workflow and authorisation

Hi,

I was thinking of using WorkFlow to advance tickets through a role and responsibility process. In this context, I wanted a ticket creator not to be able to modify a ticket, even his own, which goes from the status "New Ticket" to "in analysis" or "to be executed".

The only thing he can no longer do is to change the status of the WorkFlow because I have taken this right away from him. He can modify his ticket at any status. How to limit the authorizations to modify the tickets which have evolved in the Workflow?

Thank you.