Cryptography by the People, for the People

In this thesis we investigate methods for designing and analyzing cryptographic schemes which take into account the differences between humans and computers. On the one hand, humans have trouble performing complex algorithms and mathematical computations. On the other, humans live in a physical world, and can make use of " real " objects in ways that purely electronic algorithms cannot. Tamper-Evident Seals. In the first part of the thesis (Chapters 2 and 3), we construct cryptographic protocols that rely on the properties of everyday objects (such as locks, envelopes or scratch-off cards). In Chapter 2, we formally define the properties of tamper-evident seals. We consider several variants of the " intuitive " definition, and study their relation to basic cryptographic primitives (such as fair coin-flipping, bit-commitment and oblivious transfer), giving both positive and negative results. In Chapter 3, we use scratch-off cards and envelopes to construct protocols for securely conducting polls of sensitive questions: the responders can safely answer truthfully while maintaining plausible deniability (the pollster will never be certain of their actual answer), but at the same time cannot bias the results of the poll. The protocols are simple enough to be used in practice. Voting Protocols. The distinction between the capabilities of humans and computers is especially evident in the context of voting, where voters need to be certain that their ballots were taken into account, even if they mistrust the computers running the election. The second part of the thesis (Chapters 4 and 5) deals specifically with protocols for secure elections. We present two protocols for universally-verifiable voting; both allow voters to verify that their ballots were cast correctly and allow anyone to verify that the published final tally is correct — even if the computers running the election behave maliciously. Both protocols also have the property of everlasting privacy: the data published during the election contains no information at all about voters' choices (beyond the final tally). This ensures that even if the cryptographic assumptions used by these protocols are subsequently broken, voter privacy will not be violated. Formal Analysis. One of our major aims is to provide rigorous proofs of security for cryptographic protocols involving humans. We give formal proofs of security for our protocols in the Universal Composability model, which gives very powerful guarantees about the security of the protocols when run concurrently with other protocols and when used as building-blocks in larger protocols. …