Hash
A hash is a short, fixed-length fingerprint of data. Blockchains use hashes to link blocks, name transactions and prove work.
Photo: “Fingerprint” by kevin dooley, CC BY 2.0, via Flickr (edited: cropped and resized).
A hash is the output of a cryptographic hash function: it takes any amount of data and deterministically produces a fixed-length result, and changing even one bit of the input changes the hash completely [1]. Bitcoin uses SHA-256 [2]; Ethereum uses Keccak-256 [3].
Key points
- 1Same input, same hash, every time. Different input, unrelated-looking hash.
- 2You cannot work backwards from a hash to the data, or tweak data to hit a chosen hash.
- 3Blockchains use hashes to chain blocks, identify transactions, build addresses and run proof of work.
How does a hash work?#
A hash function is a computer algorithm that turns input of any size into an output of fixed length, the data’s “hash”. The Bitcoin Wiki notes two properties that make it useful: change any single bit of the input and the hash completely changes, and because the hash looks random it is prohibitively difficult to produce a specific hash by changing the data [1]. ethereum.org calls a hash a fixed-length fingerprint of variable-size input [3].
| Step | Value |
|---|---|
| “TokenTrail” | 3fada4676faea32c… |
| “tokentrail” (lower case) | 6ffba5f0d2efa83d… |
| “TokenTrail.” (added full stop) | c3279b014d0054c7… |
Three tiny edits, three unrelated fingerprints. That is why a stored hash proves data has not been altered.
Where do blockchains use hashes?#
- Linking blocks. Each Bitcoin block header holds the SHA256(SHA256()) hash of the previous header [4], so editing an old block breaks every link after it.
- Naming transactions. A txid is the double SHA-256 hash of a transaction [4].
- Proof of work. Miners search for a header whose hash is below a target; the whitepaper describes scanning for a value whose SHA-256 hash begins with enough zero bits [2].
- Addresses. Bitcoin and Ethereum addresses are built by hashing a public key; see address.
Which words are related?#
- What is a blockchain? — how hashes chain blocks together.
- Node — the computers that recompute hashes to check every block.
- Halving — another rule nodes enforce while validating blocks.
Frequently asked questions#
Is hashing the same as encryption?
No. Encryption is designed to be reversed with a key. A hash is one-way: there is no key that turns it back into the original data.
Can two different inputs have the same hash?
In principle, yes: there are endlessly many possible inputs but only a fixed number of outputs. What matters in practice is the property the Bitcoin Wiki describes — it is prohibitively difficult to produce a specific hash by changing the data [1].
What is Keccak-256?
The hash function Ethereum uses. ethereum.org notes it was standardised as SHA-3 [3].
Sources#
Grade A = primary source (regulator, protocol specification, client code, original author). Grade B = expert secondary source used for explanation only.
- BBitcoin Wiki. Vocabulary (Memory pool, Confirmation, Hash function, Node), 2026.
- ASatoshi Nakamoto. Bitcoin: A Peer-to-Peer Electronic Cash System, 2008.
- Aethereum.org. Ethereum Glossary, 2026.
- Abitcoin.org developer documentation. Bitcoin Developer Glossary, 2026.
- Abitcoin.org developer documentation. Developer Guide: Block Chain, 2026.