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

This works but it is impossible to verify alices

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Unformatted text preview: This is misleading and is only true for one algorithm, RSA. And different algorithms have different implementations. For example, one-way hash functions and timestamps sometimes add extra steps to the process of signing and verifying. Many algorithms can be used for digital signatures, but not for encryption. In general, I will refer to the signing and verifying processes without any details of the algorithms involved. Signing a message with private key K is: SK(M) and verifying a signature with the corresponding public key is: VK(M) The bit string attached to the document when signed (in the previous example, the one-way hash of the document encrypted with the private key) will be called the digital signature, or just the signature. The entire protocol, by which the receiver of a message is convinced of the identity of the sender and the integrity of the message, is called authentication. Further details on these protocols are in Section 3.2. Multiple Signatures How could Alice and Bob sign the same digital document? Without one-way hash functions, there are two options. One is that Alice and Bob sign separate copies of the document itself. The resultant message would be over twice the size of the original document. The second is that Alice signs the document first and then Bob signs Alice’s signature. This works, but it is impossible to verify Alice’s signature without also verifying Bob’s. With one-way hash functions, multiple signatures are easy: (1) Alice signs the hash of the document. (2) Bob signs the hash of the document. (3) Bob sends his signature to Alice. (4) Alice sends the document, her signature, and Bob’s signature to Carol. (5) Carol verifies both Alice’s signature and Bob’s signature. Alice and Bob can do steps (1) and (2) either in parallel or in series. In step (5), Carol can verify one signature without having to verify the other. Previous Table of Contents Next Products | Contact Us | About Us | Privacy | Ad Info | Home Use of this site is subject to certain Terms & Conditions, Copyright © 1996-2000 EarthWeb Inc. All...
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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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