Refactor error handling in Algorithm.prepare_key() methodsfix-key-errors

Our error handling in Algorithm.prepare_key() was previously weird and
kind of inconsistent. This change makes a number of improvements:

* Refactors RSA and ECDSA prepare_key() methods to reduce nesting and
  make the code simpler to understand
* All calls to Algorithm.prepare_key() return InvalidKeyError (or a
  subclass) or a valid key instance.
* Created a new InvalidAsymmetricKeyError class that is used to provide
  a standard message when an invalid RSA or ECDSA key is used.

1 files changed, 2 insertions, 1 deletions

diff --git a/jwt/contrib/algorithms/pycrypto.py b/jwt/contrib/algorithms/pycrypto.py
index e6afaa5..fe477db 100644
--- a/jwt/contrib/algorithms/pycrypto.py
+++ b/jwt/contrib/algorithms/pycrypto.py
@@ -7,6 +7,7 @@ from Crypto.Signature import PKCS1_v1_5
 
 from jwt.algorithms import Algorithm
 from jwt.compat import string_types, text_type
+from jwt.exceptions import InvalidAsymmetricKeyError
 
 
 class RSAAlgorithm(Algorithm):
@@ -36,7 +37,7 @@ class RSAAlgorithm(Algorithm):
 
             key = RSA.importKey(key)
         else:
-            raise TypeError('Expecting a PEM- or RSA-formatted key.')
+            raise InvalidAsymmetricKeyError
 
         return key