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If a miner mined a large amount of coins and these coins were stored offline once generated, would the balance of the addresses that hold these coins still be viewable on a blockexplorer like blockchain.info if you had the public address?

Or would a person need to use something like bitcoin core or bitcoind and add the address as a watch only address and call getbalance?

I'm trying to verify if a seller actually has the coins they claim to have. They apparently can show the wallets and their balances to the person interacting with them in the office now using bitcoin core. But is there any way for me in another country to confirm that what my colleague views is actually accurate and correct?

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Does blockchain.info show balances for addresses that are in cold storage?

Yes.

... is there any way for me in another country to confirm that what my colleague views is actually accurate and correct?

Since they use Bitcoin Core, yes, there is a way to verify that they hold the addresses that they claim. Have them sign a message with each address that they claim to have the holdings on, using Bitcoin Core you can verify that they indeed have those addresses and check them on blockchain.info to find the current balance. This feature has been in Bitcoin Core for a long time.

Only works in Bitcoin Core currently for addresses starting with a '1' (not Segwit addresses starting with a '3' and not Bech32 addresses starting with 'bc1' - the developers are aware of this and I will remind them shortly.)

In Bitcoin Core, your transaction opposite goes to File -> Sign Message and signs any message with one of the holding addresses. Copy the message, address and signature and send to you via probably plain text format email is the easiest. Repeat for each additional address holding the balance of BTC that they are offering to sell.

In Bitcoin Core, you go to File -> Verify Message and key the details provided EXACTLY - spaces, new lines and all characters must be an EXACT match. Click on verify and voilà.

I prefer the form of signed message as follows (don't key the top and bottom bar rows for the message, just the contents and you can check this yourself, the bottom row is the signature). I like to key the address used for verifying as a part of the message but that is not strictly necessary:

------------------------------
Something that I want to sign.

bitcoin:1PMUf9aaQ41M4bgVbCAPVwAeuKvj8CwxJg
------------------------------
Signture:
IGaXlQNRHHM6ferJ+Ocr3cN9dRJhIWxo+n9PGwgg1uPdOLVYIeCuaccEzDygVgYPJMXqmQeSaLaZVoG6FMHPJkg=

This contains all of the compact information necessary to verify the message.

Example of verified message:
verified message

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  • Thank you for the in-depth answer and explanation. Is it possible to use any version of bitcoin core without downloading the full blockchain to sync?
    – John Murphy
    Commented Mar 13, 2018 at 13:42
  • @JohnMurphy Yes, just about any version. Always use the official release version of the software from bitcoin.org. You do not need to download the full blockchain in order to sign/verify messages. The signer must have the key in their wallet already, that is about the only requirement.
    – Willtech
    Commented Mar 13, 2018 at 13:47
  • Sorry to keep asking questions but how do I use the bitcoin core software without downloading the full blockchain. When I attempt to run for the first time I am greeted by this screen: i.imgur.com/I9q5UQj.png Thank you for your help.
    – John Murphy
    Commented Mar 13, 2018 at 14:38
  • @JohnMurphy You can just select yes to that unless you want something custom. My suggestion is to go to Settings -> Options -> Network and set it to use a proxy that you do not have then OK and then File -> Exit and then restart it again.
    – Willtech
    Commented Mar 15, 2018 at 9:51
  • On the developer side, this use of message signing has been upgraded to '..not only just "proof I receive at this address", but also "proof of funds" (as a separate feature) since this is a popular misuse of the current message signing...' status on the [bitcoin-dev] mailing list. It should be noted that in the case of web wallets there is no such proof since a web wallet user does not really own the utxo's and they could be moved while your web wallet retains its balance but, we are speaking here about use with Bitcoin Core per the question.
    – Willtech
    Commented Mar 15, 2018 at 10:10

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