How to prove coinbase payment?

Question

If I mine blocks with daemon as "start_mining ADDRESS" then how can I check coinbase transactions to prove the payments? For example, I have mined the block 1 to address 41tYuMHTrVxCLszpbcd4b9UNA9o5Myat81KbjjtueDjrAjgrqWm4Eoibn1j9YakCkxAJx4oTJ3oxaPo9VoZZm5AE1YQcWkJ

print_block 1
timestamp: 1622650264 (2021-06-02 16:11:04)
previous hash: ca9a7c50189c41469e988a6467329d31e664d2ff732fcef233a365ceb77ab6a3
nonce: 3898888432
is orphan: 0
height: 1
depth: 70
hash: 26ea8f7e8c62917d36f2854123fa9ba20efee9bd199899325ac57d73af0f2a5d
difficulty: 1
cumulative difficulty: 2
POW hash: d010deb032bf01e1335100ae8cd47b317a0f4a706540b08e8d44859fddb46573
block size: 79
block weight: 79
long term weight: 79
num txes: 0
reward: 10000000000.00
miner tx hash: b3b2df387e10d0ffe1979c078d4b5f40c7426ff20eb90c0e369867b7c89e5845
{
  "major_version": 1,
  "minor_version": 14,
  "timestamp": 1622650264,
  "prev_id": "ca9a7c50189c41469e988a6467329d31e664d2ff732fcef233a365ceb77ab6a3",
  "nonce": 3898888432,
  "miner_tx": {
    "version": 1,
    "unlock_time": 61,
    "vin": [ {
        "gen": {
          "height": 1
        }
      }
    ],
    "vout": [ {
        "amount": 1000000000000,
        "target": {
          "key": "14b12f54354d3d3c876fbaf16850ec58f91dc9f389f549af8b2e196aacce5646"
        }
      }
    ],
    "extra": [ 1, 74, 106, 48, 234, 162, 215, 245, 229, 194, 58, 61, 147, 198, 172, 106, 186, 155, 32, 233, 94, 142, 237, 206, 249, 81, 252, 225, 209, 181, 128, 62, 139
    ],
    "signatures": [ ]
  },
  "tx_hashes": [ ]
}

For some unknown reason, the wallet does not display this transaction. Can I use a command like "check_tx_key TXID TXKEY ADDRESS" to prove the transaction? If yes then where can I get TXKEY for that?

Answer 1

How to prove coinbase payment?

You can use get_tx_proof:

[wallet xxxx]: help get_tx_proof 
Command usage: 
  get_tx_proof <txid> <address> [<message>]

Command description: 
  Generate a signature proving funds sent to <address> in <txid>, optionally with a challenge string <message>, using either the transaction secret key (when <address> is not your wallet's address) or the view secret key (otherwise), which does not disclose the secret key.

Then the person verifying can use check_tx_proof:

[wallet xxxx]: help check_tx_proof 
Command usage: 
  check_tx_proof <txid> <address> <signature_file> [<message>]

Command description: 
  Check the proof for funds going to <address> in <txid> with the challenge string <message> if any.

Your question however is ambiguous. Above is what you can use to prove you have the funds from a tx for someone else to then verify, however, from your following remark, it seems you want to simply check whether you mined a block and got rewarded.

For some unknown reason, the wallet does not display this transaction.

Use show_transfers in your wallet:

[wallet xxxx]: show_transfers coinbase

If you don't see the tx, you didn't mine a block paying out to your wallet.

Answer 2

I was trying to do the exact same thing, and I discovered it is impossible if you mine with the daemon. The reason is that the transaction (private) key for the miner transaction is discarded by the daemon as soon as the block is created. You can see that in the method construct_miner_tx in cryptonote_tx_utils.cpp.

The miners can find the outputs for them because they can check the output public key using the transaction public key (which is in the extra) and their address private view key (like in any other output). But it is impossible to retrieve the transaction private key. Therefore it is impossible to prove the transaction to other people, unless you give them your private view key.

Btw, you cannot to that with get_tx_proof for the same reason. That command will try to find the transaction private key, which is not stored anywhere.

Get_tx_key and get_tx_proof can work only within the wallet and only if the wallet saved the transaction keys. In fact, if you restore a wallet from the mnemonics, you won't be able to retrieve the transaction private keys anymore, they will be lost forever.