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Unformatted text preview: )))) Now, all Mallory has to do is decrypt the message with his private key, encrypt it with Bob’s public key, decrypt it again with his private key, and encrypt it with Alice’s public key. Voilà! Mallory has M. It is not unreasonable to imagine that Bob may automatically send Mallory a receipt. This protocol may be embedded in his communications software, for example, and send receipts automatically. It is this willingness to acknowledge the receipt of gibberish that creates the insecurity. If Bob checked the message for comprehensibility before sending a receipt, he could avoid this security problem. There are enhancements to this attack that allow Mallory to send Bob a different message from the one he eavesdropped on. Never sign arbitrary messages from other people or decrypt arbitrary messages and give the results to other people. Foiling the Resend Attack
The attack just described works because the encrypting operation is the same as the signatureverifying operation and the decryption operation is the same as the signature operation. A secure protocol would use even a slightly different operation for encryption and digital signatures. Using different keys for each operation solves the problem, as does using different algorithms for each operation; as do timestamps, which make the incoming message and the outgoing message different; as do digital signatures with oneway hash functions (see Section 2.6). In general, then, the following protocol is secure as the publickey algorithm used: (1) Alice signs a message. (2) Alice encrypts the message and signature with Bob’s public key (using a different encryption algorithm than for the signature) and sends it to Bob. (3) Bob decrypts the message with his private key. (4) Bob verifies Alice’s signature. Attacks against PublicKey Cryptography
In all these publickey cryptography protocols, I glossed over how Alice gets Bob’s public key. Section 3.1 discusses this in detail, but it is worth mentioning here. The easiest way to ge...
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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.
 Fall '10
 ALIULGER
 Cryptography

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