Knowra Block cipher Block cipher A block cipher is a symmetric-key algorithm that transforms fixed-length plaintext blocks into ciphertext blocks under a secret key. For each key, it acts as a reversible permutation on blocks of a fixed size.
Substitution–permutation network : A cipher design that repeatedly combines substitutions and permutations to spread and obscure input bits. AES uses this architecture to build its keyed block transformation.
Symmetric-key algorithm : A cryptographic algorithm in which communicating parties use the same secret key, or closely related keys. Block ciphers are one major family of algorithms built around a shared secret.
Advanced Encryption Standard : A standardized block cipher, also called Rijndael, with 128-bit blocks and 128-, 192-, or 256-bit keys. AES is the dominant modern example of a general-purpose block cipher.
Stream cipher : A symmetric-key cipher that generates a keystream combined with plaintext, typically one bit or byte at a time. Unlike a block cipher, it processes a continuous stream rather than fixed-size blocks.
Feistel network : A cipher structure that splits a block and repeatedly transforms one half using a round function. It enables reversible encryption even when the round function itself is not invertible.
Permutation : A bijection that rearranges the elements of a set without losing or duplicating any. For a fixed key, a block cipher maps every possible input block to a unique output block.
Data Encryption Standard : A historical 64-bit block cipher with a 56-bit effective key, now considered insecure against exhaustive search. DES shaped later cipher design but its short key made it obsolete.
One-time pad : An encryption method using a uniformly random secret key as long as the message, used only once. It achieves perfect secrecy under strict key conditions, unlike practical block ciphers.
Block cipher mode of operation : A method for applying a block cipher to data longer than one block. Modes determine how the primitive handles message length, repeated blocks, and initialization.
Confusion and diffusion : Design principles that obscure key relationships and spread each input bit's influence across outputs. Repeated rounds aim to hide plaintext structure and key patterns from attackers.
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