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

Explaining OP_RETURN

• 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