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

3 bob then decrypts alices message using his private

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Unformatted text preview: cryptosystem. (2) Bob sends Alice his public key. (3) Alice encrypts her message using Bob’s public key and sends it to Bob. (4) Bob decrypts Alice’s message using his private key. Notice how public-key cryptography solves the key-management problem with symmetric cryptosystems. Before, Alice and Bob had to agree on a key in secret. Alice could choose one at random, but she still had to get it to Bob. She could hand it to him sometime beforehand, but that requires foresight. She could send it to him by secure courier, but that takes time. Public-key cryptography makes it easy. With no prior arrangements, Alice can send a secure message to Bob. Eve, listening in on the entire exchange, has Bob’s public key and a message encrypted in that key, but cannot recover either Bob’s private key or the message. More commonly, a network of users agrees on a public-key cryptosystem. Every user has his or her own public key and private key, and the public keys are all published in a database somewhere. Now the protocol is even easier: (1) Alice gets Bob’s public key from the database. (2) Alice encrypts her message using Bob’s public key and sends it to Bob. (3) Bob then decrypts Alice’s message using his private key. In the first protocol, Bob had to send Alice his public key before she could send him a message. The second protocol is more like traditional mail. Bob is not involved in the protocol until he wants to read his message. Hybrid Cryptosystems The first public-key algorithms became public at the same time that DES was being discussed as a proposed standard. This resulted in some partisan politics in the cryptographic community. As Diffie described it [494]: The excitement public key cryptosystems provoked in the popular and scientific press was not matched by corresponding acceptance in the cryptographic establishment, however. In the same year that public key cryptography was discovered, the National Security Agency (NSA), proposed a conventional cryptographic system,...
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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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