What is the "avalanche effect" of a cryptographic hash function, and what does the SHA-256 of "Hallo" versus "hallo" demonstrate?

Changing even a single bit of the input flips about half the output bits — so "Hallo" and "hallo" produce completely unrelated hashes.

sha256("Hallo") = 753692ec36adb4c794c973945eb2a99c1649703ea6f76bf259abb4fb838e013e
sha256("hallo") = d3751d33f9cd5049c4af2b462735457e4d3baf130bcbb87f389e349fbaeb20b9

One capital letter → an utterly different digest. This is the avalanche effect, and it's what makes a hash useful for integrity: any change to the data, however tiny, is obvious in the hash.

Other properties of a cryptographic hash:

• Fixed length output (SHA-256 → always 256 bits / 64 hex chars), regardless of input size.
• One-way (pre-image resistance) — you can't reverse the hash back to the data.
• Collision-resistant — practically impossible to find two inputs with the same hash.

Tip: A hash is a "digital fingerprint." Same file → same fingerprint; one byte different → totally different fingerprint.