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Universally Composable Relativistic Commitments


Ciolacu,  Ines Lucia
International Max Planck Research School, MPI for Informatics, Max Planck Society;

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Ciolacu, I. L. (2011). Universally Composable Relativistic Commitments. Master Thesis, Universität des Saarlandes, Saarbrücken.

Designing communications protocols specifically adapted to relativistic situations (i.e. constrained by special relativity theory) is taking advantage of uniquely relativistic features to accomplish otherwise impossible tasks. Kent [Ken99] has demonstrated, for example, that secure bit commitment is possible using a protocol exploiting relativistic causality constraints, even though it is known to be impossible otherwise. Therefore, Kent's protocol gives a theoretical solution to the problem of finding commitment schemes secure over arbitrarily long time intervals. The functionality only requires from the committer a sequence of communications, including a post-revelation validation, each of which is guaranteed to be independent of its predecessor. We propose to verify the security of the relativistic commitment not as a stand alone protocol, but as an entity which is part of an unpredictable environment. To achieve this task we use the universal composability paradigm defined by Canetti [Can01]. The relevant property of the paradigm is the guarantee of security even when a secure protocol is composed with an arbitrary set of protocols, or, more generally, when the protocol is used as an element of a possibly complex system. Unfortunately, Kent's relativistic bit commitment satisfies universal composability only with certain restrictions on the adversarial model. However, we construct a two-party universal composable commitment protocol, also based on general relativistic assumptions.