From d605f06dd9766f481333181df86ef2101175c44d Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?David=20N=C3=BA=C3=B1ez?= <david@nucypher.com>
Date: Thu, 6 Dec 2018 17:16:30 +0100
Subject: [PATCH] Include Alice's address in the CFrag metadata and validate it

---
 nucypher/network/server.py | 11 +++++++----
 nucypher/policy/models.py  | 22 ++++++++++++++++++----
 2 files changed, 25 insertions(+), 8 deletions(-)

diff --git a/nucypher/network/server.py b/nucypher/network/server.py
index 01ebb905d..f39059366 100644
--- a/nucypher/network/server.py
+++ b/nucypher/network/server.py
@@ -315,15 +315,18 @@ def make_rest_app(
         alices_verifying_key = UmbralPublicKey.from_bytes(verifying_key_bytes)
         cfrag_byte_stream = b""
 
+        # This is Alice's verifying key as ETH address, signed by Bob
+        alice_address_signature = bytes(work_order.alice_address_signature)
+
         for capsule, capsule_signature in zip(work_order.capsules, work_order.capsule_signatures):
             # This is the capsule signed by Bob
             capsule_signature = bytes(capsule_signature)
             # Ursula signs on top of it. Now both are committed to the same capsule.
-            capsule_signed_by_both = bytes(stamp(capsule_signature))
-
+            # She signs Alice's address too.
+            ursula_signature = stamp(capsule_signature + alice_address_signature)
             capsule.set_correctness_keys(verifying=alices_verifying_key)
-            cfrag = pre.reencrypt(kfrag, capsule, metadata=capsule_signed_by_both)
-            log.info("Re-encrypting for {}, made {}.".format(capsule, cfrag))
+            cfrag = pre.reencrypt(kfrag, capsule, metadata=bytes(ursula_signature))
+            log.info(f"Re-encrypting for {capsule}, made {cfrag}.")
             signature = stamp(bytes(cfrag) + bytes(capsule))
             cfrag_byte_stream += VariableLengthBytestring(cfrag) + signature
 
diff --git a/nucypher/policy/models.py b/nucypher/policy/models.py
index 7c8bde6ce..c802c1e80 100644
--- a/nucypher/policy/models.py
+++ b/nucypher/policy/models.py
@@ -622,7 +622,7 @@ class WorkOrder:
             (self.receipt_bytes,
              msgpack.dumps(capsules_as_bytes),
              msgpack.dumps(capsule_signatures_as_bytes),
-             self.alice_address,
+             self.alice_address,  # TODO: if Ursula computes ETH address, we can remove this
              self.alice_address_signature,
              )
         )
@@ -631,13 +631,27 @@ class WorkOrder:
     def complete(self, cfrags_and_signatures):
         good_cfrags = []
         if not len(self) == len(cfrags_and_signatures):
-            raise ValueError("Ursula gave back the wrong number of cfrags.  She's up to something.")
+            raise ValueError("Ursula gave back the wrong number of cfrags.  "
+                             "She's up to something.")
+
+        alice_address_signature = bytes(self.alice_address_signature)
+        ursula_verifying_key = self.ursula.stamp.as_umbral_pubkey()
+
         for counter, capsule in enumerate(self.capsules):
             cfrag, signature = cfrags_and_signatures[counter]
-            if signature.verify(bytes(cfrag) + bytes(capsule), self.ursula.stamp.as_umbral_pubkey()):
+
+            # Validate CFrag metadata
+            capsule_signature = bytes(self.capsule_signatures[counter])
+            metadata_input = capsule_signature + alice_address_signature
+            metadata_as_signature = Signature.from_bytes(cfrag.proof.metadata)
+            if not metadata_as_signature.verify(metadata_input, ursula_verifying_key):
+                raise InvalidSignature("Invalid metadata for {}.".format(cfrag))
+
+            # Validate work order response signatures
+            if signature.verify(bytes(cfrag) + bytes(capsule), ursula_verifying_key):
                 good_cfrags.append(cfrag)
             else:
-                raise self.ursula.InvalidSignature("This CFrag is not properly signed by Ursula.")
+                raise InvalidSignature("{} is not properly signed by Ursula.".format(cfrag))
         else:
             self.completed = maya.now()
             return good_cfrags