pyln: Add code to unwrap an encrypted onion at the intended node

Changelog-Added: pyln-proto: Added pure python implementation of the sphinx onion creation and processing functionality.

contrib/pyln-proto/pyln/proto/onion.py

@@ -15,8 +15,9 @@ from cryptography.hazmat.primitives import hashes, hmac
 from cryptography.hazmat.primitives.ciphers import Cipher, algorithms
 from hashlib import sha256
 from io import BytesIO, SEEK_CUR
-from typing import List, Optional, Union
+from typing import List, Optional, Union, Tuple
 import coincurve
+import io
 import os
 import struct
 
@@ -44,7 +45,7 @@ class OnionPayload(object):
             s = s.encode('ASCII')
         return cls.from_bytes(bytes(unhexlify(s)))
 
-    def to_bytes(self):
+    def to_bytes(self, include_prefix):
         raise ValueError("OnionPayload is an abstract class, use "
                          "LegacyOnionPayload or TlvPayload instead")
 
@@ -92,20 +93,20 @@ class LegacyOnionPayload(OnionPayload):
         padding = b.read(12)
         return LegacyOnionPayload(a, o, s, padding)
 
-    def to_bytes(self, include_realm=True):
+    def to_bytes(self, include_prefix=True):
         b = b''
-        if include_realm:
+        if include_prefix:
             b += b'\x00'
 
         b += struct.pack("!Q", self.short_channel_id)
         b += struct.pack("!Q", self.amt_to_forward)
         b += struct.pack("!L", self.outgoing_cltv_value)
         b += self.padding
-        assert(len(b) == 32 + include_realm)
+        assert(len(b) == 32 + include_prefix)
         return b
 
-    def to_hex(self, include_realm=True):
-        return hexlify(self.to_bytes(include_realm)).decode('ASCII')
+    def to_hex(self, include_prefix=True):
+        return hexlify(self.to_bytes(include_prefix)).decode('ASCII')
 
     def __str__(self):
         return ("LegacyOnionPayload[scid={self.short_channel_id}, "
@@ -143,6 +144,12 @@ class TlvPayload(OnionPayload):
                 raise ValueError(
                     "Unable to read length at position {}".format(b.tell())
                 )
+
+            elif length > start + payload_length - b.tell():
+                b.seek(start + payload_length)
+                raise ValueError("Failed to parse TLV payload: value length "
+                                 "is longer than available bytes.")
+
             val = b.read(length)
 
             # Get the subclass that is the correct interpretation of this
@@ -167,9 +174,10 @@ class TlvPayload(OnionPayload):
                 return f
         return default
 
-    def to_bytes(self):
+    def to_bytes(self, include_prefix=True) -> bytes:
         ser = [f.to_bytes() for f in self.fields]
         b = BytesIO()
+        if include_prefix:
             varint_encode(sum([len(b) for b in ser]), b)
         for f in ser:
             b.write(f)
@@ -179,6 +187,40 @@ class TlvPayload(OnionPayload):
         return "TlvPayload[" + ', '.join([str(f) for f in self.fields]) + "]"
 
 
+class RawPayload(OnionPayload):
+    """A payload that doesn't deserialize correctly as TLV stream.
+
+    Mainly used if TLV parsing fails, but we still want access to the raw
+    payload.
+
+    """
+
+    def __init__(self):
+        self.content: Optional[bytes] = None
+
+    @classmethod
+    def from_bytes(cls, b):
+        if isinstance(b, str):
+            b = b.encode('ASCII')
+        if isinstance(b, bytes):
+            b = BytesIO(b)
+
+        self = cls()
+        payload_length = varint_decode(b)
+        self.content = b.read(payload_length)
+        return self
+
+    def to_bytes(self, include_prefix=True) -> bytes:
+        b = BytesIO()
+        if self.content is None:
+            raise ValueError("Cannot serialize empty TLV payload")
+
+        if include_prefix:
+            varint_encode(len(self.content), b)
+        b.write(self.content)
+        return b.getvalue()
+
+
 class TlvField(object):
 
     def __init__(self, typenum, value=None, description=None):
@@ -319,6 +361,57 @@ class RoutingOnion(object):
     def to_hex(self):
         return hexlify(self.to_bin())
 
