(amount='2msat', description='test_offer_path_self')
+    inv = l2.rpc.fetchinvoice(offer['bolt12'])['invoice']
+
+    decode = l2.rpc.decode(inv)
+    path = decode['invoice_paths'][0]
+    assert path['first_node_id'] == l2.info['id']
+    blinding = path['blinding']
+
+    encdata1 = path['path'][0]['encrypted_recipient_data']
+    # BOLT #4:
+    # 1. `tlv_stream`: `encrypted_data_tlv`
+    # 2. types:
+    # ...
+    #     1. type: 4 (`next_node_id`)
+    #     2. data:
+    #         * [`point`:`node_id`]
+    dec = l2.rpc.decryptencrypteddata(encrypted_data=encdata1, blinding=blinding)['decryptencrypteddata']
+    assert dec['decrypted'].startswith('0421' + l3.info['id'])
+
+
 def test_fetch_no_description_offer(node_factory):
     """Reproducing the issue: https://github.com/ElementsProject/lightning/issues/7405"""
     l1, l2 = node_factory.line_graph(2, opts={'experimental-offers': None,