Transition Workflow Guard, Who Initiated The Request?

I am creating a state tranisition workflow and for some transitions, I need to guard against certain users being able to change the state of an issue from one value to another. 

So far I have been unable to figure out what in the ctx object identifies the user that is attempting to make the state change. 

Sudo code of what I am trying to do as follows:

Done: {
    guard: function(ctx) {
    
        if (the user who is trying to change the state is not "UserA") {
            workflow.message('You cannot change this state');
            return false;
       }
       return true;
    },
    
},

I know I cannot use ctx.issue.updatedBy because in the guard the issue has not actually changed yet. 

So what can I use in the guard to see what user is attempting to change the state? Basically, the actor of the transition.