Adding Messages On The Bitcoin Blockchain - A How-To Guide

August 7th 2020

@bitcoin-in-actionBitcoin in Action

See on https://bitcoininaction.com

Hey!,

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Today Luca asks:

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

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

OP_RETURN

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

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

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In Action

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

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

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$ bitcoin-cli getnewaddress "" "bech32"
tb1qrggdlvezgd4uy9mntz50mpmwd6l4vk9rm4ft3d

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

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After that, I can check my mempool and find my TX ID.

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$ bitcoin-cli getrawmempool | grep c5ce66d638f1b8ca702dfb8f7d1da7a6707d9c6497212dc66829c99f69b28b9a

the c5ce66d638f1b8ca702dfb8f7d1da7a6707d9c6497212dc66829c99f69b28b9a is the faucet’s transaction.

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When can I use my UTXO?

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I can use it after the mining process.

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Please recover my UTXO with

listunspent

command.

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$ 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 retrieve the private key, as we know it’s mandatory to sign my transaction.

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$ bitcoin-cli dumpprivkey tb1qrggdlvezgd4uy9mntz50mpmwd6l4vk9rm4ft3d
cPHTHs7ERe6jDYiitj9eLVswsX3RpeKMB19eXYjpLb4CkEHd7drq

Now, we are able to create an amazing message, for instance: corsocompleto.bitcoininaction.com.

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Yep, we need to use hexadecimal!

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$ printf "corsocompleto.bitcoininaction.com" | xxd -ps
636f72736f636f6d706c65746f2e626974636f696e696e616374696f6e2e636f6d

I have the necessary to create the transaction!

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Using

help

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

createrawtransaction

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$ bitcoin-cli help createrawtransaction

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

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The address is:

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2NGZrVvZG92qGYqzTLjCAewvPZ7JE8S8VxE

Finally, I can build my transaction, in the data field I can put my hexadecimal message. Remember that it will create an operation code

OP_RETURN

as we discussed above.

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$ 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!

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Furthermore, I don’t spend my entire output because I need to pay fees (input-output = fees).

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Nice, we created the transaction data!

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Yeah!

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The next step is to sign it and send it!

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We need to use the

signrawtransactionwithkey

method.

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$ 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.

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$ bitcoin-cli sendrawtransaction 020000000001019a8bb2699fc92968c62d2197649c7d70a6a71d7d8ffb2d70cab8f138d666cec50100000000ffffffff02b88201000000000017a914ffd0dbb44402d5f8f12d9ba5b484a2c1bb47da42870000000000000000236a21636f72736f636f6d706c65746f2e626974636f696e696e616374696f6e2e636f6d0247304402205688399cb5a230f050330e2bc6d04d9864d459f85fec48a0118ca31be9239d530220228d7c04fe9e6eea3690033c01ed222284efaa01b28a9a7cae809bdb32d7ce7a0121020d12775323bbdaf0cb6e9a2b44ae7a591ef5872364e80e363a93d283c10b9e4f00000000

edee419f93521f43259b763ffb42e4b882504534494381b7e18057015a27c548

We obtained the transaction id.

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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.

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

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

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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: