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Erschienen in:
2006 | OriginalPaper | Buchkapitel
On the (Im-)Possibility of Extending Coin Toss
verfasst von : Dennis Hofheinz, Jörn Müller-Quade, Dominique Unruh
Erschienen in: Advances in Cryptology - EUROCRYPT 2006
Verlag: Springer Berlin Heidelberg
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We consider the cryptographic two-party protocol task of extending a given coin toss. The goal is to generate
n
common random coins from a single use of an ideal functionality which gives
m
<
n
common random coins to the parties. In the framework of Universal Composability we show the impossibility of securely extending a coin toss for statistical and perfect security. On the other hand, for computational security the existence of a protocol for coin toss extension depends on the number
m
of random coins which can be obtained “for free”.
For the case of stand-alone security, i.e., a simulation based security definition without an environment, we present a novel protocol for unconditionally secure coin toss extension. The new protocol works for superlogarithmic
m
, which is optimal as we show the impossibility of statistically secure coin toss extension for smaller
m
.
Combining our results with already known results, we obtain a (nearly) complete characterization under which circumstances coin toss extension is possible.