Opcode Explained

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OP_RETURN ​

INFO

Opcode number: 106
Byte representation: 0x6a
Short description: Fail the script immediatly (must be executed).

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 size of the data push is 80 bytes.
  • This means that OP_RETURN can be at most 83 bytes: 1 byte for the opcode, 1-2 bytes for the data push, and 80 bytes of data.
  • 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:

shell
# ASM script
OP_RETURN OP_PUSHBYTES_5 48454c4c4f

# Raw script
6a0548454c4c4f

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