Cipher

Cryptography works by the use of ciphers, which is a pair of functions $E$ to encrypt and $D$ to decrypt. Unencrypted messages are called plaintexts, $P$. Encrypted messages are called ciphertexts, $C$.

$$\begin{array}{rl}C& =E(P)\\ P& =D(C)\end{array}$$

However, ciphers are well-known bits of mathematics, we need to instead parameterise the cipher by a secret key $K$. And design it to twiddle bits in ways that depend on $K$.

$$\begin{array}{rl}C& =E(P,K)\\ P& =D(C,K)\end{array}$$

This only works if $K$ is kept secret.