Added SHA-512 fallback by default

Author: mitsuhiko

CHANGES.rst

@@ -4,6 +4,7 @@ Version 1.1.0
 Released 2018-10-26
 
 -   Change default signing algorithm back to SHA-1. (`#113`_)
+-   Added a default SHA-512 fallback.
 -   Add support for fallback algorithms during deserialization to
     support changing the default in the future without breaking existing
     signatures. (`#113`_)

src/itsdangerous/serializer.py

@@ -1,3 +1,5 @@
+import hashlib
+
 from ._compat import text_type
 from ._json import json
 from .encoding import want_bytes
@@ -57,7 +59,9 @@ class to the constructor as well as keyword arguments as a dict that
         the constructor.
 
     .. versionchanged:: 1.1:
-        Added support for ``fallback_signers``.
+        Added support for ``fallback_signers`` and configured a default
+        SHA-512 fallback.  This fallback is for users who used the 1.0
+        release which was yanked which had SHA-512 support in it.
     """
 
     #: If a serializer module or class is not passed to the constructor
@@ -70,6 +74,9 @@ class to the constructor as well as keyword arguments as a dict that
     #: .. versionadded:: 0.14
     default_signer = Signer
 
+    #: The default fallback signers.
+    default_fallback_signers = [{"digest_method": hashlib.sha512}]
+
     def __init__(
         self,
         secret_key,
@@ -90,7 +97,9 @@ def __init__(
             signer = self.default_signer
         self.signer = signer
         self.signer_kwargs = signer_kwargs or {}
-        self.fallback_signers = fallback_signers or ()
+        if fallback_signers is None:
+            fallback_signers = list(self.default_fallback_signers or ())
+        self.fallback_signers = fallback_signers
         self.serializer_kwargs = serializer_kwargs or {}
 
     def load_payload(self, payload, serializer=None):

src/itsdangerous/timed.py

@@ -132,6 +132,11 @@ def loads(self, s, max_age=None, return_timestamp=False, salt=None):
                 if return_timestamp:
                     return payload, timestamp
                 return payload
+            # If we get a signature expired it means we could read the
+            # signature but it's invalid.  In that case we do not want to
+            # try the next signer.
+            except SignatureExpired:
+                raise
             except BadSignature as err:
                 last_exception = err
         raise last_exception

tests/test_itsdangerous/test_serializer.py

@@ -134,12 +134,12 @@ def test_serializer_kwargs(self, serializer_factory):
         assert serializer.loads(serializer.dumps({(): 1})) == {}
 
     def test_fallback_signers(self, serializer_factory, value):
-        serializer = serializer_factory(signer_kwargs={"digest_method": hashlib.sha512})
+        serializer = serializer_factory(signer_kwargs={"digest_method": hashlib.sha256})
         signed = serializer.dumps(value)
 
         fallback_serializer = serializer_factory(
             signer_kwargs={"digest_method": hashlib.sha1},
-            fallback_signers=[{"digest_method": hashlib.sha512}],
+            fallback_signers=[{"digest_method": hashlib.sha256}],
         )
 
         assert fallback_serializer.loads(signed) == value

tox.ini

@@ -12,7 +12,7 @@ setenv =
 deps =
     pytest-cov
     freezegun
-commands = pytest --cov --cov-report= {posargs}
+commands = pytest --tb=short --cov --cov-report= {posargs}
 
 [testenv:stylecheck]
 deps = pre-commit