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Security Fundamentals & Crypto · Foundational

Cipher

An algorithm that performs encryption and decryption.

Definition

A cipher defines the mathematical transformations used to convert plaintext into ciphertext and back. Ciphers are categorized as symmetric or asymmetric, and symmetric ciphers as block or stream ciphers. Classical ciphers used substitution and transposition; modern ciphers combine many rounds of both.

Example

AES is a symmetric block cipher; ChaCha20 is a symmetric stream cipher.

Exam tip

A cipher suite is a set of algorithms negotiated in TLS, not a single cipher.

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