Avalanche Effect
The avalanche effect is when a 1-bit change in the input or key flips about half of the output bits, a key security property of block ciphers and hash functions.
Purpose
It ensures a cipher is extremely sensitive to tiny input changes so attackers cannot track outputs through local perturbations, reflecting confusion and diffusion combined.
Mechanism
The strict avalanche criterion (SAC) requires every output bit to flip with probability 0.5 whenever an input bit flips. After multiple AES rounds, a 1-bit change diffuses across all 128 bits, satisfying avalanche.
Use cases
- Block-cipher quality assessment
- Hash function design
- S-box evaluation metric