Unformatted text preview: is an entrusted undeniable signature . Imagine that Alice works for Toxins, Inc., and sends incriminating documents to a newspaper using an undeniable signature protocol. Alice can verify her signature to the newspaper reporter, but not to anyone else. However, CEO Bob suspects that Alice is the source of the documents. He demands that Alice run the disavowal protocol to clear her name, and Alice refuses. Bob maintains that the only reason Alice has to refuse is that she is guilty, and fires her. Entrusted undeniable signatures are like undeniable signatures, except that the disavowal protocol can only be run by Trent. Bob cannot demand that Alice run the disavowal protocol; only Trent can. And if Trent is the court system, then he will only run the protocol to resolve a formal dispute. 4.4 Designated Confirmer Signatures
The Alice Software Company is doing a booming business selling DEW—so good, in fact, that Alice is spending more time verifying undeniable signatures than writing new features. Alice would like a way to designate one particular person in the company to be in charge of signature verification for the whole company. Alice, or any other programmer, would be able to sign documents with an undeniable protocol. But the verifications would all be handled by Carol. As it turns out, this is possible with designated confirmer signatures [333, 1213]. Alice can sign a document such that Bob is convinced the signature is valid, but he cannot convince a third party; at the same time Alice can designate Carol as the future confirmer of her signature. Alice doesn’t even need to ask Carol’s permission beforehand; she just has to use Carol’s public key. And Carol can still verify Alice’s signature if Alice is out of town, has left the company, or just upped and died. Designated confirmer signatures are kind of a compromise between normal digital signatures and undeniable signatures. There are certainly instances where Alice might want to limit who can verify her signature. On the other hand, giving Alice complete control undermines the enforceability...
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- Fall '10
- Cryptography, Bruce Schneier, Applied Cryptography, EarthWeb, Search Search Tips