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When using the 'decode outputs' on moneroexplorer for a TX that I sent, I get this warning when it (correctly) estimates the amount that I spent:

Possible spending is: spent amount (tx fee included)
Note: without private spendkey, it is impossible to know whether this is your real spending.
So do not take this number seriously. It is probably totally wrong anyway.

What are the potential inaccuracies of simply assuming that the spent amount is equal to the sum of the user's inputs minus the sum of the user's outputs? Additionally, how could the private spendkey be used to accurately determine the amount spent?

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Because without the private spend key, the outgoing transactions will not be decoded.
It means if you send some monero, you will not see the negative transaction, but only a positive transaction corresponding to the change sent back to your address.
Therefore, the amount will be highly inaccurate.
However, if you only received and never spent any Moneroj, it will be accurate.

Example:
You received 10XMR in one output.
Your balance is 10XMR using the view key or spend key: it's accurate.
Then you send 2XMR.
You send this from the output you own and get 8XMR as change.
Using the spend key you'll see a balance of 8XMR.
Using the view key, you'll see a balance of 18XMR: the 10XMR output + the 8XMR output from change. As you do not see the first output being spend, it's inaccurate.

  • Thanks for the response! However, if you look through the ring members of each input and find the 10XMR output, what might be inaccurate about assuming that you spend 10 - 8 = 2 XMR? I'm assuming it is much less taxing just to use the spend key to check each output though – marceloneil Aug 22 '18 at 14:48
  • Or is it at all possible to receive a transaction that coincidentally mixes in one of your outputs – marceloneil Aug 22 '18 at 15:15
  • Also, don't you need the tx private key in order to decode the spent outputs using the spendkey, which if I recall is impossible to recover? – marceloneil Aug 22 '18 at 15:16
  • Looking through the ring members would need the private spend key. You need the tx key to prove payment only, not to see an outgoing transaction (to an unknown address) occurred. – el00ruobuob Aug 23 '18 at 7:36

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