--- title: "What Is a Hash? The Digital Fingerprint Behind Every Blockchain" url: https://bkvpool.com/blog/what-is-a-hash-blockchain/ date: 2026-11-13 author: Alex BKV category: Blockchain Basics --- # What Is a Hash? The Digital Fingerprint Behind Every Blockchain A hash is a short fingerprint of any data. Learn how it works, why one small change gives a totally new hash, and how blockchains use hashes to stay honest. When you leave a fingerprint on a glass, you leave a tiny pattern that belongs only to you. The fingerprint is not you. Nobody can rebuild your face from it. But if someone has your fingerprint on file, they can check in a second whether this glass was in your hand. Computers have the same trick for data. It is called a **hash**, and almost every part of a blockchain depends on it. If you have ever seen a long string like `c3004fc3f2039b84...` in a crypto wallet or explorer, you have already met one. ## What is a hash? A hash is a short, fixed-length code made from any piece of data. You put something in: a word, a contract, a photo, a whole film. A **hash function**, a special kind of math formula, turns it into a code. The code always has the same length, no matter how big the input was. A common hash function is SHA-256, used by Bitcoin and published as a standard by the US agency [NIST](https://csrc.nist.gov/pubs/fips/180-4/upd1/final). Its output is 256 bits, usually written as 64 letters and numbers. Here is a real example. I ran this sentence through SHA-256: `I owe Anna 50 euros.` The result: `c3004fc3f2039b8488db0173ca92410ea61bd5e784459ad7bf45888cddd6054c` ## What makes a good hash function? A hash function used in a blockchain must do a few things well. 1. **Same input, same output.** If you hash the same sentence tomorrow, on another computer, in another country, you get exactly the same 64 characters. 2. **A tiny change gives a totally different result.** Now I change one digit: `I owe Anna 60 euros.` The hash becomes: `6277ca836a206fa7c73580681acf29369793fe61f496114a6d31e3ef1e389dac` One number changed, and the hash has nothing in common with the first one. You cannot "nearly match" a hash. 3. **It only works in one direction.** From the hash, you cannot get back the sentence. The only way to find the input is to guess and try, again and again. 4. **Two different inputs should never give the same hash.** In theory it is possible, because there are endless inputs and a limited number of hashes. In practice, for a good function like SHA-256, nobody has ever found such a pair. ## How do blockchains use hashes? ### Every transaction gets a name When you send crypto, your transaction is hashed, and the result becomes its ID. That is the long code you can paste into an explorer to find your payment. Cardano uses a hash function called [BLAKE2b](https://www.blake2.net/) with a 256-bit output for this, so Cardano transaction IDs are also 64 characters long. ### Blocks are chained by hashes This is where the word "blockchain" comes from. Every block contains the hash of the block before it. So block 1,000 holds the fingerprint of block 999, which holds the fingerprint of block 998, and so on, back to the first block. Now imagine someone tries to change an old payment in block 500. The hash of block 500 changes. But block 501 still contains the old hash, so it no longer matches. To hide the change, the cheater would have to rebuild block 501, then 502, then every block after it, and convince thousands of independent computers to accept the fake version. That is why an old record on a big blockchain is so hard to change. ### Bitcoin mining is a hashing contest Bitcoin miners take a new block, add a random number, and hash it. They are looking for a hash that starts with a long row of zeros. Because you cannot predict a hash, the only way is to try billions of numbers per second. Whoever finds a winning hash first adds the block. That is what "proof of work" means. Cardano does not mine. It uses proof of stake instead, which I compared in [What Bitcoin and Cardano Have in Common](/blog/bitcoin-and-cardano-in-common/). ### Proving a document has not changed You can also use hashes outside of payments. On Cardano, every governance proposal links to a document, and the hash of that document is stored on the blockchain. If anyone edits the text later, the new hash will not match, and everyone can see it. I explained where that matters in [Where Does the Cardano Treasury Money Come From?](/blog/cardano-treasury-explained/). ## Hashes and your wallet address Your wallet address is also built with hashes. Your wallet has a secret key, and from it a public key is made. The address is mostly a hash of that public key, plus a few extra details. That is one reason a shared address does not reveal your secret key. But a hash does not make you anonymous. Your address is the same every time someone looks it up, so anyone can follow what it does on the public record. I wrote more about this in [Is Blockchain Private?](/blog/is-blockchain-private/). ## What a hash cannot do A hash is not encryption. Encryption locks data so the right person can unlock it later. A hash cannot be unlocked by anyone, ever. It only lets you check. A hash also says nothing about whether the data is true. If someone writes a lie and hashes it, you get a perfect fingerprint of a lie. The blockchain only promises that the record has not changed since it was written. Weak inputs are a problem too. If a password is short and common, attackers can hash millions of common passwords and compare. The hash function is fine, but the guessing is easy. This is the same reason a seed phrase must come from a proper wallet, not from words you pick yourself. See [What Is a Seed Phrase](/blog/what-is-a-seed-phrase/). And over time, hash functions can get old. Older functions like MD5 and SHA-1 were broken by researchers and are no longer trusted for security. That is why blockchains use modern ones, and why the experts keep testing them. ## Try it yourself On Linux or Mac, open a terminal and type: `echo -n "I owe Anna 50 euros." | sha256sum` (On a Mac, use `shasum -a 256` instead of `sha256sum`.) You should get the same 64 characters as above. Change one letter and run it again. Seeing a hash change completely with your own eyes explains it better than any article, including this one.
Hashes are what make Cardano's record hard to fake, and stake pools are the computers that check them every few seconds. If you want to join in, [BKV](https://bkvpool.com/#delegate) runs with a 0% margin, and your ADA stays safely in your own wallet. You can also hand your voting power to a DRep: I vote as **Alex BKV** on [cardanodrep.org](https://cardanodrep.org/) and publish a short reason with every vote. Only if it feels right: any honest pool and any active DRep make Cardano better.