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The Cryptographic Imagination 179 Material Objects In visual cryptography, assumptions regarding the relative difficulty of number-theoretic problems are traded for assumptions regarding the capabilities of the human visual system. This exchange leads to a series of schemes categorized according to trade-offs between protocol security and other design constraints. Pursuing an intriguing path, Moran and Naor have argued that it is also possible to further extend the “standard model” by including the capabilities of some of the traditional paper-and-ink objects that have provided security in the physical world—for example, sealed envelopes and scratch-cards (such as those used for instant lotteries). They model such objects as “tamper-evident seals,” defined as “a cryptographic primitive that captures the properties of a sealed envelope: while the envelope is sealed, it is impossible to tell what’s inside, but if the seal is broken the envelope cannot be resealed (so any tampering is evident).”54 Not only are the protocols designed to be implemented using actual physical envelopes, but Moran and Naor place explicit upper bounds on the number of rounds and envelopes that can be used so that the protocols remain practical for humans, rather than mere theoretical constructs. Just like the visual authentication schemes, the security of the resulting protocols does not rely on any computational assumptions but rather on the (physical) tamper-evident properties of the envelopes.55 These tamper-evident seals are introduced in the context of an information security problem with important implications: survey-based methodologies encounter elevated bias when investigating socially desirable or undesirable behaviors (e.g., voting preferences or immigration status), as survey respondents tend to lie. One solution is to use a “randomized response” technique, first proposed by Warner in 1965.56 In its simplest version, the respondent privately flips a coin before answering the question . Heads, he answers truthfully, tails, he lies. Given that only half the population surveyed will have answered truthfully, the researcher merely doubles the observed response to obtain the true proportion. Yet individual respondents are shielded by the uncertainty generated by the coin toss, as each is equally likely to have lied—that is, the protocol provides them with plausible deniability. As befits the work of cryptographers, Moran and Naor’s protocols are designed to address the issue whereby a respondent maliciously deviates ...

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