What is a hash?

Picture a file with all of a day’s transactions: how do you check whether someone changed even a single digit without comparing every line? You run the data through a cryptographic hash function and compare the result with its original fingerprint.

The same function, with exactly the same data, always produces the same hash. If even one character changes, the result usually looks completely different. Whether you process a single word or a large file, the length of the output stays fixed for that algorithm. Hashing doesn’t chop the file into pieces, despite what the word might suggest.

A hash works like a fingerprint, not a photocopy: it helps us check the data, but it doesn’t let us reconstruct the original content. With secure cryptographic functions, finding data that produces a specific hash is computationally extremely hard.

Why it matters

On a blockchain, every block includes a reference to the hash of the previous one. If someone tampers with older data, the related hashes change and the links no longer match. We see this ability to verify as a key advantage of the technology, but security doesn’t rest on hashes alone: it also takes the rules by which the network accepts blocks.

On Bitcoin, miners try different data in a block’s header, looking for a hash that meets the network’s difficulty target. In everyday use, you’ll also come across the transaction hash, or TXID: the identifier you use to look up a transaction in a block explorer.

What to watch out for

Hashing isn’t encryption that can be decrypted. Nor does it guarantee that the original data is true: a fake document has a hash too. Two different sets of data can, in theory, produce the same result, which is called a collision. Secure algorithms are designed so that finding such collisions is practically impossible.

Finally, a TXID isn’t a private key. On public networks, anyone who knows it can see the transaction’s public details, but can’t use it to access your coins.