New to Bitcoin? Start at 1.1
6.8Lesson 8 of 8
Proving a file existed
This lesson answers
- How can Bitcoin prove a document existed?
- What is a hash?
- Does timestamping a file put it on the blockchain?
- What does a Bitcoin timestamp prove?
Bitcoin can prove that a file existed by a certain time, without anyone seeing the file. Its fingerprint is written into a transaction, and the block that holds it fixes the date. Years later, anyone with the same file can check the fingerprint against the chain.
- Newest block now
- …a new stamp would go into a later one
- Average wait for a block
- …
Once the fingerprint is in a block, it is as hard to change as any payment in that block.
See it on satoshis.watch →Fingerprints, called hashes
A hash function turns any file, a contract, a photo, a song, into a short fixed-length code. The same file always gives the same code, but change even one letter and the code changes completely. And it only works one way: from the code you can't rebuild the file. Bitcoin uses the same kind of fingerprint, SHA-256, for its own blocks.
How a timestamp is made
- The file's fingerprint is worked out on your own computer. The file itself never leaves it.
- The fingerprint goes into a Bitcoin transaction, usually combined with thousands of others into a single code so one small payment covers them all.
- That transaction is mined into a block. You keep a small proof file showing how your fingerprint leads to the code in that block.
- To check, anyone takes the original file, the proof and the chain, and confirms they match.
Open standards such as OpenTimestamps do this batching, so the cost per file is close to nothing. Satoshi's paper described Bitcoin itself as a kind of timestamp server.
What it proves, and what it doesn't
| Proves | This exact file existed no later than the time of the block. |
|---|---|
| Doesn't prove | Who made the file, whether what it says is true, or that it didn't exist earlier. |
| Needs | The exact original file and the proof. Lose either, or change a single byte, and the timestamp can't be checked. |
The first block works the other way round: by including a newspaper headline from 3 January 2009, it shows it was made no earlier than that day. See what Bitcoin is.
Check yourself
Practice, not a test. Think of your answer first, then reveal it.
1Does a timestamp reveal what the file says?Reveal
No. Only the file's fingerprint goes on the chain, and the file can't be worked out from it.
2You change one word in a timestamped file. Does the old timestamp still prove it?Reveal
No. The changed file has a completely different fingerprint. The timestamp proves only the exact original.
3Does a timestamp prove you wrote the file?Reveal
No. It proves the file existed by a certain time, not who made it or whether what it says is true.
Sources
- Satoshi Nakamoto, Bitcoin: A Peer-to-Peer Electronic Cash System (2008), section 3, Timestamp Server.
- OpenTimestamps: an open standard for batching many fingerprints into one Bitcoin transaction.
- Newest block and block times: read from a Bitcoin node.