An empty or intentionally misleading TreasureMap warrants a more severe Exception than NotEnoughUrsulas: introducing TreasureMap.IsDisorienting

nucypher/policy/collections.py

@@ -23,7 +23,7 @@ from typing import List, Optional, Tuple
 
 import maya
 import msgpack
-from bytestring_splitter import BytestringSplitter, VariableLengthBytestring
+from bytestring_splitter import BytestringSplitter, VariableLengthBytestring, BytestringSplittingError
 from constant_sorrow.constants import CFRAG_NOT_RETAINED
 from constant_sorrow.constants import NO_DECRYPTION_PERFORMED
 from cryptography.hazmat.backends.openssl import backend
@@ -61,6 +61,11 @@ class TreasureMap:
         Called when no known nodes have it.
         """
 
+    class IsDisorienting(NotFound):
+        """
+        Called when an oriented TreasureMap lists fewer than m destinations.
+        """
+
     node_id_splitter = BytestringSplitter((to_checksum_address, int(PUBLIC_ADDRESS_LENGTH)), ID_LENGTH)
 
     from nucypher.crypto.signing import InvalidSignature  # Raised when the public signature (typically intended for Ursula) is not valid.
@@ -201,13 +206,15 @@ class TreasureMap:
             raise self.InvalidSignature(
                 "This TreasureMap does not contain the correct signature from Alice to Bob.")
         else:
-            m = map_in_the_clear[0]
             self._m = map_in_the_clear[0]
-            if self._m > 0:
+            try:
                 self._destinations = dict(self.node_id_splitter.repeat(map_in_the_clear[1:]))
-            else:
+            except BytestringSplittingError:
                 self._destinations = {}
 
+            if len(self._destinations) < self._m or self._m == 0:
+                raise self.IsDisorienting(f"TreasureMap lists only {len(self._destinations)} destination, but requires interaction with {self._m} nodes.")
+
     def __eq__(self, other):
         return bytes(self) == bytes(other)
 

tests/characters/control/blockchain/test_rpc_control_blockchain.py

@@ -146,5 +146,5 @@ def test_bob_rpc_character_control_retrieve_with_tmap(
             b'Wrong!')
     tmap_64 = b64encode(bytes(wrong_tmap)).decode()
     params['treasure_map'] = tmap_64
-    with pytest.raises(Ursula.NotEnoughUrsulas):
+    with pytest.raises(TreasureMap.IsDisorienting):
         bob_rpc_controller.send(request_data)