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.
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 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.
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 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 |
External resources on OP_RETURN
Viewing OP_RETURN
- coinsecrets.org: An OP_RETURN transaction explorer]
- bitcoinstrings.com: A site showing raw strings in Bitcoin transactions