Don't know, but it couldn't be a WIF private key of that address.
Because the address appears to be a "P2SH-SegWit" and the the private key looks like in WIF uncompressed format.
If anything like it, the coins can't be spent in a standard way.

If it's something like other P2SH like MultiSig, then it may be a prvKey with missing characters of a single cosigner or just a clue to the private keys,
I can't tell, try this thread: [overview] Recover Bitcoin from any old storage format

Take note that they wont be able to verify your message with other software.
They'll have to specifically use Electrum, so include it to your email/chat or the signed message itself.

It actually looks neat if we follow the forum's color-scheming style but with orange palettes.

Normal Mode:


Night Mode:


Note: That was just a simple manipulation to the forum's CSS forms for testing purposes, not a theme or plugin (not permanent).

---

I guess our founder just used the default theme of SMF "Simple Machines Forum" which is blue and didn't have the time to apply other themes.

That's one way to work around it, you've utilized from_addr= and change_addr= params.
So, it's something like?:

Input1 ═══> Output1
       ╚══> Change1/Input2 ═══> Output2
                           ╚══> Change2/Input3 ∙∙∙

You're lucky that Electrum by default spends all of the UTXO linked to a single address.
Else, it could break the chain if an address in the change/input array has more than one UTXO then a smaller UTXO was selected as an input.

That's because "sha256sum.txt.asc" isn't the signature for the binaries.
It already contains both the signature of the message and the SHA256sum of each binaries (as signed message).

You verify the signed hash file via gpg --verify sha256sum.txt.asc to see if it's legit.
After passing verification, open it using your notepad or text editor, then compare the SHA256sum of the binaries to the listed SHA256 hashes.

In my previous reply, I also mentioned that Kleopatra yields zero result at my end.
Instead, I manually imported the one suggested in the tutorial but added "&op=index" at the end of the link for the link to work.

So, instead of keyserver.ubuntu.com/pks/lookup?search=goatpig, it should be: keyserver.ubuntu.com/pks/lookup?search=goatpig&op=index
Then, click on "8c5211764922589a", copy everything on the page from "-----BEGIN PGP PUBLIC KEY BLOCK-----" to "-----END PGP PUBLIC KEY BLOCK-----".

Open Kleopatra, click on "Notepad", paste the text there and click "Import Notepad".
After that, certify that the one you're importing is correct; e.g.: ask goatpig himself if the fingerprint (below) is correct.
Lastly, tick the checkboxes and finish the rest of the process.

Try using "Detect existing accounts" in Electrum.
It will try various derivation paths and provide you a list if it found at least one with funds.
Of course, you'll have to tick "BIP39 seed" in the option for the seed phrase to work.

Just don't get your hopes up since BitShield's feature to restore an existing seed phrase then overwrite the one you currently using is ridiculous.
Not to mention that the app's UI will be chaotic if you do so since the previous wallet will still be available in "Recent" until you do something else.
It's either badly written, an utter scam or both.

Wallets do not contain bitcoins at all.
Wallets only contain private keys which can be used to unlock bitcoin which are in the blockchain that are stored in every bitcoin node.
The watching only wallet "watches" for the coins your keys can spend by using your "public keys" and the cold-storage wallet contains the "private keys".
The private keys are derived from your 24-word seed phrase deterministically so it's all you need to restore the necessary info to spend funds.

I think you mistook "Cold Wallet" with "ColdCard" Wallet.
humblewrist and ajaxtempest are both talking about another Electrum wallet in an air-gap (offline) machine.

For questions specific about hardware wallets, you can try hardware wallet board: http://bitcointalk.org/index.php?board=261.0

I just pointed out a correction to BlackHatCoiner's instruction to satisfy the OP's request.
I'll leave the questions to OP to you guys.

I've seen a couple of those transactions in mainnet but if you want to test it yourself,
you can construct a transaction in Electrum (using the instructions above) or Bitcoin Core using createrawtransaction with OP_RETURN output: {\"data\": \"555555555555555\"} w/o amount, don't forget the change.
Sign it but don't broadcast since it will be a waste of satoshi,
instead: Open Bitcoin Core's console and use the command testmempoolaccept '["RAW_TXN"]' to see if it can be accepted by your mempool.

The missed most important step is to utilize "Pay to many".
That way, he can 'send BTC' as one output and put the message in another,
OP_RETURN alone will work in "normal send" but if he wants to include another output, it wont work.

In "pay to many", the 'Pay to' field should be like this:
(no need to use 'Tools->Pay to many' if using the latest Electrum versions, just enter multiple lines)

e.g.: e3da55f62afcb69a3908324bf6eaf310f48d9366d14c5f0fd19acd3da23af3ae
Some screenshots:

That is an offline tool, you can just download the source from "Forked GitHub Repository" link below the forked version and directly run it using your offline machine's browser,
all the dependencies are already included such as the main HTML file, CSS and Javascript so it will work even in air-gap machine.

Use anything that can calculate the "SHA256" sum of texts.
The 256-bit result will be the private key, then use any tool/library to convert it into WIF Private key.

