A pass-phrase is an excellent idea. Hermione, Terry, do you have any suggestions? It needs to be something brief, something not revealing of further secrets, but not so obvious that someone might stumble across it ('Grim Truth' is too obvious, as is 'I'm with Sirius Black.' And it can't be 'I'm with the Wand Smugglers,' as then they'd realise that we're on to some of THEIR secrets, which is hardly a way to engender trust.)