Cipher

A cipher is a pair of keyed algorithms that maps a plaintext to a ciphertext and recovers the plaintext from the ciphertext when supplied with the appropriate key.

A cryptographic key is a parameter selecting a particular transformation from a family of algorithms in cryptography. For an encryption family, correctness requiresfor every allowed plaintext $m$ and appropriate encryption/decryption keys. In symmetric-key cryptography, the communicating parties share a secret key. In public-key cryptography, the public encryption or verification key is paired with a private decryption or signing key.

For a one-bit example, $E_k(m)=m\oplus k$ and $D_k(c)=c\oplus k$ recover the message because applying exclusive or twice with the same bit cancels it. The one-time pad extends this to a uniformly random independent key as long as the message. Its secrecy depends on the key distribution and use, not on hiding the XOR algorithm. The number of possible keys gives a brute-force search space, but key length alone does not establish the security of a cryptographic construction.

Plaintext is the unencrypted message supplied to a cipher.

Ciphertext is the encrypted message produced by a cipher.