noise: Implement signature verification of chat messages

Since we don't pass the public key we just rely on the pubkey recovery and the
`checkmessage` interface to tell us whether it is a publicly known `node_id`
or not.

noise/README.org

@@ -0,0 +1,21 @@
+* Protocol
+The protocol was heavily inspired by the [[https://github.com/joostjager/whatsat#protocol][WhatSat protocol]]:
+
+| record type | length (bytes) | value                                                           |
+|-------------+----------------+-----------------------------------------------------------------|
+|  5482373484 |             32 | key send preimage                                               |
+|    34349334 |       variable | chat message                                                    |
+|    34349335 |             65 | compressed signature + recovery id                              |
+|    34349339 |             33 | sender pubkey                                                   |
+|    34349343 |              8 | timestamp in nano seconds since unix epoch (big endian encoded) |
+
+The key differences are that we don't explicitly pass the sender pubkey, since
+we can recover that from the signature itself, and we use the compressed 64
+byte signature, instead of the DER encoded signature. This saves us 33 bytes
+for the pubkey and ~7 bytes for the signature, but requires that we change the
+TLV type for the signature (from ~34349337~ to ~34349335~).
+
+The signature is computed by serializing all other TLV fields, hex-encoding
+the resulting TLV payload, and signing it using ~lightning-cli signmessage~
+returning the ~zbase32~ encoded signature. The signature consists of a 1 byte
+recovery ID and the 64 byte raw signature.

noise/noise.py

@@ -11,9 +11,13 @@ import logging
 from collections import namedtuple
 import shelve
 from pyln.proto.onion import OnionPayload
+import zbase32
 
 plugin = Plugin()
 
+TLV_KEYSEND_PREIMAGE = 5482373484
+TLV_NOISE_MESSAGE = 34349334
+TLV_NOISE_SIGNATURE = 34349335
 
 class Message(object):
     def __init__(self, sender, body, signature, payment=None, id=None):
@@ -22,6 +26,7 @@ class Message(object):
         self.body = body
         self.signature = signature
         self.payment = payment
+        self.verified = None
 
     def to_dict(self):
         return {
@@ -30,6 +35,7 @@ class Message(object):
             "body": self.body,
             "signature": hexlify(self.signature).decode('ASCII'),
             "payment": self.payment,
+            "verified": self.verified,
         }
 
 
@@ -104,12 +110,16 @@ def deliver(node_id, payload, amt, max_attempts=5, payment_hash=None):
 @plugin.async_method('sendmsg')
 def sendmsg(node_id, msg, plugin, request, amt=1000, **kwargs):
     payload = TlvPayload()
-    payload.add_field(34349334, msg.encode('UTF-8'))
+    payload.add_field(TLV_NOISE_MESSAGE, msg.encode('UTF-8'))
+
+    sigmsg = hexlify(payload.to_bytes()).decode('ASCII')
 
     # Sign the message:
-    sig = plugin.rpc.signmessage(msg)['signature']
-    sig = unhexlify(sig)
-    payload.add_field(34349336, sig)
+    sig = plugin.rpc.signmessage(sigmsg)
+
+    sigcheck = plugin.rpc.checkmessage(sigmsg, sig['zbase'])
+    sig = zbase32.decode(sig['zbase'])
+    payload.add_field(TLV_NOISE_SIGNATURE, sig)
 
     res = deliver(node_id, payload.to_bytes(), amt=amt)
     request.set_result(res)
@@ -128,15 +138,23 @@ def recvmsg(plugin, request, last_id=None, **kwargs):
 def on_htlc_accepted(onion, htlc, plugin, **kwargs):
     payload = OnionPayload.from_hex(onion['payload'])
 
-    # TODO verify the signature to extract the sender
-
     msg = Message(
         id=len(plugin.messages),
-        sender="AAA",
+        sender=None,
         body=payload.get(34349334).value,
-        signature=payload.get(34349336).value,
+        signature=payload.get(34349335).value,
         payment=None)
 
