We consider problems where n people are communicating and a random subset of them is trying to leak information, without making it clear who are leaking the information. We introduce a measure of suspicion and show that the amount of leaked information will always be bounded by the expected increase in suspicion, and that this bound is tight. Suppose a large number of people have some information they want to leak, but they want to ensure that after the communication, an observer will assign probability at most c to the events that each of them is trying to leak the information. How much information can they reliably leak, per person who is leaking? We show that the answer is $$\left( \frac{-\log (1-c)}{c}-\log (e)\right) $$ bits.

We ask whether it is possible to anonymously communicate a large amount of data using only public (non-anonymous) communication together with a small anonymous channel. We think this is a central question in the theory of anonymous communication and to the best of our knowledge this is the first formal study in this direction.

Extensive-form games constitute the standard representation scheme for games with a temporal component. But do all extensive-form games correspond to protocols that we can implement in the real world? We often rule out games with imperfect recall, which prescribe that an agent forget something that she knew before. In this paper, we show that even some games with perfect recall can be problematic to implement. Specifically, we show that if the agents have a sense of time passing (say, access to a clock), then some extensive-form games can no longer be implemented; no matter how we attempt to time the game, some information will leak to the agents that they are not supposed to have. We say such a game is not exactly timeable. We provide easy-to-check necessary and sufficient conditions for a game to be exactly timeable. Most of the technical depth of the paper concerns how to approximately time games, which we show can always be done, though it may require large amounts of time. Specifically, we show that some games require time proportional to the power tower of height proportional to the number of players, which in practice would make them untimeable. We hope to convince the reader that timeability should be a standard assumption, just as perfect recall is today. Besides the conceptual contribution to game theory, we show that timeability has implications for onion routing protocols.

We consider the following cryptographic secret leaking problem. A group of players communicate with the goal of learning (and perhaps revealing) a secret held initially by one of them. Their conversation is monitored by a computationally unlimited eavesdropper, who wants to learn the identity of the secret-holder. Despite the unavailability of key, some protection can be provided to the identity of the secret-holder. We call the study of such communication problems, either from the group's or the eavesdropper's point of view, cryptogenography. We introduce a basic cryptogenography problem and show that two players can force the eavesdropper to missguess the origin of a secret bit with probability 1/3; we complement this with a hardness result showing that they cannot do better than than 3/8. We prove that larger numbers of players can do better than 0.5644, but no group of any size can achieve 0.75.

For discrete random variables X1, ..., Xnwe construct an n by n matrix. In the (i, j)-entry we put the mutual information I(Xi; Xj) between Xiand Xj. In particular, in the (i, i)-entry we put the entropy H(Xi) = I(Xi; Xi) of Xi. This matrix, called the mutual information matrix of (X1, ..., Xn), has been conjectured to be positive semidefinite. In this paper, we give counterexamples to the conjecture, and show that the conjecture holds for up to three random variables.