Abstract. We introduce the notion of resource-fair protocols. Informally, this property states that if one party learns the output of the protocol, then so can all other parties, as long as they expend roughly the same amount of resources. As opposed to similar previously proposed definitions, our definition follows the standard simulation paradigm and enjoys strong composability properties. In particular, our definition is similar to the security definition in the universal composability (UC) framework, but works in a model that allows any party to request additional resources from the environment to deal with dishonest parties that may prematurely abort. In this model we specify the ideally fair functionality as allowing parties to “invest resources ” in return for outputs, but in such an event offering all other parties a fair deal. (The formulation of fair dealings is kept independent of any particular functionality, by defining it using a “wrapper.”) Thus, by relaxing the notion of fairness, we avoid a well-known impossibility result for fair multi-party computation with corrupted majority; in particular, our definition admits constructions that tolerate arbitrary number of corruptions. We also show that, as in the UC framework, protocols in our framework may be arbitrarily and concurrently composed. Turning to constructions, we define a “commit-prove-fair-open ” functionality and design an efficient resource-fair protocol that securely realizes it, using a new variant of a cryptographic primitive known as “timelines.” With (the fairly wrapped version of) this functionality we show that some of the existing secure multi-party computation protocols can be easily transformed into resource-fair protocols while preserving their security. 1

Jan 1, 200516 citations

Composable security in relativistic quantum cryptography

V. Vilasini, Christopher Portmann, Lídia del Rio

Relativistic protocols have been proposed to overcome certain impossibility results in classical and \nquantum cryptography. In such a setting, one takes the location of honest players into account, and \nuses the signalling limit given by the speed of light to constraint the abilities of dishonest agents. \nHowever, composing such protocols with each other to construct new cryptographic resources is \nknown to be insecure in some cases. To make general statements about such constructions, a \ncomposable framework for modelling cryptographic security in Minkowski space is required. \nHere, we introduce a framework for performing such a modular security analysis of classical and \nquantum cryptographic schemes in Minkowski space. As an application, we show that (1) fair and \nunbiased coin flipping can be constructed from a simple resource called channel with delay; (2) \nbiased coin flipping, bit commitment and channel with delay through any classical, quantum or \npost-quantum relativistic protocols are all impossible without further setup assumptions; (3) it is \nimpossible to securely increase the delay of a channel, given several short-delay channels as \ningredients. Results(1) and (3) imply in particular the non-composability of existing relativistic bit \ncommitment and coin flipping protocols.

Apr 1, 201916 citations