When discussing encryption, there are a few terms with which you should be familiar. The "message" is the actual data for our concern, also frequently referred to as "plain text" (denoted as "M"). Although referred to as plain text, M is not necessarily ASCII text; it might be any type of unencrypted data. It is "plain" in the sense that it does not require decryption prior to use. The encrypted message is "cipher text" (denoted as "C").

Mathematically, encryption is simply a function from the domain of M into the range of C; decryption is just the reverse function of encryption. In practice, the domain and range of most cryptography functions are the same (that is, bit or byte sequences). We denote encryption with 'C = E(M)', and decryption with 'M = D(C)'. In order for encryption and decryption to do anything useful, the equality M = D(E(M)) will automatically hold (otherwise we do not have a way of getting plain text back out of our cipher text).