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, 4 insertions, 4 deletions

diff --git a/tests/test_algorithms.py b/tests/test_algorithms.py
index 11d8cd0..69501ee 100644
--- a/tests/test_algorithms.py
+++ b/tests/test_algorithms.py
@@ -58,11 +58,11 @@ class TestAlgorithms:
     def test_hmac_should_reject_nonstring_key(self):
         algo = HMACAlgorithm(HMACAlgorithm.SHA256)
 
-        with pytest.raises(TypeError) as context:
+        with pytest.raises(InvalidKeyError) as context:
             algo.prepare_key(object())
 
         exception = context.value
-        assert str(exception) == 'Expected a string value'
+        assert str(exception) == 'HMAC secret key must be a string type.'
 
     def test_hmac_should_accept_unicode_key(self):
         algo = HMACAlgorithm(HMACAlgorithm.SHA256)
@@ -144,7 +144,7 @@ class TestAlgorithms:
     def test_rsa_should_reject_non_string_key(self):
         algo = RSAAlgorithm(RSAAlgorithm.SHA256)
 
-        with pytest.raises(TypeError):
+        with pytest.raises(InvalidKeyError):
             algo.prepare_key(None)
 
     @pytest.mark.skipif(not has_crypto, reason='Not supported without cryptography library')
@@ -358,7 +358,7 @@ class TestAlgorithms:
     def test_ec_should_reject_non_string_key(self):
         algo = ECAlgorithm(ECAlgorithm.SHA256)
 
-        with pytest.raises(TypeError):
+        with pytest.raises(InvalidKeyError):
             algo.prepare_key(None)
 
     @pytest.mark.skipif(not has_crypto, reason='Not supported without cryptography library')