Webservice for other application

Dear All,

I am developing a Mobile application and i want to access Moodle in my application.

i want to know is there any api which i can hit to authenticate user?

e.g

url is 'https://mymoodle.tld/webservice/rest/server.php?wstoken=XXXXXXXXXXXX&wsfunction=xxxxxxx&moodlewsrestformat=json&username=xxxxx&password=xxxxx' and i want to pass username and password as parameter and get response whether the user is logged on server or user entered invalid credentials.

if credentials are correct i want to open Moodle then which should not ask for login from user.

any help will be appreciated. Thanks