OP_RETURN

OP_RETURN is a script opcode used to terminate the script and return the top value on the stack. This opcode is analogous to the return function in programming languages. The OP_RETURN opcode has a controversial history in Bitcoin and as a result has been primarily used as a means of storing data on the blockchain.

In 2010 an exploit was discovered in Bitcoin where users could steal anyone's funds leveraging the OP_RETURN opcode. Satoshi Nakamoto modified the opcode to always return false as an urgent fix.

Return to original functionality

The Genesis upgrade in February 2020 restores the original functionality of the opcode. This will not impact the ability to store arbitrary data on-chain, and it is advised to now use False Return scripts for applications that still seek to do so.

The Genesis upgrade essentially re-enables the original functionality of the push data opcodes via lifting the maximum transaction size such that using a so-called 'OP_RETURN output' is no longer the only means of adding large data items such as JSON formatted text or images into a transaction.

Reverting to the ability to return a value will enable more robust scripts to be created; for example scripts can now return different values based on conditional logic.

OP_RETURN Functionality

OP_RETURN terminates the script leaving the stack as-is and letting the result on top of the stack determine the success or failure of the script. OP_RETURN can be used in combination with flow control opcodes to implement branching, conditional logic in scripts.

Example

The below script is a metanet node that attaches the Metanet data to a UTXO with funds. This can be used to ensure that metanet nodes are left in the UTXO set

𝑇𝑥𝐼𝐷𝑛𝑜𝑑𝑒
Inputs	Outputs
<sigmetaParent> <pubKeymetaParent> <sigscript> <pubKeyscript>	DUP HASH160 <pkhscript> EQUALVERIFY CHECKSIG RETURN <meta> <pkmetaNode> <txidmetaParent>