I agree that the human intelligence factor is probably the most significant in this disaster.
Given how widely distributed the key was, it was foolish to the point of recklessness to assume that FSB operatives armed with unlimited cash and anything else they needed would not get it from someone, somewhere sooner or later. Money? Blackmail? Extortion? They had it all in abundance. It would be amazing if they failed to crack one of the hare-brained idealists sooner or later.