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Security Fundamentals & Crypto · Advanced

Confusion and Diffusion

Shannon's two properties of secure ciphers: hide the key-ciphertext relationship and spread plaintext influence across ciphertext.

Definition

Confusion makes each ciphertext bit depend on the key in complex ways, typically through substitution (S-boxes). Diffusion spreads the effect of each plaintext bit over many ciphertext bits, typically through permutation and mixing. Modern block ciphers achieve both over multiple rounds.

Example

AES's SubBytes step provides confusion while ShiftRows and MixColumns provide diffusion.

Exam tip

Substitution = confusion; transposition/permutation = diffusion.

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