Improve code coverage for pycrypto_keys.py

tests/test_pycrypto_keys.py

@@ -66,6 +66,7 @@ def test_generate_rsa_public_and_private(self):
 
   def test_create_rsa_signature(self):
     global private_rsa
+    global public_rsa
     data = 'The quick brown fox jumps over the lazy dog'.encode('utf-8')
     signature, method = pycrypto.create_rsa_signature(private_rsa, data)
 
@@ -83,9 +84,16 @@ def test_create_rsa_signature(self):
                       pycrypto.create_rsa_signature, '', data)
    
     # Check for invalid 'data'.
+    print(repr(private_rsa))
+    pycrypto.create_rsa_signature(private_rsa, '')
+    
     self.assertRaises(tuf.CryptoError,
                       pycrypto.create_rsa_signature, private_rsa, 123)
 
+    # Check for missing private key.
+    self.assertRaises(tuf.CryptoError,
+                      pycrypto.create_rsa_signature, public_rsa, data)
+
 
   def test_verify_rsa_signature(self):
     global public_rsa
@@ -209,6 +217,7 @@ def test_create_rsa_public_and_private_from_encrypted_pem(self):
     self.assertRaises(tuf.FormatError,
                       pycrypto.create_rsa_public_and_private_from_encrypted_pem,
                       pem_rsakey, ['pw'])
+    
     self.assertRaises(tuf.CryptoError,
                       pycrypto.create_rsa_public_and_private_from_encrypted_pem,
                       'invalid_pem', passphrase)

tuf/pycrypto_keys.py

@@ -585,8 +585,9 @@ def create_rsa_public_and_private_from_encrypted_pem(encrypted_pem, passphrase):
     public = rsa_pubkey.exportKey(format='PEM')
  
   # PyCrypto raises 'ValueError' if the public or private keys cannot be
-  # exported.  See 'Crypto.PublicKey.RSA'.  
-  except (ValueError):
+  # exported.  See 'Crypto.PublicKey.RSA'.  'ValueError' should not be raised
+  # if the 'Crypto.PublicKey.RSA.importKey() call above passed.
+  except (ValueError): #pragma: no cover
     message = 'The public and private keys cannot be exported in PEM format.' 
     raise tuf.CryptoError(message)