the only size limit is the block size, in other words you can inject a garbage data a little less than 1 MB in one transaction's output to the bitcoin blockchain. 20 byte (which is actually 40+OP_RETURN+push-byte size) is the standard rule that the bitcoin nodes are enforcing to reduce the chances of abuse. there is also transaction fees that will prevent such shenanigans to some extent.

the thing is that bitcoin is a payment system so it's "ledger" should only have to store payment data not arbitrary messages hence the "The best practice is don't."

I mean there are probably a few bits you could scrape from the lock time value but that'll also be very limited and it'll have to be a block height that has already passed...

20 bytes is what would be needed to store unique identifiers on the chain afaik so it's what you're going to have to put up with... But why you think others should download whatever junk you dump on there confuses me... Try it once or twice don't use 12+ kb of space that no one else can use for anything...

Are you sure it was the whitepaper? He published the title of a headline in the first block that was made (can't remember what it said now) if you open the first blk file bitcoin core downloads you can see what he wrote.

If you're after a way of storing data there are some solutions such as storj that link you up with people renting out hard drive space...

You can have other solutions as well such as only checking message that are from, to or contain a certain address... But you'd also have to check no one sabatages that so it might be better to just look at transactions FROM if you did try to go through with something like this.

Merging encoding and encryption functions is probably a good idea. As well as trying to symmetrically encrypt any private data, storing keys elsewhere...

Blockchain isn't designed for data storage. It can facilitate it but shouldn't be used directly for it... For the experienced - it's incredibly slow to access at least and for the novice it's just difficult to implement... Even not looking at the ethics of it it's still something that most consider.

time to wheel out the old lad

https://i.imgur.com/AqlQXCa.jpg

No matter what chain you use, you're almost certainly better trying to compress the data you want to store so it becomes cheaper.

I don't know about other chains but bitcoin only allows 20 bytes of optional universal data per transaction. Some encoding strategies like run length encoding or other methodologies can help maintain data in a way. Also storing text as an alphabet or alphanumeric characters can be stored in 5 bytes or 6 bytes respectively and can use sub versions of bytes to store even more.

But bitcoins blockchain isn't made for handling this and will just push fees up for everyone else...

OP,
For mastering bitcoin scripting language you need to check resources like this: https://github.com/bitcoinbook/bitcoinbook/blob/develop/ch07.asciidoc

But for creating actual scripts tailored to your needs (like the hypothetical one @ETFbitcoin suggested) without coding it you have two options:

1)lame option: Ignore the mastering phase and just storyboard your requirements right here, waiting for someone to write down a script for you.

2)Try writing down simple scripts tailored to your need and incrementally add more twists to it

In either case, you will come with something like this:
In this 'simple' script, I send funds to an address which is redeemable with 2 of 3 authorized members of a directing board or something, however, after  60 days if the board doesn't reach required 2 votes, one authorized member can join me to reclaim the funds and after 90 days I can do it by myself.

Now that the script is ready, you need the public keys to put in the placeholders and a map. What map? One that tells you the value of each opcode presented in the script. here you could find one: https://en.bitcoin.it/wiki/Script
After replacing public keys in the script you might decide to calculate its sha256 hash and ripemd160.

You can use libbitcoin-explorer (bx) to get rid of mapping and doing both hashes.  as follows:
The result would be the hash of your script and it can be used as the destination of a simple P2SH transaction but you and other people who wish to access the funds in the future should have access to the original script. reclaiming complex scripts is another story.

Not wanting to dull your thirst for knowledge... but isn't that sort of escrow scenario already handled by a simple 2-of-3 multisig wallet setup?

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