Glossary
Avalanche Effect
The avalanche effect is the property that flipping one input bit changes about half of the output bits — a hallmark of good diffusion in a hash function.
The avalanche effect describes how a tiny change in the input — even a single bit — should flip roughly half of the output bits, in an unpredictable pattern. A function with a strong avalanche effect leaves no usable statistical relationship between similar inputs and their digests.
It is a consequence of good diffusion: each round of a hash spreads every input bit across the entire internal state, so after enough rounds one flipped bit has propagated everywhere. Merkle–Damgård hashes like SHA-256 and sponge hashes like SHA-3 both aim for full avalanche.
You can see it directly: hash password and then Password in the generator — the two digests share essentially nothing, even though the inputs differ by one bit.
For the underlying theory, read how cryptographic hashing works.