Can I leave a message on Bitcoin blockchain?

4 min readAug 3, 2020

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Hey!

Today Luca asks:

Can I write on a Bitcoin Blockchain? Can I drop down a message?

Yes Luca of course you can! You can leave a message on a Bitcoin blockchain using a particular op code, called OP_RETURN.

Through this operation code your UTXO became unspendable! You can use at most 80 bytes for your message (160 hexadecimal character)!

This operation is know as “timestamping of write information” because the message will become part of a block with a timestamp as well.

Keep in mind Luca that everytime you add some information (bytes) in your transaction, it grows and you need more fees in order to send the transaction.

When a message left on the Blockchain can be useful?

For instance, if you want to leave a fingerprint’s document (check this article about it), you can do it with a timestamp.

Ok, it’s a showtime!

We will use a testnet to leave an amazing message on a Bitcoin blockchain. Then We’ll use an explorer to find it.

In Action

Can I write on Bitcoin Blockchain? — Italian Language 🇮🇹

First of all, I get a SegWit address from my testnet node.

$ bitcoin-cli getnewaddress “” “bech32”tb1qrggdlvezgd4uy9mntz50mpmwd6l4vk9rm4ft3d

I need bitcoins! I need to use a faucet service like: https://bitcoinfaucet.uo1.net/send.php

After that I can check my mempool and find my txid.

$ bitcoin-cli getrawmempool | grep c5ce66d638f1b8ca702dfb8f7d1da7a6707d9c6497212dc66829c99f69b28b9a

the c5ce66d638f1b8ca702dfb8f7d1da7a6707d9c6497212dc66829c99f69b28b9a is the faucet’s transaction.

When can I use my UTXO? I can use it after mining process. Check this article about PoW to understand it better!

Please recover my UTXO with listunspent command.

$ bitcoin-cli listunspent 1 101 ‘[“tb1qrggdlvezgd4uy9mntz50mpmwd6l4vk9rm4ft3d”]’ | jq[{“txid”: “c5ce66d638f1b8ca702dfb8f7d1da7a6707d9c6497212dc66829c99f69b28b9a”,“vout”: 1,“address”: “tb1qrggdlvezgd4uy9mntz50mpmwd6l4vk9rm4ft3d”,“label”: “”,“scriptPubKey”: “00141a10dfb322436bc2177358a8fd876e6ebf5658a3”,“amount”: 0.00100000,“confirmations”: 6,“spendable”: true,“solvable”: true,“desc”: “wpkh([3a46ecca/0'/0'/4']020d12775323bbdaf0cb6e9a2b44ae7a591ef5872364e80e363a93d283c10b9e4f)#kxjva7dw”,“safe”: true}]

Nice! I need to retrive the private key, as we know it’s a mandatory to sign my transaction.

$ bitcoin-cli dumpprivkey tb1qrggdlvezgd4uy9mntz50mpmwd6l4vk9rm4ft3dcPHTHs7ERe6jDYiitj9eLVswsX3RpeKMB19eXYjpLb4CkEHd7drq

Now, we are able to create an amazing message, for instance: corsocompleto.bitcoininaction.com. Yep we need to use hexadecimal!

$ printf “corsocompleto.bitcoininaction.com” | xxd -ps636f72736f636f6d706c65746f2e626974636f696e696e616374696f6e2e636f6d

I have the necessary to create the transaction!

Using help I can retrieve a lot of information about this method. I want to use it on createrawtransaction.

$ bitcoin-cli help createrawtransaction

The receiver is the faucet’s address, I give back bitcoins :).

The address is:

2NGZrVvZG92qGYqzTLjCAewvPZ7JE8S8VxE

Finally I can build my transaction, in data field I can put my hexadecimal message. Remember that it will create an operation code OP_RETURN as we discussed above.

$ bitcoin-cli createrawtransaction ‘[{“txid”:”c5ce66d638f1b8ca702dfb8f7d1da7a6707d9c6497212dc66829c99f69b28b9a”,”vout”:1}]’ ‘[{“2NGZrVvZG92qGYqzTLjCAewvPZ7JE8S8VxE”:0.00099000},{“data”:”636f72736f636f6d706c65746f2e626974636f696e696e616374696f6e2e636f6d”}]’02000000019a8bb2699fc92968c62d2197649c7d70a6a71d7d8ffb2d70cab8f138d666cec50100000000ffffffff02b88201000000000017a914ffd0dbb44402d5f8f12d9ba5b484a2c1bb47da42870000000000000000236a21636f72736f636f6d706c65746f2e626974636f696e696e616374696f6e2e636f6d00000000

n.b. As you can see I have just one input, for that reason I don’t need of change address, because I don’t have any change! Furthermore, I don’t spend my entire output because I need to pay fees (input-output = fees).

Nice, we created the transaction data! Yeah!

The next step is to sign it and send it!

We need to use the signrawtransactionwithkey method.

$ bitcoin-cli signrawtransactionwithkey 02000000019a8bb2699fc92968c62d2197649c7d70a6a71d7d8ffb2d70cab8f138d666cec50100000000ffffffff02b88201000000000017a914ffd0dbb44402d5f8f12d9ba5b484a2c1bb47da42870000000000000000236a21636f72736f636f6d706c65746f2e626974636f696e696e616374696f6e2e636f6d00000000 ‘[“cPHTHs7ERe6jDYiitj9eLVswsX3RpeKMB19eXYjpLb4CkEHd7drq”]’ ‘[{“txid”:”c5ce66d638f1b8ca702dfb8f7d1da7a6707d9c6497212dc66829c99f69b28b9a”,”vout”:1,”scriptPubKey”:”00141a10dfb322436bc2177358a8fd876e6ebf5658a3",”amount”:0.00100000}]’{“hex”: “020000000001019a8bb2699fc92968c62d2197649c7d70a6a71d7d8ffb2d70cab8f138d666cec50100000000ffffffff02b88201000000000017a914ffd0dbb44402d5f8f12d9ba5b484a2c1bb47da42870000000000000000236a21636f72736f636f6d706c65746f2e626974636f696e696e616374696f6e2e636f6d0247304402205688399cb5a230f050330e2bc6d04d9864d459f85fec48a0118ca31be9239d530220228d7c04fe9e6eea3690033c01ed222284efaa01b28a9a7cae809bdb32d7ce7a0121020d12775323bbdaf0cb6e9a2b44ae7a591ef5872364e80e363a93d283c10b9e4f00000000”,“complete”: true}

Nice, the signature is valid, now we gonna send the transaction with sendrawtransaction method.

$ bitcoin-cli sendrawtransaction 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

We obtained the transaction id.

We can use a block explorer (https://tbtc.bitaps.com/) with the transaction id in order to find out the transaction and check its structure.

Our message! Amazing Bitcoin course 🚀 — corsocompleto.bitcoininaction.com

The transaction has been sent properly and now it’s waiting for mining.

It contains two outputs. One of them is for faucet’s address, the other one is an unspendable UTXO, because it contains our message with operation code: OP_RETURN.

Through this example, We can understand which operation code use to “write” on Bitcoin blockchain.

You can find the code in our GitHub!

Do you want more?

You rock 🎸!

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