Mastering Algorithms with C

Description of DES

DES (Data Encryption Standard) is one of the most popular symmetric ciphers. DES is symmetric because it uses a single key both to encipher and decipher data. This is useful in situations in which parties that encipher data are allowed to decipher data as well. DES is a block cipher , which means that it processes data in fixed-size sections called blocks. The block size of DES is 64 bits. If the amount of data to be encrypted is not an even multiple of 64 bits, it is padded in some application-specific way.

DES is considered reasonably secure, and it runs fast, even in software. However, as with many ciphers, the security of DES has never been proven publicly. Nevertheless, the algorithm has stood up to years of cryptanalysis, which does suggest a certain level of confidence. Even so, as computing speeds continue to increase, DES becomes less and less secure. Today, its security is challenged regularly in contests that offer cash prizes to those who can crack messages encrypted with DES the fastest.

At its essence, the security of DES revolves around smoke and mirrors, or in cryptographic lingo, the principles of confusion and diffusion . The goal of confusion is to hide any relationship between the plaintext, the ciphertext, and the key. The goal of diffusion is to spread the effect of bits in the plaintext and the key over as much of the ciphertext as possible. Together, these make cryptanalysis very difficult.

With DES, we encipher a block of plaintext by ...