Glossary

Hash

Plain-language definition Crypto glossary

Key takeaways

A hash is a fixed-length string produced by running data through a cryptographic hash function, acting as a unique digital fingerprint of that data.
The same input always produces the same hash, but even a tiny change to the input yields a completely different output, and the process cannot practically be reversed.
Hashes are fundamental to blockchains: each block is identified by its hash and references the previous block's hash, which makes the chain tamper-evident.

Definition

A hash is a fixed-length string of characters produced by running data through a cryptographic hash function. It acts as a unique digital fingerprint of that data.

How it works

A hash function takes an input of any size and returns an output of fixed length. The same input always yields the same hash, but even a tiny change to the input produces a completely different result, and the process cannot practically be reversed to recover the original data. These properties make hashes ideal for verifying integrity.

Why it matters

Hashes are fundamental to blockchains: each block is identified by its hash and references the previous block’s hash, which is what makes the chain tamper-evident. Proof-of-work mining is essentially a race to find an input that produces a hash meeting a target.

Example

Changing a single character in a block’s data would change its hash entirely, instantly signalling that the data was altered.

FAQ

Frequently asked questions

What makes a hash useful for blockchains?

Because the same input always yields the same hash and any change produces a completely different one, a hash works as a tamper-evident fingerprint. Each block references the previous block's hash, so altering an old record would break every block that follows, instantly signalling that something was changed.

Can a hash be reversed to recover the original data?

No, not in practice. A cryptographic hash function is designed to be one-way, so you cannot work backwards from the output to reconstruct the input. This is precisely why hashes can verify data integrity without exposing the underlying data itself.

How is hashing related to mining?

Proof-of-work mining is essentially a race to find an input that produces a hash meeting a target value. Miners repeatedly hash candidate data until one of them stumbles on a qualifying result, which is what secures the network and earns the block reward.