avalancheeffect

The avalanche effect is a property of cryptographic algorithms in which a small change in the input—such as flipping a single bit—causes a substantial and unpredictable change in the output, typically affecting about half of the output bits. This behavior, called diffusion, is a core principle in secure design to ensure that each output bit depends on many input bits.

In practice, avalanche effects are desirable in block ciphers and hash functions. Within block ciphers, designers aim for each round to propagate input bit changes to many output bits, so that after several rounds a tiny modification yields a drastically different ciphertext. In hash functions, a single-bit alteration in the input should change roughly half of the output bits, making the hash appear random and preventing partial collisions.

Measuring avalanche involves perturbing the input and observing the Hamming distance between outputs, or performing statistical tests across many inputs. The property complements other security goals such as diffusion, nonlinearity, and key sensitivity. While a strong avalanche effect is necessary for resistance to differential cryptanalysis and related attacks, it is not by itself a guarantee of security; a design may exhibit good avalanche characteristics yet remain vulnerable if other weaknesses exist, such as poor key schedules or exploitable linear relations.