You could utilise eternitywall.
The have an electrum plugin that streamline the process.
Read more here:
sign-to-contract
How to timestamp with zero marginal cost
If in reality you don't want to actually write on the blockchain, but have a proof of existence (saving on the blockchain the hash of a document, instead of the document itself), you can try https://opentimestamps.org/
It is basically an alternative to the website provided above by ETFbitcoin.
Topic: How to write a message to the Bitcoin Core Blockchain? (NOT BitcoinSV!) (Read 266 times)
February 27, 2020, 09:45:27 AM
February 27, 2020, 05:24:54 AM
As others mentioned, OP_RETURN is the answer. You could either
1. Use website / tools such as https://proofofexistence.com/ (even though it's specifically created hash of a file) and https://github.com/coinspark/php-OP_RETURN
2. Create it manually from Bitcoin Core CLI, see https://bitcoin.stackexchange.com/a/56416
If you want to write message which is far bigger than 80 bytes and you don't want or can't afford tons of Bitcoin, you should consider Ethereum.
1. Use website / tools such as https://proofofexistence.com/ (even though it's specifically created hash of a file) and https://github.com/coinspark/php-OP_RETURN
2. Create it manually from Bitcoin Core CLI, see https://bitcoin.stackexchange.com/a/56416
If you want to write message which is far bigger than 80 bytes and you don't want or can't afford tons of Bitcoin, you should consider Ethereum.
February 27, 2020, 02:16:55 AM
Does anyone know of another service like www.cryptograffiti.info that posts to the Bitcoin Core blockchain? I would like a similar simple interface if possible (to post a message and maybe a photo!). When researching, all articles pointed to Cryptograffiti.info but I failed to find notes or comments confirming it no longer supported BTC core and had moved on as BTC forked.
You can "post" your messages using a "text-to-HEX converter" & Electrum, and it will be saved to Bitcoin's blockchain after getting a confirmation, but the transaction can already be seen on blockexplorers right after it was broadcast.Although, you won't be able to see the messages directly like in that website. You'll have to convert the hex string into txt, image or other formats manually.
There are limitations though, it will be non-standard and wont easily propagate nor mined if it didn't follow any of the standard rules: Standard-transactions | Non-standard-transactions.
Like converting an image into hex, the size will not even fit the default relay size of 83bytes.
TL;DR: In comparison, that website is using BitcoinSV's chain that has a maximum OP_RETURN relay size of 100kB (just because?) while Bitcoin's default standard limit is 83 Bytes.
In short, something like that site that can 'casually' post long messages, codes, images, etc. using Bitcoin's [BTC] Blockchain isn't possible.
(Call bitcoin: Bitcoin, not Bitcoin Core, it's the full node client's name or a name of another misleading fork - BTCC).
(Do not try doing this in the mainnet without sufficient research, use testnet for your tests).
February 26, 2020, 10:31:57 PM
bitcoin core doesn't have a blockchain because it is a full node not a cryptocurrency and only cryptocurrencies have a blockchain of their own which full nodes download, verify and store.
FYI this is with a purpose and I do not intend to just spam the blockchian.
the site you linked doesn't seem to be doing anything special, if your "purpose" is serious then write your own code to simply add a new txout to your transactions that uses OP_RETURN script and then sign it with your favorite bitcoin client such as bitcoin core or electrum. then broadcast it to bitcoin network. 
February 26, 2020, 06:54:17 PM
Hi All,
I thought I was publishing a message to the original Bitcoin blockchain but after paying, it appears to be the the Bitcoin SV blockchain (I know, rookie mistake - sorry). I am a little disappointed to be honest because www.cryptograffiti.info does not really make it that clear (the fee flicks between BTC and BSV which just adds to the confusion for a newbie). I get it now that if I paid in BSV it was for the BSV blockchain, but I just thought the website preferred being paid in BSV for blockchain posting.
Does anyone know of another service like www.cryptograffiti.info that posts to the Bitcoin Core blockchain? I would like a similar simple interface if possible (to post a message and maybe a photo!). When researching, all articles pointed to Cryptograffiti.info but I failed to find notes or comments confirming it no longer supported BTC core and had moved on as BTC forked.
FYI this is with a purpose and I do not intend to just spam the blockchian.
Thank you in advance.
I thought I was publishing a message to the original Bitcoin blockchain but after paying, it appears to be the the Bitcoin SV blockchain (I know, rookie mistake - sorry). I am a little disappointed to be honest because www.cryptograffiti.info does not really make it that clear (the fee flicks between BTC and BSV which just adds to the confusion for a newbie). I get it now that if I paid in BSV it was for the BSV blockchain, but I just thought the website preferred being paid in BSV for blockchain posting.
Does anyone know of another service like www.cryptograffiti.info that posts to the Bitcoin Core blockchain? I would like a similar simple interface if possible (to post a message and maybe a photo!). When researching, all articles pointed to Cryptograffiti.info but I failed to find notes or comments confirming it no longer supported BTC core and had moved on as BTC forked.
FYI this is with a purpose and I do not intend to just spam the blockchian.
Thank you in advance.
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