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Key K

Key K

FIGURE 11.2 Secret key cryptography

Key K

Key K

on large scale. This suggests adopting a standard mathematical algorithm that can be readily implemented. On the other hand, the algorithm must provide security to all of its users. It is here that the use of a key plays an essential role. The key must uniquely specify a particular variation of the algorithm that will produce secure ciphertext. Indeed the best algorithms should prevent an attacker from deriving the key even when a large sample of the plaintext and corresponding ciphertext is known. In general, the number of possible keys must be very large. Otherwise, a brute force approach of trying all possible keys may be successful.

Clearly, secret key cryptography addresses the privacy requirement. A message that needs to be kept confidential is encrypted prior to transmission, and any eavesdropper that manages to gain access to the ciphertext will be unable to access the contents of the plaintext message. The Data Encryption Standard (DES) is a well-known example of a secret key system and is discussed in a later section.

A traditional method of authentication involves demonstrating possession of a secret. For example, in a military setting a messenger might be confirmed to be authentic if he or she can produce the correct answer to the specific question. A similar procedure can be used over a network, using secret key cryptography. Suppose that a transmitter wants to communicate with a receiver as shown in Figure 11.3 and that the receiver and transmitter share a secret key. The transmitter sends a message identifying itself. The receiver replies with a message that contains a random number r. This action is called a challenge. The transmitter sends a response with an encrypted version of the random number. The receiver applies the shared key to decrypt the number. If the decrypted number is r, then the receiver knows that it is communicating with the given transmitter. If the transmitter also wishes to authenticate the receiver, the transmitter can then issue a challenge by sending its own random number. It is extremely important that the random numbers used in each challenge be different; otherwise, an eaves-


John to Jane, "let's talk"



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