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 40 bytes, but does not provide a way to create OP_RETURN transactions.
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, and OP_RETURN is an efficient way to do it. [bitcointalk discussion]
Resources on OP_RETURN
- coinsecrets.org: An OP_RETURN transaction explorer
- 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
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 registry of OP_RETURN prefixes.
|Prefix (Ascii-Encoded)||Protocol/Owner||Brief description|
|?||Open Assets||Issuance and transfer of user-created assets|
|MG||Monegraph||Digital work licensing|
|ASCRIBE||ASCRIBE||Digital work licensing|