BITCOIN BLOCK HASH
LAST BLOCK · 0000000000…ee78f7
Every block gets a fingerprint. It is 64 characters long and it comes out of the block's own contents. Change one letter of one payment inside and the fingerprint changes completely. The one above belongs to block 965,970, found 12 minutes ago.
- BLOCK
- 965,970
- ZEROS AT THE FRONT
- 19
- FOUND MINUTES AGO
- 12
- PAYMENTS INSIDE
- 3,983
- ITS SIZE
- 1.69 MB
- DIFFICULTY IT BEAT
- 127.5 trillion
WHAT A HASH IS
Feed anything into the same recipe and 64 characters come out. Same input, same 64 characters, every time, on every computer in the world.
Change the input by one letter and what comes out looks completely different. There is no way to work backwards from the result to the input, and no way to aim for a particular result except by trying inputs until one lands.
That is a hash. Bitcoin uses it as the fingerprint of every block.
WHY IT STARTS WITH ZEROS
Look at the fingerprint above. It begins with 19 zeros. That is not luck.
A miner is allowed to change one meaningless number inside the block and hash the whole thing again. And again. Billions of times a second. They are hunting for a version whose fingerprint happens to start with enough zeros.
The more zeros the rules demand, the more tries it takes. That is the difficulty, and that is what the electricity buys. The zeros are proof the work was done, because there is no other way to get them.
WHY THE PAST IS HARD TO CHANGE
Every block carries the fingerprint of the block before it. That is what makes the chain a chain.
So if you edit an old payment, that block's fingerprint changes. The block after it is now pointing at something that does not exist. And so is the one after that, all the way to today.
To make your version stick you would have to redo the zeros for every block since, faster than the rest of the world is adding new ones. Not impossible on paper. Absurdly expensive in practice, and it gets worse every ten minutes.
QUESTIONS
What is a Bitcoin block hash?
The fingerprint of a block: 64 characters worked out from everything inside it. The latest one is 00000000000000000001a772b2deb5993999f503dd5ae3ef8c012db566ee78f7, for block 965,970.
Why does a Bitcoin block hash start with zeros?
Because miners keep re-hashing the block with a different throwaway number until the result happens to begin with enough zeros. The current block found 19. There is no shortcut, so the zeros are proof of how many tries it took.
What happens if you change an old Bitcoin block?
Its fingerprint changes, so the block after it stops pointing at anything real, and so does every block after that. Your version is rejected unless you redo all of that work faster than the whole network.
Can two Bitcoin blocks have the same hash?
In theory yes, in practice no. There are more possible fingerprints than there are atoms in the visible universe, and nobody has ever found two inputs that produce the same one.
Is a block hash the same as a transaction ID?
No, but they are made the same way. A transaction ID is the fingerprint of one payment. A block hash is the fingerprint of a whole block.
SEE IT RUNNING
The block hash is one of the nine parts of the machine on this site: a press that casts the last block's fingerprint into the face of the new one.
THE PARTS NEXT TO IT
- BLOCKCHAINThe line all those fingerprints hold together.
- MININGThe machines doing the guessing.
- MINING DIFFICULTYThe rule that decides how many zeros are needed.
SMALL PRINT
This page describes a public network. It is not financial advice and quotes no price. The block shown comes from public Bitcoin APIs and may be a few minutes old.