A hash is a fixed-size output produced by a one-way function that maps any input to a seemingly random value, used for integrity, addressing, and proof-of-work.
A cryptographic hash function deterministically maps input data of arbitrary length to a fixed-size output (the hash). Good hash functions are preimage-resistant, second-preimage-resistant, and collision-resistant. They are widely used for data integrity, digital signatures, block headers, Merkle trees, and mining.
"Bitcoin and Bitcoin Cash use double SHA-256 for block headers; changing any bit would produce a completely different hash."
"Hashes are used as addresses in content-addressed systems; the same content always produces the same identifier."
Key properties include avalanche effect (small input changes yield large output changes), uniform distribution, and computational efficiency. Hashes are not encryption—they cannot be reversed to reveal the original message.
"A 1-byte change in a transaction alters the entire block hash due to the avalanche effect."
"Collision resistance means it is infeasible to find two different inputs with the same hash."
In blockchains, hashes secure links between blocks, form Merkle roots of transactions, and gate mining difficulty targets. Wallets and signatures rely on hashing messages before signing to standardize inputs and protect against certain attacks.
"Miners vary a nonce to search for a block header hash below the difficulty target."
"ECDSA signs the hash of a message, not the raw message, for performance and safety."
All terms and definitions may update as the Cryptionary improves.