+    # Filter out the signature so we can check it against the rest of the payload
+    sigpayload = TlvPayload()
+    sigpayload.fields = filter(lambda x: x.typenum != TLV_NOISE_SIGNATURE, payload.fields)
+    sigmsg = hexlify(sigpayload.to_bytes()).decode('ASCII')
+
+    zsig = zbase32.encode(msg.signature).decode('ASCII')
+    sigcheck = plugin.rpc.checkmessage(sigmsg, zsig)
+    msg.sender = sigcheck['pubkey']
+    msg.verified = sigcheck['verified']
+
     plugin.messages.append(msg)
     for r in plugin.receive_waiters:
         r.set_result(msg.to_dict())

noise/test_chat.py

@@ -1,6 +1,8 @@
 from pyln.testing.fixtures import *
 from pyln.testing.utils import wait_for
 from pprint import pprint
+import zbase32
+
 
 plugin = os.path.join(os.path.dirname(__file__), 'noise.py')
 
@@ -20,6 +22,9 @@ def test_sendmsg_success(node_factory, executor):
     # They should be the same :-)
     assert(m1 == m2)
 
+    assert(m2['sender'] == l1.info['id'])
+    assert(m2['verified'] == True)
+
 
 def test_sendmsg_retry(node_factory, executor):
     opts = [{'plugin': plugin}, {}, {'fee-base': 10000}, {'plugin': plugin}]
@@ -52,3 +57,10 @@ def test_sendmsg_retry(node_factory, executor):
     print(recv.result(10))
 
     msg = l4.rpc.recvmsg(last_id=-1)
+
+
+def test_zbase32():
+    zb32 = b'd75qtmgijm79rpooshmgzjwji9gj7dsdat8remuskyjp9oq1ugkaoj6orbxzhuo4njtyh96e3aq84p1tiuz77nchgxa1s4ka4carnbiy'
+    b = zbase32.decode(zb32)
+    enc = zbase32.encode(b)
+    assert(enc == zb32)

noise/zbase32.py

@@ -0,0 +1,51 @@
+import bitstring
+
+zbase32_chars = b'ybndrfg8ejkmcpqxot1uwisza345h769'
+zbase32_revchars = [
+    255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255,
+    255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255,
+    255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255,
+    255, 255, 255, 255, 18, 255, 25, 26, 27, 30, 29, 7, 31, 255, 255, 255, 255,
+    255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255,
+    255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255,
+    255, 255, 255, 255, 255, 24, 1, 12, 3, 8, 5, 6, 28, 21, 9, 10, 255, 11, 2,
+    16, 13, 14, 4, 22, 17, 19, 255, 20, 15, 0, 23, 255, 255, 255, 255, 255, 255,
+    255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255,
+    255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255,
+    255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255,
+    255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255,
+    255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255,
+    255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255,
+    255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255,
+    255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255,
+    255, 255, 255, 255, 255, 255, 255
+]
+
+def bitarray_to_u5(barr):
+    assert len(barr) % 5 == 0
+    ret = []
+    s = bitstring.ConstBitStream(barr)
+    while s.pos != s.len:
+        ret.append(s.read(5).uint)
+    return ret
+
+def u5_to_bitarray(arr):
+    ret = bitstring.BitArray()
+    for a in arr:
+        ret += bitstring.pack("uint:5", a)
+    return ret
+
+def encode(b):
+    uint5s = bitarray_to_u5(b)
+    res = [zbase32_chars[c] for c in uint5s]
+    return bytes(res)
+
+def decode(b):
+    if isinstance(b, str):
+        b = b.encode('ASCII')
+
+    uint5s = []
+    for c in b:
+        uint5s.append(zbase32_revchars[c])
+    dec = u5_to_bitarray(uint5s)
+    return dec.bytes