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
the committer a sequence of communications, including a post-revelation
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.