use pycrpyto if it is installed

README.rst

@@ -31,6 +31,18 @@ Installation
     $ pip install python-jose
 
 
+Custom Backends
+---------------
+
+As of 2.0.0, python-jose uses pycryptodome by default for RSA signing and verification. If
+necessary, other RSA backends are supported. Both pycrpyto and crytography are options.
+
+In order to use a custom backend, install python-jose with the appropriate extra.
+
+::
+    $ pip install python-jose[pycrypto]
+    $ pip install python-jose[crytography]
+
 Usage
 -----
 

jose/backends/pycrypto_backend.py

@@ -14,12 +14,15 @@
 from jose.exceptions import JWKError
 from jose.utils import base64url_decode
 
-# PyCryptodome's RSA module doesn't have PyCrypto's _RSAobj class
-# Instead it has a class named RsaKey, which serves the same purpose.
-if hasattr(RSA, '_RSAobj'):
-    _RSAKey = RSA._RSAobj
-else:
+
+# We default to using PyCryptodome, however, if PyCrypto is installed, it is
+# used instead. This is so that environments that require the use of PyCrypto
+# are still supported.
+if hasattr(RSA, 'RsaKey'):
     _RSAKey = RSA.RsaKey
+else:
+    _RSAKey = RSA._RSAobj
+
 
 
 class RSAKey(Key):