What is a hash function in blockchain?
Change one character of the input and the fingerprint changes beyond recognition. That one property holds much of a blockchain together.
On this page
A hash function turns data of any size into a fixed-length string called a hash or digest — a fingerprint of the data.[3] It is easy to compute, practically impossible to reverse, and any change to the input changes the output completely.[1] Blockchains use hashes to link blocks and derive addresses.[1]
What makes a hash function secure?
NIST's Secure Hash Standard, FIPS 180-4, describes digests as a way to detect whether a message has changed since its digest was made.[2] Its blockchain overview lists the three properties that make a cryptographic hash function trustworthy[1]:
| Property | In plain English |
|---|---|
| Preimage resistance | Given a hash, you cannot work out the input — it is one-way |
| Second preimage resistance | Given an input, you cannot find a different one with the same hash |
| Collision resistance | You cannot find any two inputs that share a hash |
SHA-256, the function NIST uses in its examples, always outputs 256 bits, written as 64 hexadecimal characters.[1] Ethereum uses a different function, Keccak-256, which ethereum.org's glossary lists as its hash function.[3]
What does a hash look like?
How do blockchains use hash functions?
NIST explains that each block header contains the hash of the previous block's header.[1] Alter an old block and its hash changes, which breaks the link in every block after it — that is what makes the chain tamper-evident.[1] Read more in what is a blockchain.
Hashes do other jobs too. A block's transactions can be summarised in a single root hash, for example with a Merkle tree.[1] Your address is derived by hashing a public key.[1] And in proof of work, miners keep changing a number called the nonce until the block header's hash falls below a target — hard to find, easy for everyone else to check.[1] See how Bitcoin mining works.
Questions readers ask
Is hashing the same as encryption?
No. Encryption is designed to be reversed by someone with the key; a cryptographic hash is designed to be one-way, with no key that turns the digest back into the input.[1]
Can two different inputs have the same hash?
In theory yes, because inputs are unlimited and outputs are fixed-length. A secure hash function makes finding such a pair computationally infeasible, which is what collision resistance means.[1]
A hash function is a one-way fingerprint machine. Its predictability for the same input and wild change for any edit are what let blockchains chain blocks together, derive addresses and run mining — see proof of work vs proof of stake.
Sources
- National Institute of Standards and Technology, NISTIR 8202: Blockchain Technology Overview (2018)Primary source
- NIST Computer Security Resource Center, FIPS 180-4, Secure Hash Standard (SHS) (2015)Primary source
- ethereum.org, Ethereum glossary (2026)Primary source
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Read next
- Crypto Basics · Beginner
What is a blockchain?
A record book copied across many computers, where changing old pages is easy to spot.
- Mining, Staking & Infrastructure · Beginner
How does Bitcoin mining work?
What miners compute, how difficulty keeps blocks near 10 minutes, and why most miners join pools.
- Glossary · Definition · Beginner
Address
The public string that receives coins and tokens, how it is derived and how to check it.
- Mining, Staking & Infrastructure · Beginner
Proof of work vs proof of stake
Two ways to pick who writes the next block — and what each one asks an attacker to put on the line.
