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I am thinking of broadcasting a Bitcoin transaction with a very important message. The message is so important that I want it to be the only transaction (other than coinbase) in the block.

For example if my transaction were to be included in block 740000 than the block should contain only the

  1. Coinbase Transaction
  2. My Transaction with message.

I am willing to pay an additional fee of 1BTC but only after the block is mined.

Is there a way to make sure this?

2 Answers 2

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OP_RETURN

The generally approved way to include arbitrary content in a transaction is by use of an OP_RETURN script. See Explanation of what an OP_RETURN transaction looks like

There is nothing you can write in a Bitcoin script that checks the number of other transactions in a block. The only way to ensure exclusivity is to make the transaction sufficiently large.

Transaction size

To be sure your transaction is the only non-coinbase transaction in a block it would have to be large enough to almost fill the block. See What is the maximum size of a transaction?

Personally, I don't see how the presence of other transactions could in any way affect the perceived importance of a transaction, but I'll disregard that aspect.

Payment Conditions

To be sure that you only pay the miner after the transaction is included in a valid block, you only have to use one output of your transaction to pay the miner a large transaction fee. For example with inputs of 1.0001 BTC and an output to yourself of 0.0001 BTC. leaving 1.0000 BTC to be collected by the miner.

Minimum relay fee

Large transactions will not be relayed by many intermediary nodes unless the mining fee exceeds a minimum amount. I believe initially this was 1000 satoshis per vkByte. This could mean that you either need to pay more than 1 BTC or need to make a special arrangement with a miner to deliver your unconfirmed transaction to them using some direct method.

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Theoretically you could just jam some string as a "message" in the Tx Witness.

To answer your question though, if you're willing to pay the miner a whopping 1BTC fee they'd probably accept it, but if you want to claim the whole block to yourself the transaction's weight will have to be heavy enough that the miner is forced accept your transaction into their block exclusively should they choose to incorporate it.

Nonetheless though, unless the fee is huge, virtually no miner will accept the transaction.

I am willing to pay an additional fee of 1BTC but only after the block is mined.

Fees are always "paid" after the block is mined. A miner cant claim the fee if its an unmined block.

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