Workflow - Simple user validation

Hi,

Does anyone have a simple solution for user validation. I was thinking of some kind of popup / prompt for username and password when starting the worfklow. It does not need to be highsecure but it would be good that the password is masked in the input. I was looking at some javascript but there it seems if you have a prompt the password is unmasked.

The idea is that the workflow can be started from every users computers but I want to avoid user running the workflow and therefore have some challange wich only admin / support knows.

Stefan