Visual cryptography
datagenetics.com
datagenetics.com
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.
Therefore one has to flip the coin always twice.
“Flip this coin twice. If both flips agree, answer truthfully. If they're different, then answer ‘Yes’ if the 2nd flip was heads, ‘No’ if it was tails.”
I wrote the example thinking that people were filling in the survey in private (sort of like you, probably, do when voting).
Anyway, you get the concept :)
/\/ick
"You have a coin. Before flipping it, decide which flips you will count. Now flip it sixteen times, only paying attention to the nth and mth flips. If n and m agree, then answer truthfully, otherwise answer according to m's result, in a manner that you personally pre-designated"
:) That's when the survey-taker looks at you cross-eyed and just lies on the question.