Ciphers

A cipher is an algorithm for performing encryption and decryption. It converts plaintext into ciphertext to conceal information, and the reverse process recovers the original message.

In modern usage, a cipher is the algorithm component of a cryptographic system. A usable system also requires keys, protocols, and careful operation; security arises from the combination, not from the algorithm alone.

Ciphers are often classified by operation. Substitution ciphers replace symbols; transposition ciphers rearrange them. They may be used in stream modes (bit-by-bit) or block modes (fixed-size blocks). Symmetric ciphers use the same key for encryption and decryption; asymmetric ciphers use different keys.

Classical examples include the Caesar shift, monoalphabetic substitutions, the Vigenère cipher, and the Playfair cipher. These illustrate historical methods and the emergence of cryptanalysis.

Modern ciphers include AES and DES as symmetric-key algorithms, and RSA and elliptic-curve cryptography as public-key algorithms. Real systems also combine ciphers with authentication and key-exchange protocols.

Key terms are plaintext, ciphertext, key, encryption, and decryption. Security goals include confidentiality, integrity, authenticity, and non-repudiation. Attacks range from brute-force searches to cryptanalysis and side-channel attacks.