Mastering Algorithms with C

Chapter 15. Data Encryption

Data encryption, or cryptography, is the science of secrecy. Its purpose is to keep information in the hands of those who should have it and out of the hands of those who should not. Considering such a statement, it probably comes as no surprise that cryptographic algorithms, called ciphers , historically have had profound political, social, and ethical implications. Data encryption, like data compression, is another product of information theory, an area of mathematics that addresses various ways to manage and manipulate information. Data encryption entails two processes: in one process we encipher recognizable data, called plaintext, into an unrecognizable form, called ciphertext ; in a second process we decipher the ciphertext back into the original plaintext. The main idea behind a cipher is that the transformation from ciphertext to plaintext should be easy if we are allowed to read the data, yet impractical if we are not.

Ciphers use a special piece of information, called a key, for security. Once a key has been used to encipher some data, only someone who knows the correct key can decipher it. In fact, a fundamental characteristic of any good cipher is that its security revolves around a key, or even several. Furthermore, the security of a good cipher does not rely on keeping the cipher’s algorithm a secret. This idea is similar to the security offered by a safe: even though everyone knows how a safe works, we cannot get inside without the combination ...