Aleksandr Yampolskiy

A verifiable random function with short proofs and keys

We give a simple and efficient construction of a verifiable random function (VRF) on bilinear groups. Our construction is direct. In contrast to prior VRF constructions [14,15], it avoids using an inefficient Goldreich-Levin transformation, thereby saving several factors in security. Our proofs of security are based on a decisional bilinear Diffie-Hellman inversion assumption, which seems reasonable given current state of knowledge. For small message spaces, our VRFs proofs and keys have constant size. By utilizing a collision-resistant hash function, our VRF can also be used with arbitrary message spaces. We show that our scheme can be instantiated with an elliptic group of very reasonable size. Furthermore, it can be made distributed and proactive.

Towards a Theory of Data Entanglement

We give a formal model for systems that store data in entangled form. We propose a new notion of entanglement, called all-or-nothing integrity (AONI) that binds the users’ data in a way that makes it hard to corrupt the data of any one user without corrupting the data of all users. AONI can be a useful defense against negligent or dishonest storage providers who might otherwise be tempted to discard documents belonging to users without much clout. We show that, if all users use the standard recovery algorithm, we can implement AONI using a MAC, but, if some of the users adopt the adversary’s non-standard recovery algorithm, AONI can no longer be achieved. However, even for the latter scenario, we describe a simple entangling mechanism that provides AONI for a restricted class of destructive adversaries.

Spreading Alerts Quietly and the Subgroup Escape Problem

We introduce a new cryptographic primitive called a blind coupon mechanism (BCM). In effect, a BCM is an authenticated bit commitment scheme, which is AND-homomorphic. We show that a BCM has natural and important applications. In particular, we use it to construct a mechanism for transmitting alerts undetectably in a message-passing system of

Inoculation strategies for victims of viruses and the sum-of-squares partition problem

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Threshold and proactive pseudo-random permutations

n nodes. Our algorithms allow an alert to quickly propagate to all nodes without its source or existence being detected by an adversary, who controls all message traffic. Our proofs of security are based on a new subgroup escape problem, which seems hard on certain groups with bilinear pairings and on elliptic curves over the ring

Efficient cryptographic tools for secure distributed computing

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