Cryptography is the practice of encryption and decryption. The purpose is simple: encrypted data is encrypted so that if someone sees the data he can't do anything with it. It holds no value for that person, and the intended participant should be the only person or entity that can decrypt the data, specifically because he has the key to do so. A system for encrypting and decrypting data is a cryptosystem. A cryptosystem is comprised of one or more algorithms used to combine original data ("plaintext") with one or more "keys" numbers or strings of characters known only to the sender and/or recipient. The resulting output is encrypted data known as cipher-text. From here on out, any data referred to as plaintext is not encrypted and is deemed insecure. Plaintext data refers to the state of encryption and not to the content of the data. Plaintext data isn't necessarily text or ASCII or Unicode. It could be a binary program, a music file, or even a stream of bytes floating across the network.

All data referred to as cipher-text is encrypted, and is only as secure as the cryptosystem that was used to encrypt the original plaintext data.

The security of a cryptosystem usually depends on the secrecy of one or more keys rather than the supposed secrecy of the algorithm. A strong cryptosystem has a large range of possible keys so it is impossible to try all of them (a brute force approach).