Unformatted text preview: ve to write their own algorithms and implementations. If no one in the group is a good cryptographer, then they won’t know if they have a secure algorithm. Despite these major drawbacks, restricted algorithms are enormously popular for lowsecurity applications. Users either don’t realize or don’t care about the security problems inherent in their system. Modern cryptography solves this problem with a key, denoted by K. This key might be any one of a large number of values. The range of possible values of the key is called the keyspace. Both the encryption and decryption operations use this key (i.e., they are dependent on the key and this fact is denoted by the k subscript), so the functions now become: EK(M) = C DK(C) = M Those functions have the property that (see Figure 1.2): DK(EK(M)) = M Some algorithms use a different encryption key and decryption key (see Figure 1.3). That is, the encryption key, K1, is different from the corresponding decryption key, K2. In this case: EK1(M) = C DK2(C) = M DK2(EK1 (M)) = M All of the security in these algorithms is based in the key (or keys); none is based in the details of the algorithm. This means that the algorithm can be published and analyzed. Products using the algorithm can be massproduced. It doesn’t matter if an eavesdropper knows your algorithm; if she doesn’t know your particular key, she can’t read your messages. Figure 1.2 Encryption and decryption with a key. Figure 1.3 Encryption and decryption with two different keys. A cryptosystem is an algorithm, plus all possible plaintexts, ciphertexts, and keys. Symmetric Algorithms
There are two general types of keybased algorithms: symmetric and publickey. Symmetric algorithms, sometimes called conventional algorithms, are algorithms where the encryption key can be calculated from the decryption key and vice versa. In most symmetric algorithms, the encryption key and the decryption key are the same. These algorithms, also called secretkey algorithms, singlekey algorithms, or onekey algorithms, require that the sender and receiver agree on a key before they can communicate securely. The security of a symmetric algorithm rests in the key; divulging the key means that anyone could encrypt and decryp...
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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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