Cipher

A cipher is a method for performing encryption, turning plaintext into ciphertext to conceal information. It can also refer to the specific algorithm or process that implements this transformation. In traditional usage, ciphers were algorithms used to hide messages, often relying on a key or secret parameter.

Classical ciphers include substitution ciphers, which replace elements of the plaintext with other symbols (such as the Caesar cipher that shifts letters), and transposition ciphers, which reorder characters. More sophisticated classic systems include monoalphabetic substitution and polyalphabetic methods like the Vigenère cipher.

In modern cryptography, ciphers are implemented as algorithms and organized into two broad families: symmetric and asymmetric. Symmetric (secret-key) ciphers use the same key to encrypt and decrypt, and include block ciphers (DES, AES) and stream ciphers (RC4, Grain). Asymmetric (public-key) ciphers use a key pair, enabling secure key exchange and digital signatures; common examples are RSA and elliptic-curve-based schemes.

Ciphers are used to achieve confidentiality, and when combined with authentication and integrity mechanisms to form cryptographic protocols. Security depends on properties such as key length, algorithm design, and proper key management. Cryptanalysis is the study of breaking ciphers, employing techniques like brute force, frequency analysis, and mathematical attacks. Distinctions are sometimes made between ciphers, encodings, and codes; a cipher is specifically a method for transforming text under a key to conceal meaning, not merely representation.