[otrs] API call protocol/way to connect to OTRS?

Dear listreaders! We're programming an external application that needs to autheticate users (agents) through OTRS. We need to know the best way and protocol to connect API calls to OTRS and authenticate users this way. Can you help us with some inputs on this or direct us to some relevant documentation? I have searched, but not found exactly what I'm looking for. Thanks! Best regards, Andy