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 .    
It was launched in 2013 as an open source project. It was developed by Manuel Araoz and Esteban Ordano. 
On May 21, 2017, the “Blockchain Notary Service” was acquired by Canaan .
- “I’m really happy to close this deal with Canaan,” Araoz said in a statement. “They’ve been around for years, and they’re making it hard to build the chips that secure the blockchain.” 
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 certain point in time. ”  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.”  On May 8, 2014 reporter Daniel Cawrey from CoinDesk wrote that the service ” Blueprint for a New Economy from O’Reilly Media , author Melanie Swan wrote it was “One of the first services to offer blockchain attestation”.  Blueprint for a New Economy from O’Reilly Media , author Melanie Swan wrote it was “One of the first services to offer blockchain attestation”. 
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 0x444f4350524f4f4 (which is ‘DOCPROOF’ when converted to UTF-8 / ASCII ) with the sha256sum of the document which exists at the time of the 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 does not hold any value. The other transaction outlets 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:
- Calculate the sha256sum σ of the document.
- Search the blockchain for an output script containing 0x444f4350524f4f4. Σ where the period is the concatenation operator. When searching for many documents, the process is sped up by first restricting the search to an OP_RETURN output script and 0x444f4350524f4f4 in it.
- 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 more than the median timestamp of previous 11 blocks, and less than the network-adjusted time + 2 hours “.  However, a more accurate estimate is necessary.
- Digital Sign Agreement without revealing actual content.
- Demonstrating data ownership without revealing actual data.
- Document time stamping.
- Proving ownership
- Checking for document integrity
- ^ Jump up to:a b Daniel Cawrey (May 8, 2014). “Bitcoin’s Technology Could Revolutionize Intellectual Property Rights” . CoinDesk .
- ^ Jump up to:a b Jeremy Kirk (May 24, 2013). “Could the Bitcoin network be used as an ultrasecure notary service?” . IDG News Service via Computerworld .
- ^ Jump up to:a b Melanie Swan (2015). “Blockchain: Blueprint for a New Economy” . O’Reilly Media . pp. 38-39.
- Jump up^ Ram (February 2, 2015). “Anonymous Proof Of Existence – Bitcoin Tech” . Newsbtc.com .
- Jump up^ https://proofofexistence.com/about
- Jump up^ Higgins, Stan (2017-05-21). “Bitcoin Miner Canaan Acquires Blockchain Notary Service” . Coindesk . Coindesk . Retrieved 2017-05-26 .
- Jump up^ Rob Wile (April 22, 2014). “SATOSHI’S REVOLUTION: How The Creator Of Bitcoin May Have Stumbled Onto Something Much, Much Bigger” . Business Insider .
- Jump up^ https://en.bitcoin.it/wiki/Block_timestamp