Block cipher

As explained in Real-World Cryptography

A block cipher is a keyed symmetric cipher that encrypts fixed-size blocks of data.

A block cipher is a keyed symmetric cipher that encrypts fixed-size plaintext blocks into equally sized ciphertext blocks. AES is the central example: it accepts 128-bit plaintext and returns 128-bit ciphertext, while decryption reverses the mapping. section 4.2.2 section 4.2

A block cipher works through a few linked ideas:

• A key selects a reversible permutation from every possible plaintext block to a corresponding ciphertext block. section 4.2.2
• AES repeatedly applies round functions to a byte state, deriving distinct round keys from the main key. section 4.2.3
• SubBytes, ShiftRows, MixColumns, and AddRoundKey transform the state; repeated rounds create diffusion and the avalanche effect. section 4.2.3

Figure 4.2 A cipher with a key can be seen as a permutation: it maps all the possible plaintexts to all the possible ciphertexts.

Visualize encryption as a key-selected permutation connecting every plaintext block to one ciphertext block. section 4.2.2

How Real-World Cryptography covers it

How deep it goes

Defines the interface and permutation model, then sketches AES rounds but defers exact subfunction mechanics.

Examples

Uses AES-128-bit blocks, a 4-by-4 byte state, repeated rounds, and the four named AES round functions.

What you can do after

Evaluate how AES maps blocks, reverses encryption, and transforms state through keyed rounds.

How it connects

Connects block ciphers to AES, symmetric cryptography, pseudorandom permutations, key schedules, diffusion, and cryptanalysis.

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Also explained in

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  liveBookmanning.com
  Software Security for DevelopersA block cipher is an encryption algorithm that divides data into fixed-size blocks and transforms each block using a secret key.
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  Hacking CryptographyA block cipher is a keyed cryptographic algorithm that transforms fixed-size blocks of data.

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Related concepts

• AESincludes · AES is a standardized symmetric block cipher that encrypts and decrypts data in 128-bit blocks using 128-, 192-, or 256-bit keys.
• Davies–Meyer methodused by · The Davies–Meyer method is a compression-function construction that combines a block-cipher operation with a chaining value.
• message authentication codeused by · A message authentication code is a keyed cryptographic value generated from a message to verify its integrity and authenticity.
• stream ciphercontrasts with · A stream cipher is an encryption algorithm that combines plaintext sequentially with a generated keystream, typically using XOR.
• paddinguses · Padding is the addition of extra space, values, or data around or within a structure to satisfy size, alignment, layout, or processing requirements.

This explainer was generated by Manning's knowledge graph from the text of Real-World Cryptography. The section links point to the parts of the book it draws on.