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| OP_RETURN is a [[script]] opcode used to mark a transaction output as invalid. Since the data after OP_RETURN are irrelevant to Bitcoin payments, arbitrary data can be added into the transaction by following the OP_RETURN with an OP_PUSHDATA. Since any outputs with OP_RETURN are provably unspendable, OP_RETURN outputs can be used to [[Proof of burn|burn]] bitcoins. | | '''OP_RETURN''' is a [[script]] opcode used to mark a transaction output as invalid. Since any outputs with OP_RETURN are provably unspendable, OP_RETURN outputs can be used to [[Proof of burn|burn]] bitcoins. |
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| Currently, the default Bitcoin client relays OP_RETURN transactions up to 80 bytes [https://github.com/bitcoin/bitcoin/search?utf8=%E2%9C%93&q=MAX_OP_RETURN_RELAYa], but does not provide a way for users to create OP_RETURN transactions.
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| == Is storing data in the blockchain acceptable? == | | == Is storing data in the blockchain acceptable? == |
| Some members of the Bitcoin community believe that use of OP_RETURN violates the contract of Bitcoin, because Bitcoin was intended to provide a record for financial transactions, not a record for arbitrary data. Despite this, use of OP_RETURN may continue unabated because there is no easy way to stop people from embedding arbitrary data in the blockchain if they want to, and OP_RETURN is reasonably efficient in terms of [http://i.imgur.com/VAGZWBK.png data bytes stored as a fraction of blockchain space consumed]. Compared to some other ways of storing data in the blockchain, OP_RETURN has the advantage of not creating bogus UTXO entries. [https://github.com/bitcoin/bitcoin/pull/5286 Discussion on GitHub pull request]
| | Many members of the Bitcoin community believe that use of OP_RETURN is irresponsible in part because Bitcoin was intended to provide a record for financial transactions, not a record for arbitrary data. Additionally, it is trivially obvious that the demand for external, massively-replicated data store is essentially infinite. Despite this, OP_RETURN has the advantage of not creating bogus [[UTXO]] entries, compared to some other ways of storing data in the blockchain. |
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| From [https://bitcoin.org/en/release/v0.9.0#opreturn-and-data-in-the-block-chain Bitcoin Core release 0.9.0]: | | From [https://bitcoin.org/en/release/v0.9.0#opreturn-and-data-in-the-block-chain Bitcoin Core release 0.9.0]: |
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| Storing arbitrary data in the blockchain is still a bad idea; it is less costly and far more efficient to store non-currency data elsewhere. | | Storing arbitrary data in the blockchain is still a bad idea; it is less costly and far more efficient to store non-currency data elsewhere. |
| </blockquote> | | </blockquote> |
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| == OP_RETURN applications == | | == OP_RETURN applications == |
| OP_RETURN is used for writing human-language messages, digital asset proof-of-ownership, and storing data. Its use has been proposed for P2P application discovery. See the "prefixes" table below. | | OP_RETURN can be used for digital asset proof-of-ownership, and has at times been used to convey additional information needed to send transactions (see [[ECDH address]]). |
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| == OP_RETURN prefixes ==
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| Often, OP_RETURN transactions include a prefix to identify which protocol they belong to. There is no standardized method of claiming OP_RETURN prefixes, and not all OP_RETURN transactions use prefixes. At the time of writing, this wiki page is probably the most complete list of OP_RETURN prefixes. Note that this table is an attempt to catalog OP_RETURN prefixes that are already in use, *not* a system for reserving OP_RETURN prefixes!
