<i>Give the people taking your questionnaire a coin. When the question appears e.g. “Have you ever made a mistake that has cost your company thousands of dollars?”, ask the subject to flip a coin. If the coin comes up HEADS, tell the person to answer the question truthfully. If the coin comes up TAILS, tell the person to flip the coin again and if the coin lands HEADS to answer ”Yes” and if the second flip comes up TAILS to answer the question ”No”.</i><p>This protocol has small flaw as it is open to the "side channel" attack. I.e. when it is possible to count number of coin flips then observer can directly deduce the truthfulness of the answer.<p>Therefore one has to flip the coin always twice.
I think we used to have these in old cereal box surprises or kids books for secret decoder games. There would be a message visually encrypted in a book and the book came with a transparent cellophane key sheet you'd place on top of the image to decode the secret message.