Cipher.

A cipher is the algorithm that performs encryption and decryption. Modern ciphers fall into symmetric (single shared key, e.g., AES, ChaCha20, 3DES) and asymmetric (public/private key pair, e.g., RSA, ECC) families, and into block ciphers (fixed-size blocks, e.g., AES-128) or stream ciphers (one bit/byte at a time, e.g., ChaCha20). A cipher is typically composed with a mode of operation (CBC, GCM, CTR) plus key-derivation, MAC, or AEAD construction to achieve confidentiality and integrity together.

From classical to standardized ciphers

Ciphers predate computing — substitution (Caesar), polyalphabetic (Vigenere), and rotor machines (Enigma) were all eventually broken, which is why modern designs are public and peer-reviewed rather than secret (Kerckhoffs's principle). AES was standardized in FIPS 197 (2001) after an open NIST competition and remains the default symmetric cipher. Older algorithms have been formally retired: NIST SP 800-131A Rev. 2 disallowed three-key 3DES for encryption after 31 December 2023, and RC4 was banned in TLS by RFC 7465 once keystream biases enabled plaintext-recovery attacks. Choosing a strong cipher today means AEAD constructions such as AES-GCM, AES-GCM-SIV (RFC 8452), or ChaCha20-Poly1305 (RFC 8439) with correct IV/nonce handling — nonce reuse in GCM is catastrophic, leaking the authentication key and enabling forgery. Cipher agility (the ability to swap algorithms without re-architecting) is now a design requirement, driven by the migration toward post-quantum primitives such as ML-KEM.

flowchart LR
  P[Plaintext] --> Enc["Cipher + mode (encrypt)"]
  K1[Key] --> Enc
  Enc --> C[Ciphertext]
  C --> Dec["Cipher + mode (decrypt)"]
  K2[Key] --> Dec
  Dec --> P2[Plaintext]
  C -. intercepted .-> A[Attacker sees only ciphertext]