The keywords here are “randomized response”. There are some interesting variations (from the Wikipedia page):
The sensitive question is worded in two dichotomous alternatives, and chance decides, unknown to the interviewer, which one is to be answered honestly.
Alternative 1: “I have consumed marijuana.”
Alternative 2: “I have never consumed marijuana.”
The interviewed are asked to secretly throw a die and answer the first question only if they throw a 6, otherwise the second question.
The keywords here are “randomized response”. There are some interesting variations (from the Wikipedia page):