Proof of Existence

Proof of Existence is an online service that verifies the existence of computer files as of a specific time via timestamped transactions in the bitcoin blockchain.^[1][2]

It was launched in 2013 as an open source project. It was developed by Manuel Araoz and Esteban Ordano.^[3]

Reception

On May 24, 2013 reporter Jeremy Kirk from IDG News Service wrote that "It's essentially a notary public service on the Internet, an inexpensive way of using Bitcoin's distributed computing power to allow people to verify that a document existed at a certain point in time."^[1]

Terence Lee from Tech in Asia said, "Notaries — people with legal training that are licensed by the state to authenticate the signing of documents — could use this to timestamp contractual agreements."^[4] In November 2013, Proof of Existence also received attention in Spanish-language Genbeta in "Proof of Existence, certificando documentos con Bitcoin".^[5]

On April 22, 2014 reporter Rob Wile from Business Insider wrote that it is "Perhaps the most straightforward example of a post-Bitcoin service using Satoshi's blockchain".^[6] In her 2015 book Blockchain: Blueprint for a New Economy from O'Reilly Media, author Melanie Swan wrote it was "One of the first services to offer blockchain attestation".^[2]

Since 2014, O'Reilly author Andreas Antonopoulos uses Proof of Existence as an example in his book, Mastering Bitcoin.^[7]

Service

The service enters a sha256 cryptographic hash of a document into the blockchain.

The service costs 5 mBTC per use. As of this writing (2016-12-24), each use of the service creates 2 transaction outputs. One of them holds the identifier 0x444f4350524f4f46 (which is 'DOCPROOF' when converted to UTF-8 / ASCII) with the sha256sum of the document whose existence at the time of admission of the transaction into the blockchain is proven appended. This transaction output is provably unspendable because it's marked as such via an OP_RETURN at the beginning of the output script and doesn't hold any value. The other transaction output holds 4.9 mBTC and pays the service's operators. 0.1 mBTC is paid as a fee to the miner admitting the transaction into the blockchain.

To verify the existence of a document at the contended time, one proceeds as follows:

1. Calculate the sha256sum σ of the document.
2. Search the blockchain for an output script containing 0x444f4350524f4f46 . σ where the period is the concatenation operator. When searching for many documents, the process is sped up by first restricting the search space to transactions containing an OP_RETURN output script and 0x444f4350524f4f46 in it.
3. If the block time minus the block time variation is at most the time the document is claimed to have existed, the claim is valid.

Note that the block time of each block may be slightly inaccurate as it "is accepted as valid if it is greater than the median timestamp of previous 11 blocks, and less than the network-adjusted time + 2 hours".^[8] However, a more accurate time can be calculated from the surrounding blocks if increased precision is necessary.

Usage

• Digital Sign Agreement without revealing actual content.
• Demonstrating data ownership without revealing actual data.
• Document time stamping.
• Proving ownership.
• Checking for document integrity.

References

External links

• proofofexistence on GitHub

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