Incorrect circuit digest for higher protocol versions

I only got stem.client working for link protocol 3 when cobbling it together. We attempt to conform with the spec for higher protocol versions but it's mostly untested. While looking into #26060 I realized that we're using the wrong length for cell headers when the link protocol is higher than three. Link v4 and greater use a long for circuit identifiers rather than a short... circ_id (4 bytes) + command (1 byte) = 5 bytes

Incorrect circuit digest for higher protocol versions
e3fab7bf · Damian Johnson · ee3bca1e · e3fab7bf
Commit e3fab7bf authored May 24, 2018 by Damian Johnson
--- a/stem/client/__init__.py
+++ b/stem/client/__init__.py
@@ -235,20 +235,26 @@ class Circuit(object):
      orig_digest = self.forward_digest.copy()
      orig_key = copy.copy(self.forward_key)

+      # Digests and such are computed using the RELAY cell payload. This
+      # doesn't include the initial circuit id and cell type fields.
+      # Circuit ids vary in length depending on the protocol version.
+
+      header_size = 5 if self.relay.link_protocol > 3 else 3
+
      try:
        cell = stem.client.cell.RelayCell(self.id, command, data, 0, stream_id)
-        payload_without_digest = cell.pack(self.relay.link_protocol)[3:]
+        payload_without_digest = cell.pack(self.relay.link_protocol)[header_size:]
        self.forward_digest.update(payload_without_digest)

        cell = stem.client.cell.RelayCell(self.id, command, data, self.forward_digest, stream_id)
-        header, payload = split(cell.pack(self.relay.link_protocol), 3)
+        header, payload = split(cell.pack(self.relay.link_protocol), header_size)
        encrypted_payload = header + self.forward_key.update(payload)

        reply = []
        self.relay._orport.send(encrypted_payload)

        for cell in stem.client.cell.Cell.unpack(self.relay._orport.recv(), self.relay.link_protocol):
-          decrypted = self.backward_key.update(cell.pack(self.relay.link_protocol)[3:])
+          decrypted = self.backward_key.update(cell.pack(self.relay.link_protocol)[header_size:])
          reply.append(stem.client.cell.RelayCell._unpack(decrypted, self.id, self.relay.link_protocol))

        return reply