Transaction ID showing MORE than I sent - Double spend attempt?

[tdcooper99]

Hi,

I created a new receiving address yesterday in my local bitcoin-qt wallet, and received some BTC from an exchange.

Today I have sent 10 BTC to another exchange, which I can see in my history, and as pending on the exchange... but, when I check the transaction ID for this send, in addition to the 10 sent to my exchange wallet address, it looks like another 5 (4.9999774) has been sent to another address that I do not recognise....  and of course this send does not show up in my bitcoin-qt history.

Is this potentially a malicious double spend attempt? How do I fix it?

As an aside, the 10 was sent with the minimum fee by accident, which is why I've been watching the transaction fairly closely to see if I need to take measures to fix it...  but the extra 5 is concerning me more at the moment.

Details:

The receiving address in my local wallet:

https://live.blockcypher.com/btc/address/1GpotiZtcJcD8rcPrpuMfXvPD5GEPzRRJp/

Transaction ID for 10 BTC sent today:

https://live.blockcypher.com/btc/tx/884b508555942d9e7cc6694da905ccb99eb1098a3ccc5edca5cb1c55d2e557b0/

[achow101]

No. That "extra spend" that you see is called change. The Bitcoin is yours and is being sent back to your wallet.

 Bitcoin works by creating and spending transaction outputs. When you receive Bitcoin, you get a transaction output which you can spend from. When you spend from that output, it must be spent from fully. Thus when you spend from that output, some of it goes to whoever you wanted the Bitcoin to go to, the rest returns to you as change. You still control that Bitcoin and your wallet handles everything in the background, which is why you don't see the change anywhere in the GUI.

Change for Bitcoin is much like change with fiat money. For example, you have a $20 bill and buy something that only costs $8. You will receive $12 as change. The same happens with Bitcoin, except that the "bills" are outputs and can have variable value.

[tdcooper99]

Ok, I think I understand...  so because I received 15 BTC in a single send from the exchange, and then decided to send 10 to another exchange... it has used the 15 I received to send the 10, and is returning me the 5, minus the TX fee? Makes sense now...

[achow101]

Ok, I think I understand...  so because I received 15 BTC in a single send from the exchange, and then decided to send 10 to another exchange... it has used the 15 I received to send the 10, and is returning me the 5, minus the TX fee? Makes sense now...

Yes. That is exactly what happened.

[DannyHamilton]

Ok, I think I understand...  so because I received 15 BTC in a single send from the exchange, and then decided to send 10 to another exchange... it has used the 15 I received to send the 10, and is returning me the 5, minus the TX fee? Makes sense now...

Yes. That is exactly what happened.

And if you want to see that this is what has happened, for your own peace of mind, you can go to the "Debug Window" under the "Help" menu and choose "Console".  Then type:

Code:

listunspent

And hit the [ENTER] or [RETURN] key on your keyboard.

This will list all the unspent outputs that your wallet has access to and can spend in the future.

You should see your transaction as a "txid" in the list that you receive.  You should also see the address that the 4.9999774 BTC were sent to, and it should indicate that it is "spendable".

To prove to yourself that your wallet actually is maintaining the private key for that address even though it doesn't display the address in your list of receiving addresses in the User Interface you can then type:

Code:

dumpprivkey 129mvCH4gmnWvYhsbEipoPiR1erKhZ9TLx

And hit the [ENTER] or [RETURN] key on your keyboard.

If your wallet doesn't have the private key and therefore doesn't have access to that 4.9999774 BTC, then you'll receive an error in red that says:
"Private key for address 129mvCH4gmnWvYhsbEipoPiR1erKhZ9TLx is not known (code -4)"

If your wallet does have the private key and therefore does have access to that 4.9999774 BTC, then you'll receive a 52 character long string of letters and numbers in black that starts with either a K or L.

Note: THAT STRING OF LETTERS AND NUMBERS IS THE PRIVATE KEY TO THAT ADDRESS, DO NOT SHARE IT WITH ANYONE. IF YOU DO THEN THEY CAN TAKE OR SPEND THOSE BITCOINS.