Decryption is simply the reverse of encryption, the process by which ordinary data, or plain text, is converted into a cipher. A cipher, often incorrectly identified as a code, is a system in which every letter of a plain text message is replaced with another letter so as to obscure its meaning. To decipher a message requires a key, an algorithm that provides the method by which the message was encrypted.
Ciphers, algorithms, and keys. In one of the earliest and simplest ciphers, Julius Caesar sent messages in which each letter was substituted by the letter three places after it in the alphabet. In place of A, then, one would use a D. The key for such a cipher would be simply, "Shift right by three," or something similar.
A key is an algorithm, or a method for solving a mathematical problem by using a finite number of computations, usually involving repetition of certain operations or steps. An excellent example of an algorithm is f( x ) = y, a formula by which a relationship between two elements is shown on a Cartesian coordinate system. It is said that " y is a function of x, " meaning that for every value of x, there is a corresponding value of y. Suppose it is established that 2 x = y; then the key for the function has been established, and all possible values of x and y can be mapped.
Brute force and weak and strong encryption. In a simplified form, this is what occurs in decryption. The example shown is one that could easily be solved by what are called "brute-force" means. Brute force is a method of decryption in which a cryptanalyst, lacking a key, solves a cipher by testing all possible keys. This tends to be impractical for most ciphers without the use of a computer, and for the most sophisticated modern ciphers, brute force is all but impossible.
Suppose, however, one were shown a graph with the following coordinates for x and y: 1, 2; 2, 4; 3, 6, and so on. It would be fairly easy to determine from these values, using brute force, that 2 x = y, even if one did not have the key. This is an example of "weak" encryption. By contrast, some of the systems in use today for encryption of bank transactions or cellular phone communications and other purposes are extremely "strong". The ultimate example of strong encryption would be a situation in which decryption would be impossible without knowing the key.
Strong encryption is a controversial matter, due to the concerns of law-enforcement and intelligence authorities that such ciphers could be used by terrorists or other illegal groups. This has led to a move on the part of several governments, including that of the United States, to set up "key-escrow" arrangements, whereby all developers of ciphers would be required to give authorities a "back door" or key into the cipher. The government would maintain decryption keys in a secure location, and use them only when given a court order.
█ FURTHER READING:
Kahn, David. The Codebreakers: The Story of Secret Writing. New York: Macmillan, 1967.
Kippenhahn, Rudolf. Code Breaking: A History and Exploration. Woodstock, NY: Overlook Press, 1999.
Levy, Steven. Crypto: How the Code Rebels Beat the Government, Saving Privacy in the Digital Age. New York: Viking, 2001.
Schneier, Bruce. Secrets and Lies: Digital Security in a Networked World. New York: John Wiley, 2000.