Entropy and Hashing Lab
Learn how hashing works and why tiny input changes can produce completely different outputs. Includes a simple entropy estimate and an avalanche demo.
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SHA-1 is included for learning and compatibility only. Use SHA-256, SHA-384, or SHA-512 for modern systems.
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Understanding Cryptographic Hashing
What is a Hash?
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.
Avalanche Effect
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.
Security Properties
- 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
Common Use Cases
- 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