Add getter for Curve25519 shared secret (#159)

* add getter for Curve25519 shared secret

* remove blank line at EOF

docs/public.rst

@@ -167,3 +167,15 @@ Reference
         :param encoder: A class that is able to decode the plaintext.
 
         :return bytes: The decrypted plaintext.
+
+    .. method:: shared_key()
+
+        Returns the Curve25519 shared secret, that can then be used as a key in
+        other symmetric ciphers.
+
+        .. warning:: It is **VITALLY** important that you use a nonce with your
+            symmetric cipher. If you fail to do this, you compromise the
+            privacy of the messages encrypted. Ensure that the key length of
+            your cipher is 32 bytes.
+
+        :return bytes: The shared secret.

src/nacl/public.py

@@ -206,3 +206,17 @@ def decrypt(self, ciphertext, nonce=None, encoder=encoding.RawEncoder):
         )
 
         return plaintext
+
+    def shared_key(self):
+        """
+        Returns the Curve25519 shared secret, that can then be used as a key in
+        other symmetric ciphers.
+
+        .. warning:: It is **VITALLY** important that you use a nonce with your
+            symmetric cipher. If you fail to do this, you compromise the
+            privacy of the messages encrypted. Ensure that the key length of
+            your cipher is 32 bytes.
+        :rtype: [:class:`bytes`]
+        """
+
+        return self._shared_key