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| {| class="wikitable"
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| |-
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| ! Prefix !! Protocol
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| | SPK || [http://coinspark.org/developers/ CoinSpark]
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| | DOCPROOF || [http://www.proofofexistence.com/ Proof of Existence]
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| |-
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| | CryptoTests- || [http://crypto-copyright.com/ Crypto Copyright]
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| | CryptoProof- || [Crypto Copyright http://crypto-copyright.com/
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| | BS || [http://blocksignit.com/ BlockSign]
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| |-
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| | OA || [https://github.com/OpenAssets/open-assets-protocol/blob/master/specification.mediawiki Open Assets]
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| |-
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| | STAMPD## || [http://stampd.io/ stampd]
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| |-
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| | Factom!! || [http://factom.org/ Factom]
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| |-
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| | FACTOM00 || [http://factom.org/ Factom]
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| |-
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| | Fa || [http://factom.org/ Factom]
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| | FA || [http://factom.org/ Factom]
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| |-
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| | tradle || [http://tradle.io/ Tradle]
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| | LaPreuve || [http://www.lapreuve.net/ LaPreuve]
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| | hex:5888 || [http://blog.onename.com/blockstore-bitcoin/ Blockstore]
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| | hex:5808 || [http://blog.onename.com/blockstore-bitcoin/ Blockstore]
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| | id || [http://blog.onename.com/blockstore-bitcoin/ Blockstore]
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| | BITPROOF || [https://bitproof.io/ Bitproof]
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| | S1 || [https://stampery.co/ Stampery]
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| | ASCRIBE || [https://www.ascribe.io/ Ascribe]
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| | ProveBit || [https://github.com/thereal1024/ProveBit ProveBit]
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| | EW || [http://eternitywall.it/ Eternity Wall]
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| | CC || [http://colu.co/ Colu]
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| | omni || [http://www.omnilayer.org/ Omni Layer]
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| | MG || [http://monegraph.com/ Monegraph]
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| | RMBd || [https://app.remembr.io/ Remembr]
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| | RMBe || [https://app.remembr.io/ Remembr]
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| | ORIGMY || [http://originalmy.com/ OriginalMy]
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| |}
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| == External resources on OP_RETURN ==
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| === Viewing OP_RETURN ===
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| * [http://coinsecrets.org/ coinsecrets.org]: An OP_RETURN transaction explorer]
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| * [http://bitcoinstrings.com/ bitcoinstrings.com]: A site showing raw strings in Bitcoin transactions
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| === Explaining OP_RETURN ===
| | {{DISPLAYTITLE:OP_RETURN}} |
| * [https://github.com/coinspark/python-OP_RETURN python-OP_RETURN]
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| * [http://bitcoin.stackexchange.com/questions/29554/explanation-of-what-an-op-return-transaction-looks-like StackExchange: Explanation of what an OP_RETURN transaction looks like]
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| * [http://www.slideshare.net/coinspark/bitcoin-2-and-opreturns-the-blockchain-as-tcpip Metadata in the Blockchain: The OP_RETURN Explosion]
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| * [http://wlangiewicz.com/blog/2014/10/24/how-to-put-custom-messages-into-bitcoin-blockchain-op-return/ How to Put Custom Messages Into Bitcoin Blockchain - OP_RETURN]
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OP_RETURN is a script opcode used to mark a transaction output as invalid. Since any outputs with OP_RETURN are provably unspendable, OP_RETURN outputs can be used to burn bitcoins.
Is storing data in the blockchain acceptable?
Many members of the Bitcoin community believe that use of OP_RETURN is irresponsible in part because Bitcoin was intended to provide a record for financial transactions, not a record for arbitrary data. Additionally, it is trivially obvious that the demand for external, massively-replicated data store is essentially infinite. Despite this, OP_RETURN has the advantage of not creating bogus UTXO entries, compared to some other ways of storing data in the blockchain.
From Bitcoin Core release 0.9.0:
This change is not an endorsement of storing data in the blockchain. The OP_RETURN change creates a provably-prunable output, to avoid data storage schemes – some of which were already deployed – that were storing arbitrary data such as images as forever-unspendable TX outputs, bloating bitcoin's UTXO database.
Storing arbitrary data in the blockchain is still a bad idea; it is less costly and far more efficient to store non-currency data elsewhere.
OP_RETURN applications
OP_RETURN can be used for digital asset proof-of-ownership, and has at times been used to convey additional information needed to send transactions (see ECDH address).