Entropy and Hashing Lab

A cryptographic hash function takes an input of any size and produces a fixed-size output (digest). SHA-256 always produces a 256-bit (64 hex character) output, regardless of whether you hash a single character or an entire file.

A key property of secure hash functions is the avalanche effect. Changing even a single bit of the input completely changes the output. This makes it impossible to predict how the hash will change or to find inputs that produce similar hashes.

• Pre-image resistance means you cannot work backwards from a hash to the input
• Second pre-image resistance means you cannot find a different input with the same hash
• Collision resistance means you cannot find two different inputs with the same hash
• Deterministic means the same input always produces the same hash

• Password storage uses slow password hashing and a salt, not a raw hash
• Data integrity uses a known hash to spot tampering
• Digital signatures usually sign a hash rather than a full document
• Blockchains link blocks using hashes as references