The Clarendon Cipher

Sifting the key: keeping only matching bases

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

Pay attention, recruit; we're auditing a rather sensitive quantum key exchange between Alice and Bob, and that meddling spy, Eve, may or may not be lurking on the line like a fresher at a free wine tasting.

Now, the instinct you arrived with is wrong, so let us dispose of it early. You will not be spotting Eve by finding a mismatch. Honest fibre produces mismatches all by itself: a warm connector, a jittery detector, a cosmic ray with poor timing. And if a single disagreement were proof of espionage you would be aborting the protocol every four minutes for the rest of your career.

What you are looking for is a rate. Set how many sifted bits Alice and Bob are willing to sacrifice, let them read that sample out over the open line, and compare the error rate against the threshold. Eve's intercept-resend sits at about 25%. Honest noise sits at a couple of percent. The two are very far apart, but at a sample of eight bits they overlap often enough to ruin you in both directions.

So find your sample size. Drag it low and watch how often Eve strolls past you. Drag it high and watch the false alarms vanish, along with a great deal of key you could have been using. That trade is the job. Chop chop!