Encryption is the process of transforming normal text, called plaintext, into unreadable gibberish, called ciphertext. Decryption reverses this process, transforming ciphertext back into plaintext.
Figure 7-7 shows that both a key and algorithm are used to encrypt plaintext into ciphertext and to decrypt the ciphertext back into plaintext. Computers represent plaintext and ciphertext as a series of binary digits (0s and 1s). The key is also a string of binary digits of a fixed length; for example, a 128-bit key consists of a string of 128 0s and 1s.The algorithm is a formula for combining the key and text. Most document are longer then the key, so the computer first divides the plaintext or ciphertext into blocks, each block being of equal length to the key. It then applies the algorithm to each block of text.