
Visual cryptography - squidlogic
http://www.datagenetics.com/blog/november32013/
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eveningcoffee
_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”._

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.

~~~
function_seven
Good point.

“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.”

~~~
squeakynick
I guess both of you are right. But then even this is open to the side attack
if someone looks over your shoulder and views the state of the coins and knows
the protocol :)

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

~~~
function_seven
Ok, I'll try to solve that...

"You have a coin. Before flipping it, decide which flips you will count. Now
flip it sixteen times, only paying attention to the _n_ th and _m_ th 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.

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mozumder
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.

~~~
BostonEnginerd
Those were usually red color filters which filtered out one of the noise
channels, leaving the message.

