Encryption of Radius acct-req authenticator (implemented by python)

# acct_req_msg: hexlified acct-req msg

def recalculate_authenticator(self, acct_req_msg):
        acct_req_msg = list(acct_req_msg)
        acct_req_msg[4*2:(4+16)*2] = '0'*32
        acct_req_msg = ''.join(acct_req_msg)
       
        mixed_str = acct_req_msg+hexlify(self.m_shared_secret)
        m = md5.new(unhexlify(mixed_str))
        new_ator = m.hexdigest()
        
        acct_req_msg = list(acct_req_msg)
        acct_req_msg[4*2:(4+16)*2] = new_ator
        new_msg = ''.join(acct_req_msg)

        return new_msg