The Jericho Miscalculation

Why classical encryption alone isn't enough

Mission 5: Quantum Key Distribution. Part of the Quantum Courier Service, a free course from the Oxford University Quantum Information Society.

Pull up a stool at the Jericho, recruit, because we need to be honest about what we are defending.

You have two lines of defence, and neither is what you would call load-bearing. The first is a brute-force mathematical cipher: secure not because anyone proved it unbreakable, but because factoring a large number takes longer than anyone is prepared to wait. That is not a law. That is a queue. Shor's algorithm on a large enough quantum computer walks straight past it, and every message the Echo Network recorded this morning is still sitting on their disks waiting for that day.

The second is a rather damp-looking human courier with a locked briefcase. Physically unbreakable, philosophically hopeless: the Echo Network's coffers are deeper than his salary, and always will be.

That is the miscalculation. Classical security rests on how hard a sum is and how loyal a man is, and both of those are matters of budget. What we want instead is a guarantee that does not depend on anyone's patience or anyone's price. Chop chop!