prefer ctypes over m2crypto in rc4-md5

shadowsocks/crypto/rc4_md5.py

@@ -28,28 +28,22 @@ import hashlib
 
 __all__ = ['ciphers']
 
-m2_not_found = False
-
 
 def create_cipher(alg, key, iv, op, key_as_bytes=0, d=None, salt=None,
                   i=1, padding=1):
-    global m2_not_found
-
     md5 = hashlib.md5()
     md5.update(key)
     md5.update(iv)
     rc4_key = md5.digest()
 
-    if not m2_not_found:
     try:
+        from shadowsocks.crypto import ctypes_openssl
+        return ctypes_openssl.CtypesCrypto(b'rc4', rc4_key, b'', op)
+    except:
         import M2Crypto.EVP
         return M2Crypto.EVP.Cipher(b'rc4', rc4_key, b'', op,
                                    key_as_bytes=0, d='md5', salt=None, i=1,
                                    padding=1)
-        except:
-            m2_not_found = True
-    from shadowsocks.crypto import ctypes_openssl
-    return ctypes_openssl.CtypesCrypto(b'rc4', rc4_key, b'', op)
 
 
 ciphers = {