Tim Cook's letter to customers about the FBI request to assist in unlocking an iPhone

Some would argue that building a backdoor for just one iPhone is a simple, clean-cut solution. But it ignores both the basics of digital security and the significance of what the government is demanding in this case … In the physical world, it would be the equivalent of a master key, capable of opening hundreds of millions of locks — from restaurants and banks to stores and homes.

It’s worse than that. If someone steals a physical key, at least you know it’s missing and can go round and change the locks.

But a digital master key can be infinitely copied, instantaneously distributed across the world and you would never ever know.