Encryption is the science of making a text unreadable in order to pass it securely and secretly to the intended recipient, who uses a key to decrypt the message. An encryption algorithm is a defined series of steps for encrypting data. The original text is referred to as plaintext, and after the encryption algorithm has been applied to it, it is called ciphertext. The encryption algorithm may be referred to as a cipher, and is often given a name. For example, there is one encryption algorithm named Blowfish and another named AES (Advanced Encryption Standard), for example.
An encryption algorithm can depend on substitution, as in a substitution cipher, or transposition, as in a transposition cipher. Examples of early ciphers that are known relied on simple encryption algorithms. For example, Julius Caesar used a cipher that involved substitution in which he shifted the letters of the alphabet so that an A was represented by a D, a B was represented by an E, and so on. Since there are 26 letters in the alphabet, 25 ciphers of this kind can be created—the 26th would have A replace A, which isn’t very secret. The key to this cipher can be thought of as the number of places shifted.
It is not necessary, however, for an encryption algorithm to use letters. It can use numbers and other symbols as substitutes for letters. In the short story “The Gold Bug,” Edgar Allen Poe used a substitution cipher in which the encryption algorithm involves this type of substitution, using symbols.
If one were to solve this cipher, one would see that Poe did not assign the symbols to the letters in an orderly fashion with A = 1, B = 2, etc. In this way, the encryption algorithm is a bit more complicated than the alphabet-shifting algorithm. If you figure out one letter in an alphabet shift, then you can easily derive them all. With Poe’s algorithm, however, knowing that A = 5, tells you nothing about B or any other letter.
The encryption algorithms used in modern applications, like securing credit card transactions on the Internet, are much more complex. The keys can be as long as 256 bits or longer. If they are to be cracked, it will only be with the help of computers. If one were to obtain the cipher key, however, the ciphertext could be read immediately.