Add custom challenge argument to register_begin.

fido2/server.py

@@ -94,6 +94,17 @@ def _default_attestations():
     ]
 
 
+def _validata_challenge(challenge):
+    if challenge is None:
+        challenge = os.urandom(32)
+    else:
+        if not isinstance(challenge, bytes):
+            raise TypeError("Custom challenge must be of type 'bytes'.")
+        if len(challenge) < 16:
+            raise ValueError("Custom challenge length must be >= 16.")
+    return challenge
+
+
 class Fido2Server(object):
     """FIDO2 server
 
@@ -127,6 +138,7 @@ class Fido2Server(object):
         resident_key=False,
         user_verification=USER_VERIFICATION.PREFERRED,
         authenticator_attachment=None,
+        challenge=None,
     ):
         """Return a PublicKeyCredentialCreationOptions registration object and
         the internal state dictionary that needs to be passed as is to the
@@ -138,12 +150,14 @@ class Fido2Server(object):
         :param user_verification: The desired USER_VERIFICATION level.
         :param authenticator_attachment: The desired AUTHENTICATOR_ATTACHMENT
             or None to not provide a preference (and get both types).
+        :param challenge: A custom challenge to sign and verify or None to use
+            OS-specific random bytes.
         :return: Registration data, internal state."""
         if not self.allowed_algorithms:
             raise ValueError("Server has no allowed algorithms.")
 
         uv = USER_VERIFICATION(user_verification)
-        challenge = os.urandom(32)
+        challenge = _validata_challenge(challenge)
 
         # Serialize RP
         rp_data = {"id": self.rp.ident, "name": self.rp.name}
@@ -237,9 +251,9 @@ class Fido2Server(object):
     def authenticate_begin(
         self, credentials, user_verification=USER_VERIFICATION.PREFERRED, challenge=None
     ):
-        """Return a PublicKeyCredentialRequestOptions assertion object and
-        the internal state dictionary that needs to be passed as is to the
-        corresponding `authenticate_complete` call.
+        """Return a PublicKeyCredentialRequestOptions assertion object and the internal
+        state dictionary that needs to be passed as is to the corresponding
+        `authenticate_complete` call.
 
         :param credentials: The list of previously registered credentials.
         :param user_verification: The desired USER_VERIFICATION level.
@@ -247,12 +261,7 @@ class Fido2Server(object):
             OS-specific random bytes.
         :return: Assertion data, internal state."""
         uv = USER_VERIFICATION(user_verification)
-
-        if challenge is None:
-            challenge = os.urandom(32)
-        else:
-            if not isinstance(challenge, bytes):
-                raise TypeError("Custom challenge must be bytes.")
+        challenge = _validata_challenge(challenge)
 
         data = {
             "publicKey": {

test/test_server.py

@@ -47,6 +47,23 @@ class TestFido2Server(unittest.TestCase):
         }
         self.assertEqual(request["publicKey"]["rp"], data)
 
+    def test_register_begin_custom_challenge(self):
+        rp = RelyingParty("example.com", "Example")
+        server = Fido2Server(rp)
+
+        challenge = b"1234567890123456"
+        request, state = server.register_begin({}, challenge=challenge)
+
+        self.assertEqual(request["publicKey"]["challenge"], challenge)
+
+    def test_register_begin_custom_challenge_too_short(self):
+        rp = RelyingParty("example.com", "Example")
+        server = Fido2Server(rp)
+
+        challenge = b"123456789012345"
+        with self.assertRaises(ValueError):
+            request, state = server.register_begin({}, challenge=challenge)
+
     def test_authenticate_complete_invalid_signature(self):
         rp = RelyingParty("example.com", "Example")
         server = Fido2Server(rp)