OP_RETURN ​

OP_RETURN is an opcode in bitcoin's scripting system that allows for the inclusion of a small amount of arbitrary data in a transaction. This opcode immediately marks the transaction output as unspendable, ensuring that the output can't be used as an input in a subsequent transaction. It's primarily used to embed arbitrary data in the blockchain.

Operation ​

1. Terminate the script execution unsuccessfully. Any transaction output with this opcode is not spendable, but the data after OP_RETURN remains permanently on the blockchain. It's a way of declaring that the output can't be spent, and it's only here for the data.

Notes ​

• OP_RETURN is commonly followed by a pushdata operation that determines the length of the data to be embedded, and then the data itself.
• The maximum number of bytes that can follow an OP_RETURN opcode is 80.
• OP_RETURN outputs are used for various purposes, from proving the existence of a particular piece of data at a certain point in time to more complex protocols built on top of bitcoin.
• Historically, there were debates around the use of OP_RETURN because embedding data into the bitcoin blockchain adds to its size without directly supporting the transfer of bitcoin.

Examples ​

Example 1 ​

Embedding the string "HELLO" into the blockchain:

# ASM script
OP_RETURN OP_PUSHBYTES_5 48454c4c4f

# Raw script
6a0548454c4c4f

# Script result
Output is unspendable and contains the data "HELLO".