refactor exceptions, add code linting and more tests

Author: purificant

.gitignore

@@ -3,4 +3,6 @@
 # pycharm
 .idea
 # bytecode
-__pycache__
+__pycache__
+# pytest-cov
+.coverage

paseto/exceptions.py

@@ -0,0 +1,13 @@
+
+
+class PasetoException(Exception):
+    """ Base exception for all paseto related errors"""
+    pass
+
+
+class InvalidFooter(PasetoException):
+    pass
+
+
+class InvalidHeader(PasetoException):
+    pass

paseto/protocol/version2.py

@@ -2,6 +2,7 @@
 import hmac
 import hashlib
 from .util import pae, b64, b64decode
+from paseto.exceptions import InvalidHeader, InvalidFooter
 from nacl.bindings import (
     crypto_aead_xchacha20poly1305_ietf_encrypt,
     crypto_aead_xchacha20poly1305_ietf_decrypt,
@@ -66,13 +67,13 @@ def decrypt(message: bytes, key: bytes, footer: bytes = None):
         #        they do so using a constant-time string compare function.
         if footer is not None:
             if not hmac.compare_digest(b64(footer), message.split(b".")[-1]):
-                raise Exception("Unexpected footer found")
+                raise InvalidFooter("Invalid message footer")
 
         # 2.  Verify that the message begins with "v2.local.", otherwise throw
         #        an exception.  This constant will be referred to as "h".
         header = Version2.HEADER_LOCAL
         if not message.startswith(header):
-            raise Exception("Unexpected header found")
+            raise InvalidHeader("Invalid message header")
 
         # 3.  Decode the payload ("m" sans "h", "f", and the optional trailing
         #        period between "m" and "f") from base64url to raw binary.  Set:

poetry.lock

@@ -0,0 +1,2 @@
+[pylama:pycodestyle]
+max_line_length = 90

pylama.ini

@@ -13,6 +13,7 @@ pynacl = "^1.3"
 pytest = "^3.9"
 pytest-cov = "^2.6"
 black = "^18.3-alpha.0"
+pylama = "^7.6"
 
 [build-system]
 requires = ["poetry>=0.12"]

pyproject.toml

@@ -1,8 +1,11 @@
+import pytest
 from paseto.protocol.version2 import Version2
+from paseto.exceptions import InvalidFooter, InvalidHeader
 
 
 class TestVersion2(object):
-    def test_encrpy_decrypt(self):
+
+    def test_encrypt_decrypt(self):
         key = b"0" * 32
         message = b"foo"
         footer = b"baz"
@@ -11,3 +14,11 @@ def test_encrpy_decrypt(self):
 
         plain_text = Version2.decrypt(token, key, footer)
         assert plain_text == message
+
+    def test_decrypt_invalid_footer(self):
+        with pytest.raises(InvalidFooter):
+            Version2.decrypt(b'header.message.footer', b'a key', b'some_other_footer')
+
+    def test_decrypt_invalid_header(self):
+        with pytest.raises(InvalidHeader):
+            Version2.decrypt(b'some_incorrect_header.message.footer', b'a key')