Create a webservice using DNN's authentication module

Hi every body
 I must develop a new class on an existing software develop on VB.Net. In this new class i must use the DotNetNuke login and password. But those login and PWD are encoded with 3DES in a table used by DNN.
So my question, Can i create a webservice able to use the DNN's authentication module, or if it's not possible i will create a method to decode those login and password ?

...