applied cryptography - protocols, algorithms, and source code in c

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Unformatted text preview: of signatures: Alice might refuse to cooperate in either confirming or denying, she might claim the loss of keys for confirming or denying, or she might just be unavailable. Designated confirmer signatures can give Alice the protection of an undeniable signature while not letting her abuse that protection. Alice might even prefer it that way: Designated confirmer signatures can help prevent false applications, protect her if she actually does lose her key, and step in if she is on vacation, in the hospital, or even dead. This idea has all sorts of possible applications. Carol can set herself up as a notary public. She can publish her public key in some directory somewhere, and people can designate her as a confirmer for their signatures. She can charge a small fee for confirming signatures for the masses and make a nice living. Carol can be a copyright office, a government agency, or a host of other things. This protocol allows organizations to separate the people who sign documents from the people who help verify signatures. 4.5 Proxy Signatures Designated confirmer signatures allows a signer to designate someone else to verify his signature. Alice, for instance, needs to go on a business trip to someplace which doesn’t have very good computer network access—to the jungles of Africa, for example. Or maybe she is incapacitated after major surgery. She expects to receive some important e-mail, and has instructed her secretary Bob to respond accordingly. How can Alice give Bob the power to sign messages for her, without giving him her private key? Proxy signatures is a solution [1001]. Alice can give Bob a proxy, such that the following properties hold: — Distinguishability. Proxy signatures are distinguishable from normal signatures by anyone. — Unforgeability. Only the original signer and the designated proxy signer can create a valid proxy signature. — Proxy signer’s deviation. A proxy signer cannot create a valid proxy signature not detected as a proxy signature....
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