If I want to prove that I wrote a particular document before a certain point in time, it would be sufficient to generate the sha256 hash of the document and somehow arrange for that hash to appear in the blockchain.

Is there an easy way to do that simply by sending a transaction and leaving it to the miners to arrange for it to get into the blockchain for me?

One way that occurs to me is that I can treat the hash as a private key (bitaddress.org will tell me the address and private key that corresponds to any given input), and send a Bitcoin to the corresponding address. Then when I publish the document anyone can go through the same process and see the date of the transaction in the blockchain.

That seems a little convoluted. Is there a simpler way? Ideally free, not computationally intensive, and not requiring any special software to make the timestamp or to verify it.


You can encode any arbitrary data using the Bitcoin script's OP_PUSH and OP_DROP commands. For this, however, you would need some custom Bitcoin client, as the standard one does not allow you to send messages like this.

Alternatively, you could just use the hash as a part of the generation of a Bitcoin address and store it in the Block chain by destroying one satoshi on it.

There should be a topic covering encoding of any data into the Blockchain somewhere on this StackExchange, so you can also have a look at that.

  • 2
    Thanks. Now that bitaddress.org has added a 'brain wallet' tab, I think it's easiest just to paste the document hash (or even the whole document if it's short enough) into the brain wallet tab, and deposit a coin to the resulting address. Then if I need to prove that the document existed at that time I can point people at bitaddress.org to validate it. – Chris Moore Jul 30 '12 at 20:32

One way to insert an arbitrary SHA-256 hash into the blockchain is by using it to generate a Bitcon address, then sending a very small amount of Bitcoin to that address. The Bitcoin wiki does a good job of covering the actual process of generating an address from an ECDSA private key here and here.

If you'd prefer not to have to perform the computation manually, however, there's a useful command-line utility called bitcoin-tool. Suppose we want to generate a Bitcoin address from the SHA-256 hash of the string "test":

$ ./bitcoin-tool \
  --input-file <(echo -n "test" | openssl dgst -sha256 -binary) \
  --input-format raw \
  --input-type private-key \
  --network bitcoin \
  --output-type all \
  --public-key-compression uncompressed

We would then proceed to send a small amount of Bitcoin to the address on the third line: 1HKqKTMpBTZZ8H5zcqYEWYBaaWELrDEXeE. Sure enough, this address has seen a lot of activity over the years. Unsurprisingly, it has a zero balance. If someone were to send coins to that address, we'd be able to spend them, since we have the private key.

Notice that, in the above example, we opted to output an uncompressed public key. We could have instead chosen to compress the public key, and the output would've been different:

$ ./bitcoin-tool \
  --input-file <(echo -n "test" | openssl dgst -sha256 -binary) \
  --input-format raw \
  --input-type private-key \
  --network bitcoin \
  --output-type all \
  --public-key-compression compressed

Note that the resulting address is different: 19eA3hUfKRt7aZymavdQFXg5EZ6KCVKxr8. However, it's still an equally valid Bitcoin address, and it too has seen a bit of activity over the years, though not nearly as much as the address associated with the uncompressed key.

It's important to understand that, even though the public keys and addresses are different, the private keys are the same. Observe that the last three lines of each output are identical:


Nonetheless, you should be careful when converting raw private keys to addresses, since you can easily end up with different addresses.

So far, we've covered converting an arbitrary SHA-256 hash into a private key, but we can also convert one into a public key. This may be more suitable for public proofs. For example, if you publish a document, and you want others to be able to verify that it existed prior to a certain point in time, you can take the SHA-256 hash of that file, convert it into a public key, then send some small amount of Bitcoin to the associated address. There is, in fact, a real life example of this. Isis Agora Lovecruft, a core developer for the Tor Project, was being harassed by the FBI. She documented this in a post on her blog. In the post, she wrote:

In case the FBI is seeking data on Tor users or Tor bridges, and especially in case the subpoena turns out to be sealed or accompanied by an NSL: the original published contents of this post are archived as a PDF here, and the RIPE160(SHA256(PDF)) is equal to 5541405e08048658cf457b3c59bf42a51f84a1a3 and hence Bitcoin address 18mnc4BCud3vjAdLbCc3QhyrjN84VTT1iM, in order to prove in a cryptographically verifiable manner that I published before that point in time.

In this case, the SHA-256 hash of the PDF was converted to a uncompressed public key, which we can verify like so:

$ ./bitcoin-tool \
  --input-file <(curl https://fyb.patternsinthevoid.net/blog.patterns-in-the-void-2016-04-30.pdf | openssl dgst -sha256 -binary) \
  --input-format raw \
  --input-type public-key-sha \
  --network bitcoin \
  --output-type address \
  --output-format base58check \
  --public-key-compression uncompressed
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100 4022k  100 4022k    0     0  1228k      0  0:00:03  0:00:03 --:--:-- 1229k

Sure enough, public-key-ripemd160.hex:5541405e08048658cf457b3c59bf42a51f84a1a3 matches the posted value of RIPE160(SHA256(PDF)), and address.base58check:18mnc4BCud3vjAdLbCc3QhyrjN84VTT1iM matches the posted Bitcoin address. Finally, the deposit of 0.001 BTC to this address on 2016-05-01 proves that the address existed as of that date. Since generating a PDF after the fact (i.e., to match an existing Bitcoin address) would require, at the very least, a successful preimage attack against full round SHA-256, the conjunction of these facts prove beyond any reasonable (cryptographically-informed) doubt that the document in question existed at least as early as 2016-05-01.


While I understand the temptation to re-/abuse the blockchain for such things, why not use a service which is built on purpose for this? Have a look at guardtime.com, specifically http://www.guardtime.com/software/developer-tools/


Please check out this blockchain timestamping tool, it does exactly what you require for free (this blockchain has 0 fees for writing this data), and is technically better.


There is no way to attach meta data to the block-chain without forming an alternate chain, which as ThePiachu pointed out requires a modified client. However, you could

  • Generate a wallet
  • Send 1 BTC to that wallet
  • Add both the public bitcoin wallet address and the block of your 1 BTC deposit
  • Use the private key of the Bitcoin wallet to sign the article contents

You can now prove to third parties you authored the document by sending 0.01 BTC to interested parties.

  • 2
    Completely misses the point. – Rotsor Aug 8 '12 at 19:29

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