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

4 the signed document is unalterable were bob to try

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Unformatted text preview: and not from some imposter? He infers it from the message’s encryption. Since only he and Alice share their secret key, only Alice could encrypt a message using it. Is this as good as a paper signature? Let’s look at the characteristics we want: 1. This signature is authentic. Trent is a trusted arbitrator and Trent knows that the message came from Alice. Trent’s certification serves as proof to Bob. 2. This signature is unforgeable. Only Alice (and Trent, but everyone trusts him) knows KA, so only Alice could have sent Trent a message encrypted with KA. If someone tried to impersonate Alice, Trent would have immediately realized this in step (2) and would not certify its authenticity. 3. This signature is not reusable. If Bob tried to take Trent’s certification and attach it to another message, Alice would cry foul. An arbitrator (it could be Trent or it could be a completely different arbitrator with access to the same information) would ask Bob to produce both the message and Alice’s encrypted message. The arbitrator would then encrypt the message with KA and see that it did not match the encrypted message that Bob gave him. Bob, of course, could not produce an encrypted message that matches because he does not know KA. 4. The signed document is unalterable. Were Bob to try to alter the document after receipt, Trent could prove foul play in exactly the same manner just described. 5. The signature cannot be repudiated. Even if Alice later claims that she never sent the message, Trent’s certification says otherwise. Remember, Trent is trusted by everyone; what he says is true. If Bob wants to show Carol a document signed by Alice, he can’t reveal his secret key to her. He has to go through Trent again: (1) Bob takes the message and Trent’s statement that the message came from Alice, encrypts them with KB, and sends them back to Trent. (2) Trent decrypts the bundle with KB. (3) Trent checks his database and confirms that the original message came from Alice. (4)...
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This note was uploaded on 10/18/2010 for the course MATH CS 301 taught by Professor Aliulger during the Fall '10 term at Koç University.

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