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Unformatted text preview: ocks. A 64-bit block of plaintext goes in one end of the algorithm and a 64-bit block of ciphertext comes out the other end. DES is a symmetric algorithm: The same algorithm and key are used for both encryption and decryption (except for minor differences in the key schedule). The key length is 56 bits. (The key is usually expressed as a 64-bit number, but every eighth bit is used for parity checking and is ignored. These parity bits are the least-significant bits of the key bytes.) The key can be any 56-bit number and can be changed at any time. A handful of numbers are considered weak keys, but they can easily be avoided. All security rests within the key. At its simplest level, the algorithm is nothing more than a combination of the two basic techniques of encryption: confusion and diffusion. The fundamental building block of DES is a single combination of these techniques (a substitution followed by a permutation) on the text, based on the key. This is known as a round. DES has 16 rounds; it applies the same combination of techniques on the plaintext block 16 times (see Figure 12.1). The algorithm uses only standard arithmetic and logical operations on numbers of 64 bits at most, so it was easily implemented in late 1970s hardware technology. The repetitive nature of the algorithm makes it ideal for use on a special-purpose chip. Initial software implementations were clumsy, but current implementations are better. Outline of the Algorithm
DES operates on a 64-bit block of plaintext. After an initial permutation, the block is broken into a right half and a left half, each 32 bits long. Then there are 16 rounds of identical operations, called Function f, in which the data are combined with the key. After the sixteenth round, the right and left halves are joined, and a final permutation (the inverse of the initial permutation) finishes off the algorithm. In each round (see Figure 12.2), the key bits are shifted, and then 48 bits are selected from the 56 bits of the key. The right half of the data is expa...
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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