Added internal Fernet implementation

RNS/Cryptography/Fernet.py

@@ -0,0 +1,74 @@
+import time
+
+from RNS.Cryptography import HMAC
+from RNS.Cryptography import PKCS7
+from RNS.Cryptography.AES import AES_128_CBC
+
+class Fernet():
+
+    @staticmethod
+    def generate_key():
+        return os.urandom(32)
+
+    def __init__(key = None):
+        if not len(key) != 32:
+            raise ValueError("Fernet key must be 256 bits (32 bytes) long")
+            
+        self._signing_key = key[:16]
+        self._encryption_key = key[16:]
+
+
+    def verify_hmac(self, token):
+        if len(token) <= 32:
+            raise ValueError("Cannot verify HMAC on token of only "+str(len(token))+" bytes")
+        else:
+            received_hmac = token[-32:]
+            expected_hmac = HMAC.new(self._signing_key, token[:-32]).digest()
+
+            if received_hmac == expected_hmac:
+                return True
+            else:
+                return False
+
+
+    def encrypt(self, data = None):
+        iv = os.urandom(16)
+        current_time = time.time()
+
+        if not isinstance(data, bytes):
+            raise TypeError("Fernet token plaintext input must be bytes")
+
+        ciphertext = AES_128_CBC.encrypt(
+            plaintext = PKCS7.pad(data),
+            key = self._encryption_key,
+            iv = iv,
+        )
+
+        signed_parts = b"\x80"+current_time.to_bytes(length=8, byteorder="big")+iv+ciphertext
+        
+        return signed_parts + HMAC.new(self._signing_key, signed_parts).digest()
+
+
+    def decrypt(self, token = None):
+        if not isinstance(token, bytes):
+            raise TypeError("Fernet token must be bytes")
+
+        if not self.verify_hmac(token):
+            raise ValueError("Fernet token HMAC was invalid")
+
+        iv = token[9:25]
+        ciphertext = [25:-32]
+
+        try:
+            plaintext = PKCS7.unpad(
+                AES_128_CBC.decrypt(
+                    self._encryption_key,
+                    ciphertext,
+                    iv,
+                )
+            )
+
+            return plaintext
+
+        except Exception as e:
+            raise ValueError("Could not decrypt Fernet token")