Do not reinitialise cipher class every time

Now CipherState holds instance of Cipher wrapper and manages
initialization of underlying cipher method with keys.

Closes #6

noise/functions.py

@@ -65,28 +65,22 @@ class Cipher(object):
             self.rekey = self._default_rekey
         else:
             raise NotImplementedError('Cipher method: {}'.format(method))
+        self.cipher = None
 
     def _aesgcm_encrypt(self, k, n, ad, plaintext):
-        # Might be expensive to initialise AESGCM with the same key every time. The key should be (as per spec) kept in
-        # CipherState, but we may as well hold an initialised AESGCM and manage reinitialisation on CipherState.rekey
-        cipher = self._cipher(k)
-        return cipher.encrypt(nonce=self._aesgcm_nonce(n), data=plaintext, associated_data=ad)
+        return self.cipher.encrypt(nonce=self._aesgcm_nonce(n), data=plaintext, associated_data=ad)
 
     def _aesgcm_decrypt(self, k, n, ad, ciphertext):
-        cipher = self._cipher(k)
-        return cipher.decrypt(nonce=self._aesgcm_nonce(n), data=ciphertext, associated_data=ad)
+        return self.cipher.decrypt(nonce=self._aesgcm_nonce(n), data=ciphertext, associated_data=ad)
 
     def _aesgcm_nonce(self, n):
         return b'\x00\x00\x00\x00' + n.to_bytes(length=8, byteorder='big')
 
     def _chacha20_encrypt(self, k, n, ad, plaintext):
-        # Same comment as with AESGCM
-        cipher = self._cipher(k)
-        return cipher.encrypt(nonce=self._chacha20_nonce(n), data=plaintext, associated_data=ad)
+        return self.cipher.encrypt(nonce=self._chacha20_nonce(n), data=plaintext, associated_data=ad)
 
     def _chacha20_decrypt(self, k, n, ad, ciphertext):
-        cipher = self._cipher(k)
-        return cipher.decrypt(nonce=self._chacha20_nonce(n), data=ciphertext, associated_data=ad)
+        return self.cipher.decrypt(nonce=self._chacha20_nonce(n), data=ciphertext, associated_data=ad)
 
     def _chacha20_nonce(self, n):
         return b'\x00\x00\x00\x00' + n.to_bytes(length=8, byteorder='little')
@@ -94,6 +88,9 @@ class Cipher(object):
     def _default_rekey(self, k):
         return self.encrypt(k, MAX_NONCE, b'', b'\x00' * 32)[:32]
 
+    def initialize(self, key):
+        self.cipher = self._cipher(key)
+
 
 class Hash(object):
     def __init__(self, method):
@@ -209,8 +206,8 @@ dh_map = {
 }
 
 cipher_map = {
-    'AESGCM': Cipher('AESGCM'),
-    'ChaChaPoly': Cipher('ChaCha20')
+    'AESGCM': partial(Cipher, 'AESGCM'),
+    'ChaChaPoly': partial(Cipher, 'ChaCha20')
 }
 
 hash_map = {

noise/state.py

@@ -9,11 +9,14 @@ class CipherState(object):
     Implemented as per Noise Protocol specification (rev 32) - paragraph 5.1.
 
     The initialize_key() function takes additional required argument - noise_protocol.
+
+    This class holds an instance of Cipher wrapper. It manages initialisation of underlying cipher function
+    with appropriate key in initialize_key() and rekey() methods.
     """
     def __init__(self, noise_protocol):
         self.k = Empty()
         self.n = None
-        self.noise_protocol = noise_protocol
+        self.cipher = noise_protocol.cipher_fn()
 
     def initialize_key(self, key):
         """
@@ -21,6 +24,8 @@ class CipherState(object):
         """
         self.k = key
         self.n = 0
+        if self.has_key():
+            self.cipher.initialize(key)
 
     def has_key(self):
         """
@@ -41,7 +46,7 @@ class CipherState(object):
         if not self.has_key():
             return plaintext
 
-        ciphertext = self.noise_protocol.cipher_fn.encrypt(self.k, self.n, ad, plaintext)
+        ciphertext = self.cipher.encrypt(self.k, self.n, ad, plaintext)
         self.n = self.n + 1
         return ciphertext
 
@@ -59,12 +64,13 @@ class CipherState(object):
         if not self.has_key():
             return ciphertext
 
-        plaintext = self.noise_protocol.cipher_fn.decrypt(self.k, self.n, ad, ciphertext)
+        plaintext = self.cipher.decrypt(self.k, self.n, ad, ciphertext)
         self.n = self.n + 1
         return plaintext
 
     def rekey(self):
-        self.k = self.noise_protocol.cipher_fn.rekey(self.k)
+        self.k = self.cipher.rekey(self.k)
+        self.cipher.initialize(self.k)
 
 
 class SymmetricState(object):