Cryptography

Cryptography is the science of encrypting, or hiding, information—something people have sought to do since they began using language. Although language allowed them to communicate with one another, people in power attempted to hide information by controlling who was taught to read and write. Eventually, more complicated methods of concealing information by shifting letters around to make the text unreadable were developed.
The Spartans of ancient Greece would write on a ribbon wrapped around a specific gauge cylinder. When the ribbon was unwrapped, it revealed a strange string of letters.
The message could be read only when the ribbon was wrapped around the same gauge cylinder. This is an example of a transposition cipher, where the same letters are used but the order is changed.
The Romans typically used a different method known as a shift cipher. In this case,
one letter of the alphabet is shifted a set number of places in the alphabet for another letter. A common modern-day example of this is the ROT13 cipher, in which every letter is rotated 13 positions in the alphabet: n is written instead of a, o instead of b, and so on.
These ciphers were simple to use and also simple to break. Because hiding information was still important, more advanced transposition and substitution ciphers were required. As systems and technology became more complex, ciphers were frequently automated by some mechanical or electromechanical device. A famous example of a modern encryption machine is the German Enigma machine from World War II. This machine used a complex series of substitutions to perform encryption, and interestingly enough it gave rise to extensive research in computers.
Cryptanalysis, the process of analyzing available information in an attempt to return the encrypted message to its original form, required advances in computer technology for complex encryption methods. The birth of the computer made it possible to easily execute the calculations required by more complex encryption algorithms. Today, the computer almost exclusively powers how encryption is performed. Computer technology has also aided cryptanalysis, allowing new methods to be developed, such as linear and differential cryptanalysis. Differential cryptanalysis is done by comparing the input plaintext to the output ciphertext to try and determine the key used to encrypt the information.
Linear cryptanalysis is similar in that it uses both plaintext and ciphertext, but it puts the plaintext through a simplified cipher to try and deduce what the key is likely to be in the full version of the cipher.
This chapter examines the most common symmetric and asymmetric algorithms in use today, as well as some uses of encryption on computer networks.