'Will encrypt an assertion
:param statement: A XML document that contains the assertion to encrypt
:param enc_key: File name of a file containing the encryption key
:param template: A template for the encryption part to be added.
:param key_type: The type of session key to use.
:return: The encrypted text'

def encrypt_assertion(self, statement, enc_key, template, key_type='des-192', node_xpath=None):

    raise NotImplemented()
