OP_RETURN
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 burn bitcoins.
Currently, the default Bitcoin client relays OP_RETURN transactions up to 80 bytes [1], but does not provide a way for users to create OP_RETURN transactions.
Philosophy
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 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. Discussion on GitHub pull request
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.
Resources on OP_RETURN
- coinsecrets.org: An OP_RETURN transaction explorer
- python-OP_RETURN
- StackExchange: Explanation of what an OP_RETURN transaction looks like
- Metadata in the Blockchain: The OP_RETURN Explosion
- How to Put Custom Messages Into Bitcoin Blockchain - OP_RETURN
OP_RETURN prefixes
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!
| Prefix | Protocol |
|---|---|
| SPK | CoinSpark |
| DOCPROOF | Proof of Existence |
| CryptoTests- | Crypto Copyright |
| CryptoProof- | [Crypto Copyright http://crypto-copyright.com/ |
| BS | BlockSign |
| OA | Open Assets |
| STAMPD## | stampd |
| Factom!! | Factom |
| FACTOM00 | Factom |
| Fa | Factom |
| FA | Factom |
| tradle | Tradle |
| LaPreuve | LaPreuve |
| hex:5888 | Blockstore |
| hex:5808 | Blockstore |
| id | Blockstore |
| BITPROOF | Bitproof |
| S1 | Stampery |
| ASCRIBE | Ascribe |
| ProveBit | ProveBit |
| EW | Eternity Wall |
| CC | Colu |
| omni | Omni Layer |
| MG | Monegraph |
| RMBd | Remembr |
| RMBe | Remembr |
| ORIGMY | OriginalMy |