Timed Commitments and Applications

Dan Boneh and Moni Naor


We introduce and construct  timed commitment   schemes, an extension to the standard notion of commitments in which a potential forced opening phase permits the receiver to recover (with effort) the committed value without the help of the committer. The result of a forced opening should be identical to normal ``reveal", including a proof of the committed value.

An important application of our timed-commitment scheme is contract signing: two mutually suspicious parties wish to exchange signatures on a contract. We show a two-party protocol that allows them to exchange RSA or Rabin signatures, without changing the PKI or the semantics of the signature. The protocol is  strongly  fair: if one party quits the protocol early, then the  two parties must invest comparable amounts of time to retrieve the signatures.  This statement holds even if one party has many more machines than the other.

Other applications, including honesty preserving auctions and collective coin-flipping, are discussed. Applications, connected to 3-round zero-knowledge protocols are discussed in the paper Zaps and Their Application (FOCS'2000).

Postscript , gzipped Postscript .

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