Merge pull request #203 from alex/fernet

[WIP][POC] Implement Fernet

1 files changed, 131 insertions, 0 deletions

diff --git a/cryptography/fernet.py b/cryptography/fernet.py
new file mode 100644
index 00000000..c19309d5
--- /dev/null
+++ b/cryptography/fernet.py
@@ -0,0 +1,131 @@
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#    http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or
+# implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+import base64
+import binascii
+import os
+import struct
+import time
+
+import six
+
+from cryptography.exceptions import InvalidSignature
+from cryptography.hazmat.backends import default_backend
+from cryptography.hazmat.primitives import padding, hashes
+from cryptography.hazmat.primitives.hmac import HMAC
+from cryptography.hazmat.primitives.ciphers import Cipher, algorithms, modes
+
+
+class InvalidToken(Exception):
+    pass
+
+
+_MAX_CLOCK_SKEW = 60
+
+
+class Fernet(object):
+    def __init__(self, key, backend=None):
+        if backend is None:
+            backend = default_backend()
+
+        key = base64.urlsafe_b64decode(key)
+        if len(key) != 32:
+            raise ValueError(
+                "Fernet key must be 32 url-safe base64-encoded bytes"
+            )
+
+        self._signing_key = key[:16]
+        self._encryption_key = key[16:]
+        self._backend = backend
+
+    @classmethod
+    def generate_key(cls):
+        return base64.urlsafe_b64encode(os.urandom(32))
+
+    def encrypt(self, data):
+        current_time = int(time.time())
+        iv = os.urandom(16)
+        return self._encrypt_from_parts(data, current_time, iv)
+
+    def _encrypt_from_parts(self, data, current_time, iv):
+        if isinstance(data, six.text_type):
+            raise TypeError(
+                "Unicode-objects must be encoded before encryption"
+            )
+
+        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
+        ).encryptor()
+        ciphertext = encryptor.update(padded_data) + encryptor.finalize()
+
+        basic_parts = (
+            b"\x80" + struct.pack(">Q", current_time) + iv + ciphertext
+        )
+
+        h = HMAC(self._signing_key, hashes.SHA256(), backend=self._backend)
+        h.update(basic_parts)
+        hmac = h.finalize()
+        return base64.urlsafe_b64encode(basic_parts + hmac)
+
+    def decrypt(self, token, ttl=None):
+        if isinstance(token, six.text_type):
+            raise TypeError(
+                "Unicode-objects must be encoded before decryption"
+            )
+
+        current_time = int(time.time())
+
+        try:
+            data = base64.urlsafe_b64decode(token)
+        except (TypeError, binascii.Error):
+            raise InvalidToken
+
+        if six.indexbytes(data, 0) != 0x80:
+            raise InvalidToken
+
+        try:
+            timestamp, = struct.unpack(">Q", data[1:9])
+        except struct.error:
+            raise InvalidToken
+        if ttl is not None:
+            if timestamp + ttl < current_time:
+                raise InvalidToken
+        if current_time + _MAX_CLOCK_SKEW < timestamp:
+            raise InvalidToken
+        h = HMAC(self._signing_key, hashes.SHA256(), backend=self._backend)
+        h.update(data[:-32])
+        try:
+            h.verify(data[-32:])
+        except InvalidSignature:
+            raise InvalidToken
+
+        iv = data[9:25]
+        ciphertext = data[25:-32]
+        decryptor = Cipher(
+            algorithms.AES(self._encryption_key), modes.CBC(iv), self._backend
+        ).decryptor()
+        plaintext_padded = decryptor.update(ciphertext)
+        try:
+            plaintext_padded += decryptor.finalize()
+        except ValueError:
+            raise InvalidToken
+        unpadder = padding.PKCS7(algorithms.AES.block_size).unpadder()
+
+        unpadded = unpadder.update(plaintext_padded)
+        try:
+            unpadded += unpadder.finalize()
+        except ValueError:
+            raise InvalidToken
+        return unpadded