Address dreid's comments

2 files changed, 28 insertions, 18 deletions

diff --git a/cryptography/fernet.py b/cryptography/fernet.py
index 8bcaa40a..c0c5631f 100644
--- a/cryptography/fernet.py
+++ b/cryptography/fernet.py
@@ -33,12 +33,16 @@ _MAX_CLOCK_SKEW = 60
 
 
 class Fernet(object):
-    def __init__(self, key):
+    def __init__(self, key, backend=None):
+        if backend is None:
+            backend = default_backend()
+
         key = base64.urlsafe_b64decode(key)
         assert len(key) == 32
-        self.signing_key = key[:16]
-        self.encryption_key = key[16:]
-        self.backend = default_backend()
+
+        self._signing_key = key[:16]
+        self._encryption_key = key[16:]
+        self._backend = backend
 
     @classmethod
     def generate_key(cls):
@@ -58,7 +62,7 @@ class Fernet(object):
         padder = padding.PKCS7(algorithms.AES.block_size).padder()
         padded_data = padder.update(data) + padder.finalize()
         encryptor = Cipher(
-            algorithms.AES(self.encryption_key), modes.CBC(iv), self.backend
+            algorithms.AES(self._encryption_key), modes.CBC(iv), self._backend
         ).encryptor()
         ciphertext = encryptor.update(padded_data) + encryptor.finalize()
 
@@ -66,7 +70,7 @@ class Fernet(object):
             b"\x80" + struct.pack(">Q", current_time) + iv + ciphertext
         )
 
-        h = HMAC(self.signing_key, hashes.SHA256(), self.backend)
+        h = HMAC(self._signing_key, hashes.SHA256(), backend=self._backend)
         h.update(basic_parts)
         hmac = h.finalize()
         return base64.urlsafe_b64encode(basic_parts + hmac)
@@ -93,14 +97,14 @@ class Fernet(object):
                 raise InvalidToken
         if current_time + _MAX_CLOCK_SKEW < timestamp:
             raise InvalidToken
-        h = HMAC(self.signing_key, hashes.SHA256(), self.backend)
+        h = HMAC(self._signing_key, hashes.SHA256(), backend=self._backend)
         h.update(data[:-32])
         hmac = h.finalize()
         if not constant_time.bytes_eq(hmac, data[-32:]):
             raise InvalidToken
 
         decryptor = Cipher(
-            algorithms.AES(self.encryption_key), modes.CBC(iv), self.backend
+            algorithms.AES(self._encryption_key), modes.CBC(iv), self._backend
         ).decryptor()
         plaintext_padded = decryptor.update(ciphertext)
         try:
diff --git a/tests/test_fernet.py b/tests/test_fernet.py
index af64175e..48df867c 100644
--- a/tests/test_fernet.py
+++ b/tests/test_fernet.py
@@ -24,6 +24,7 @@ import pytest
 import six
 
 from cryptography.fernet import Fernet, InvalidToken
+from cryptography.hazmat.backends import default_backend
 
 
 def json_parametrize(keys, fname):
@@ -40,8 +41,8 @@ class TestFernet(object):
     @json_parametrize(
         ("secret", "now", "iv", "src", "token"), "generate.json",
     )
-    def test_generate(self, secret, now, iv, src, token):
-        f = Fernet(secret.encode("ascii"))
+    def test_generate(self, secret, now, iv, src, token, backend):
+        f = Fernet(secret.encode("ascii"), backend=backend)
         actual_token = f._encrypt_from_parts(
             src.encode("ascii"),
             calendar.timegm(iso8601.parse_date(now).utctimetuple()),
@@ -52,29 +53,34 @@ class TestFernet(object):
     @json_parametrize(
         ("secret", "now", "src", "ttl_sec", "token"), "verify.json",
     )
-    def test_verify(self, secret, now, src, ttl_sec, token, monkeypatch):
-        f = Fernet(secret.encode("ascii"))
+    def test_verify(self, secret, now, src, ttl_sec, token, backend,
+                    monkeypatch):
+        f = Fernet(secret.encode("ascii"), backend=backend)
         current_time = calendar.timegm(iso8601.parse_date(now).utctimetuple())
         monkeypatch.setattr(time, "time", lambda: current_time)
         payload = f.decrypt(token.encode("ascii"), ttl=ttl_sec)
         assert payload == src.encode("ascii")
 
     @json_parametrize(("secret", "token", "now", "ttl_sec"), "invalid.json")
-    def test_invalid(self, secret, token, now, ttl_sec, monkeypatch):
-        f = Fernet(secret.encode("ascii"))
+    def test_invalid(self, secret, token, now, ttl_sec, backend, monkeypatch):
+        f = Fernet(secret.encode("ascii"), backend=backend)
         current_time = calendar.timegm(iso8601.parse_date(now).utctimetuple())
         monkeypatch.setattr(time, "time", lambda: current_time)
         with pytest.raises(InvalidToken):
             f.decrypt(token.encode("ascii"), ttl=ttl_sec)
 
-    def test_unicode(self):
-        f = Fernet(base64.urlsafe_b64encode(b"\x00" * 32))
+    def test_unicode(self, backend):
+        f = Fernet(base64.urlsafe_b64encode(b"\x00" * 32), backend=backend)
         with pytest.raises(TypeError):
             f.encrypt(six.u(""))
         with pytest.raises(TypeError):
             f.decrypt(six.u(""))
 
     @pytest.mark.parametrize("message", [b"", b"Abc!", b"\x00\xFF\x00\x80"])
-    def test_roundtrips(self, message):
-        f = Fernet(Fernet.generate_key())
+    def test_roundtrips(self, message, backend):
+        f = Fernet(Fernet.generate_key(), backend=backend)
         assert f.decrypt(f.encrypt(message)) == message
+
+    def test_default_backend(self):
+        f = Fernet(Fernet.generate_key())
+        assert f._backend is default_backend()