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

Certainly protocols should be secure against passive

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Unformatted text preview: rmance, corrupting existing information, or gaining unauthorized access to resources. Active attacks are much more serious, especially in protocols in which the different parties don’t necessarily trust one another. The attacker does not have to be a complete outsider. He could be a legitimate system user. He could be the system administrator. There could even be many active attackers working together. Here, the part of the malicious active attacker will be played by Mallory. It is also possible that the attacker could be one of the parties involved in the protocol. He may lie during the protocol or not follow the protocol at all. This type of attacker is called a cheater. Passive cheaters follow the protocol, but try to obtain more information than the protocol intends them to. Active cheaters disrupt the protocol in progress in an attempt to cheat. It is very difficult to maintain a protocol’s security if most of the parties involved are active cheaters, but sometimes it is possible for legitimate parties to detect that active cheating is going on. Certainly, protocols should be secure against passive cheating. 2.2 Communications Using Symmetric Cryptography How do two parties communicate securely? They encrypt their communications, of course. The complete protocol is more complicated than that. Let’s look at what must happen for Alice to send an encrypted message to Bob. (1) Alice and Bob agree on a cryptosystem. (2) Alice and Bob agree on a key. (3) Alice takes her plaintext message and encrypts it using the encryption algorithm and the key. This creates a ciphertext message. (4) Alice sends the ciphertext message to Bob. (5) Bob decrypts the ciphertext message with the same algorithm and key and reads it. What can Eve, sitting between Alice and Bob, learn from listening in on this protocol? If all she hears is the transmission in step (4), she must try to cryptanalyze the ciphertext. This passive attack is a ciphertext-only attack; we have algorithms that are resistant (as far as we know) to...
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