But take note that line-spaces (enter) are also calculated by some tools but brainwalletX wont, so it took the string:
With SHA256 sum: 34ae22f3cbaf0b0adfea0bc52ded6eb4a39c1b325c45c8283ffa04612ed3e273
When converted to WIF (uncompressed): 5JDVE834PAQ6dPnwLNDfVhrtpvUnLrjMd542K2v6xaxBMJmx5dZ

instead of:
With SHA256 sum: 9243d83a0f17d54df5c35d42015d81f9cf3207c0dc7b4c0aa10f16bd07345db7
When converted to WIF (uncompressed): 5Jvhj1T7Gqe23KCQkxDXKWVvanfJqotjE9tVJLHFN752XcHrNRk

Just do it for research purposes only.

Have you tried this with the new "Descriptor Wallets"?

AFAIK, those are comparably lightweight compared to the old BDB wallet.dat counterparts.
Although I haven't tested loading 100s of them at once.

To fully understand it, you need to learn at least these two things:

• learnmeabitcoin.com/technical/mining
• learnmeabitcoin.com/technical/target

As a summary, the "target" is a number which is set to be higher or lower to maintain the 10-minute block average.
Miners are trying to get a hash with lower value than the target, each try can be the winning block, be it within a minute, ten (on average) or even an hour, it's unpredictable.

So they immediately fixed this after o-e-l-e-o submitted the issue in GitHub.
Gotta take note of that version in case some newbie experience the same issue like this one: https://bitcointalk.org/index.php?topic=5391541.msg60068150#msg60068150

For the record, the "official" documentation in Bitcoin's GitHub repository instructs to build using the git clone.

These for example: github.com/bitcoin/bitcoin/blob/master/doc/build-windows.md and /doc/build-unix.md
From: github.com/bitcoin/bitcoin/tree/master/doc

It makes sense since it's in GitHub where you can assume that the readers have audited the code.
However, 'all those tutorials' might have just imitated what's written there.

Is the actual code followed by broadcast() after all the payto() loop?
Because you'll have to broadcast after each 'payto' instead of loop 'payto' to generate signed raw transactions and broadcast all later.

Here's to reproduce the error using console commands:

• Use payto("address1","amount1")
• Save the created "raw transaction1" for later.
• Use payto("address2","amount1")
• Save the created "raw transaction2" for later.
• Use broadcast("raw transaction1") and it will be sent
• Use broadcast("raw transaction2") and it will be rejected with the same error because it's created by spending raw transaction1's input
  since transaction1's input(s) wasn't spent still (not broadcasted) while transaction 2 is being generated.

If not, it could be the above, everything's too fast that the client/server is having trouble identifying unspent UTXO.

Generally, it's safe even in Android as long as the device itself isn't compromised and the wallet isn't malicious.
If you can risk the amount and you're confident with your Android phone's security, do it.
But if you're dealing with huge amounts, it's best to proceed with caution.

---

If you're interested in Electrum cold storage, you can use your PC as "temporary" air-gap PC until you fully spent its funds.
Disconnect the PC's internet communication devices like WiFi, Bluetooth, LAN cables, etc.
And use your android phone as the online "watching only" wallet.

Download PC and Android Electrum from electrum.org/#download.
Transfer the files to the specific devices to install (recommended: verify their signature)

Setting the wallets is easy, in the offline PC Electrum, create a new imported wallet by selecting the option: "Import bitcoin addresses or private keys"
And type the private key there (you have to append the script type described in "?" if the address isn't legacy '1 address').
In the online Android Electrum, create a new wallet and select the same option but paste the address instead so it will be created as a 'watching only' wallet.
The balance should appear in the online watching only wallet.
With that you've created a cold-storage setup.

To spend it, create a transaction on the online Android Electrum, find a way to transfer the PSBT file to the offline PC (e.g. Flash drive or QR Code);
sign it using the offline PC, transfer the signed raw transaction back to the online Electrum and "broadcast" it.

Read these for the details, you can skip to the "create txn" part (the buttons are different due to the difference in versions):

• electrum.readthedocs.io/en/latest/coldstorage.html#create-an-unsigned-transaction
• bitcoinelectrum.com/creating-a-cold-storage-wallet-in-electrum/
  
  

After spending the funds completely (e.g.: sent to a hardware wallet/exchange) with 6 confirmations or more,
you can now use the offline PC normally since the private key there is now empty.

That was not totally cold-storage setup but it's a lot better than exposing the private key online.

Thanks, then it would only be an average of 19,674 years for 1trillion combinations per second bruteforce speed for a disarranged 24-words seed then.
And only 479,001,600 combinations for a disarranged 12-word seed.

Yes, the last word is part of the "checksum". It has to be the right one to make the seed phrase valid.

The simplest bruteforce formula is 24^24 for disordered 24-word seed, which is 1,333,735,776,850,284,124,449,081,472,843,776 combinations.
Even if you can get up to 1trillions of combinations per second, it would still take 42,292,484,045,227 years to find the right order.
Even 10000-times that speed isn't any better.

Compare it to 12 which is only 8,916,100,448,256 combinations.

If nothing works and you still want to use that older version of Windows,
you can try running Electrum from source, it requires Python and some commands.

Follow the links in this reply for instructions: /index.php?topic=5391692.msg59669529#msg59669529

My previous reply to this thread: