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

On a secure telephone the key should exist for the

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Unformatted text preview: s to generate a new key from the old key; this is sometimes called key updating. All it takes is a one-way function. If Alice and Bob share the same key and they both operate on it using the same one-way function, they will get the same result. Then they can take the bits they need from the results to create the new key. Key updating works, but remember that the new key is only as secure as the old key was. If Eve managed to get her hands on the old key, she can perform the key updating function herself. However, if Eve doesn’t have the old key and is trying a ciphertext-only attack on the encrypted traffic, this is a good way for Alice and Bob to protect themselves. 8.7 Storing Keys The least complex key storage problem is that of a single user, Alice, encrypting files for later use. Since she is the only person involved, she is the only person responsible for the key. Some systems take the easy approach: The key is stored in Alice’s brain and never on the system. Alice is responsible for remembering the key and entering it every time she needs a file encrypted or decrypted. An example of this system is IPS [881]. Users can either directly enter the 64-bit key or enter the key as a longer character string. The system then generates a 64-bit key from the character string using a key-crunching technique. Another solution is to store the key in a magnetic stripe card, plastic key with an embedded ROM chip (called a ROM key), or smart card [556,557,455]. A user could then enter his key into the system by inserting the physical token into a special reader in his encryption box or attached to his computer terminal. While the user can use the key, he does not know it and cannot compromise it. He can use it only in the way and for the purposes indicated by the control vector. A ROM key is a very clever idea. People understand physical keys, what they signify and how to protect them. Putting a cryptographic key in the same physical form makes storing and protecting that key more intuitive. This technique is made more secure by splitt...
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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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