Unformatted text preview: ce has n different messages. Bob can choose one of the n messages for Alice to sign, and Alice will have no way of knowing which one she signed. 2. Alice has one message. Bob can choose one of n keys for Alice to use in signing the message, and Alice will have no way of knowing which key she used. It’s a neat idea; I’m sure it has a use somewhere. 5.7 Simultaneous Contract Signing
Contract Signing with an Arbitrator Alice and Bob want to enter into a contract. They’ve agreed on the wording, but neither wishes to sign unless the other signs as well. Face to face, this is easy: Both sign together. Over a distance, they could use an arbitrator. (1) Alice signs a copy of the contract and sends it to Trent. (2) Bob signs a copy of the contract and sends it to Trent. (3) Trent sends a message to both Alice and Bob indicating that the other has signed the contract. (4) Alice signs two copies of the contract and sends them to Bob. (5) Bob signs both copies of the contract, keeps one for himself, and sends the other to Alice. (6) Alice and Bob both inform Trent that they each have a copy of the contract signed by both of them. (7) Trent tears up his two copies of the contract with only one signature each. This protocol works because Trent prevents either of the parties from cheating. If Bob were to refuse to sign the contract in step (5), Alice could appeal to Trent for a copy of the contract already signed by Bob. If Alice were to refuse to sign in step (4), Bob could do the same. When Trent indicates that he received both contracts in step (3), both Alice and Bob know that the other is bound by the contract. If Trent does not receive both contracts in steps (1) and (2), he tears up the one he received and neither party is bound. Simultaneous Contract Signing without an Arbitrator (Face-to-Face)
If Alice and Bob were sitting face-to-face, they could sign the contract this way : (1) Alice signs the first letter of her name and passes the contract to Bob. (2) Bob signs the first letter of his name and passes the contract to Alice. (3) Ali...
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- Fall '10
- Cryptography, Bruce Schneier, Applied Cryptography, EarthWeb, Search Search Tips