Library · Crypto, word by word · The basics
What is a hash?
A hash is a fixed-length fingerprint of any piece of data, produced by a hash function. Changing even a small part of the data produces a different hash. Blockchains use hashes to link each block to the one before it and to identify transactions.
What it means for you
A transaction hash is the public reference for a payment: pasting it into a block explorer shows whether it was confirmed. It is not a secret and cannot move funds, so sharing it is nothing like sharing a private key or seed phrase.
How it works
A hash function turns input of any size into an output of fixed size, and the US standard that defines SHA-256 describes digests as a way to detect whether data has changed since the digest was made. Blockchains use hashes everywhere: each Bitcoin block header includes the previous header's hash, and is itself hashed twice with SHA-256; proof of work means finding a header whose hash falls below a target, which takes trial and error but can be checked with a single hash. Ethereum addresses are the last 20 bytes of the Keccak-256 hash of a public key.
Sources: ethereum.org: Glossary, ethereum.org: Blocks, NIST FIPS 180-4: Secure Hash Standard, Bitcoin developer guide: Block chain, ethereum.org: Ethereum accounts · checked 4 October 2026
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