+    def unwrap(self, privkey: PrivateKey, assocdata: Optional[bytes]) \
+            -> Tuple[OnionPayload, Optional['RoutingOnion']]:
+        shared_secret = ecdh(privkey, self.ephemeralkey)
+        keys = generate_keyset(shared_secret)
+
+        h = hmac.HMAC(keys.mu, hashes.SHA256(),
+                      backend=default_backend())
+        h.update(self.payloads)
+        if assocdata is not None:
+            h.update(assocdata)
+        hh = h.finalize()
+
+        if hh != self.hmac:
+            raise ValueError("HMAC does not match, onion might have been "
+                             "tampered with: {hh} != {hmac}".format(
+                                 hh=hexlify(hh).decode('ascii'),
+                                 hmac=hexlify(self.hmac).decode('ascii'),
+                             ))
+
+        # Create the scratch twice as large as the original packet, since we
+        # need to left-shift a single payload off, which may itself be up to
+        # ROUTING_INFO_SIZE in length.
+        payloads = bytearray(2 * ROUTING_INFO_SIZE)
+        payloads[:ROUTING_INFO_SIZE] = self.payloads
+        chacha20_stream(keys.rho, payloads)
+
+        r = io.BytesIO(payloads)
+        start = r.tell()
+
+        try:
+            payload = OnionPayload.from_bytes(r)
+        except ValueError:
+            r.seek(start)
+            payload = RawPayload.from_bytes(r)
+
+        next_hmac = r.read(32)
+        shift_size = r.tell()
+
+        if next_hmac == bytes(32):
+            return payload, None
+        else:
+            b = blind(self.ephemeralkey, shared_secret)
+            ek = blind_group_element(self.ephemeralkey, b)
+            payloads = payloads[shift_size:shift_size + ROUTING_INFO_SIZE]
+            return payload, RoutingOnion(
+                version=self.version,
+                ephemeralkey=ek,
+                payloads=payloads,
+                hmac=next_hmac,
+            )
+
 
 KeySet = namedtuple('KeySet', ['rho', 'mu', 'um', 'pad', 'gamma', 'pi'])
 

contrib/pyln-proto/tests/test_onion.py

@@ -12,7 +12,7 @@ def test_legacy_payload():
         b'00000067000001000100000000000003e800000075000000000000000000000000'
     )
     payload = onion.OnionPayload.from_bytes(legacy)
-    assert(payload.to_bytes(include_realm=True) == legacy)
+    assert(payload.to_bytes(include_prefix=True) == legacy)
 
 
 def test_tlv_payload():
@@ -325,3 +325,21 @@ def test_sphinx_path_compile():
     o = sp.compile()
 
     assert(o.to_bin() == unhexlify(v['onion']))
+
+
+def test_unwrap():
+    f = 'tests/vectors/onion-test-multi-frame.json'
+    sp, v = sphinx_path_from_test_vector(f)
+    o = onion.RoutingOnion.from_hex(v['onion'])
+    assocdata = unhexlify(v['generate']['associated_data'])
+    privkeys = [onion.PrivateKey(unhexlify(h)) for h in v['decode']]
+
+    for pk, h in zip(privkeys, v['generate']['hops']):
+        pl, o = o.unwrap(pk, assocdata=assocdata)
+
+        b = hexlify(pl.to_bytes(include_prefix=False))
+        if h['type'] == 'legacy':
+            assert(b == h['payload'].encode('ascii') + b'00' * 12)
+        else:
+            assert(b == h['payload'].encode('ascii'))
+    assert(o is None)