Paper 2005/444

Privacy-Preserving Polling using Playing Cards

Sid Stamm and Markus Jakobsson

Abstract

Visualizing protocols is not only useful as a step towards understanding and ensuring security properties, but is also a beneficial tool to communicate notions of security to decision makers and technical people outside the field of cryptography. We present a simple card game that is a visualization for a secure protocol for private polling where it is simple to see that individual responses cannot be traced back to a respondent, and cheating is irrational. We use visualization tricks to illustrate a somewhat complex protocol, namely the Cryptographic Randomized Response Technique protocol of Lipmaa et al. While our tools --- commitments and cut-and-choose --- are well known, our construction for oblivious transfer using playing cards is new. As part of visualizing the protocol, we have been able to show that, while cut-and-choose protocols normally get more secure with an increasing number of choices, the protocol we consider --- surprisingly --- does not. This is true for our visualization of the protocol and for the real protocol.