Help! Odd unauthorized payment by my address in Blockchain!? Fraudulent? Error?

[padre999]

Hello

I have a rather odd and worrying problem. An unauthorized payment was made from my address "on top" of another payment I made.

I transferred 2BTC using Bitcoin-Qt on OSX to one recipient. When I looked it up in the Blockchain I saw that there was another recipient added to the transfer, receiving 0.04… BTC!??

You can see it here http://blockexplorer.com/t/6MY6KQyb8k under Outputs, or here http://blockchain.info/de/tx/8d48f8d48825b925ae49494e2c132bbba5e5dc1200b2a6103b1045a07e89d288

I did NOT MAKE this payment of 0.04 BTC!!! There was no transfer fee setup in my client for my 2BTC payment. I have no idea who this recipient is and where it comes from.

This payment of 0.04… BTC is also not showing up in my Bitcoin-Qt client. There is now a discrepancy between the total amount for my address 17PmuT7rHcJh5DVenTRD7Nw5ZTjhjWXcyJ in the Blockchain and in my wallet. While Blockchain shows a total of 0,99 BTC there is a total of 1.0307206 showing in my wallet.

I find this really scary and I have no idea how this could happen. Can anybody shed some light on this please. Has my Bitcoin client been hacked? I downloaded it from github as linked in the Bitcoin Wiki. Or its this some Blockchain error?

Thank you.

[greyhawk]

Read this:
https://en.bitcoin.it/wiki/Change

Sync your client.

[DannyHamilton]

I sent an amount to one address for x btc. blockchain.info shows the transaction confirmed with a second address listed for an additional amount.  how can this happen? . . .

With the way bitcoin works, you can't partially spend a previous output that you received.  It has to be completely spent.  So, if you receive 10 BTC all in a single transaction from someone/somewhere at an address, then that address now has a single 10 BTC output.

Later if you want to send 2 BTC to somebody, the program can use the previous 10 BTC output as an input into the transaction, and create a 2 BTC output to the address where you want to send it.  This leaves 8 BTC leftover in the transaction.  If you don't send those 8 BTC somewhere, then they become miner fees and end up in the coinbase transaction of the new block for the miner to keep.  If you don't want to give those 8 BTC away as fees to the miner, then you need to include a second output in your transaction sending the 8 BTC back to your own wallet.  As an analogy, think of pulling a $10 bill out of your pocket to pay for something that costs $2, you get $8 back in "change".

The way Bitcoin-Qt does this is to create a new bitcoin address that it doesn't tell you about and keep track of that address and the associated private key in your wallet.dat.  Since this isn't an address that you requested in the "Receive Coins" tab of the user interface, it doesn't show up in that list.  Bitcoin-Qt knows about the address since it is in the wallet.dat file, so it includes it in the balance that it shows you.  Bitcoin-Qt uses a new change address for every transaction.  So if you create 10 different transactions, each sending bitcoins somewhere, then your wallet will have used 10 different change addresses (although some of them may be empty if Bitcoin-Qt used the bitcoins associated with some of those address as inputs into other transactions).

[padre999]

Thank you for your explanations. As a newbie I am still not able to fully grasp what is going on, but at least seems that this is a rather "normal" an legitimate thing.

Still there remains the discrepancy between blockchain and my wallet. My client is fully synced.

I guess the the total amount of 1.03 in my wallet is now made up of the 0.99 total of my address 17PmuT7rHcJh5DVenTRD7Nw5ZTjhjWXcyJ plus the 0.04 of the mysterious recipient 1KGnRoxAqffvszcffzf6xfJqKfzNwrdMVP (which is really a new address of mine now).

Bitcoin-Qt just doesn't tell me about the split that happened.

Correct?

Thank you!

[greyhawk]

Pretty much, yeah. BT-qt knows internally about the new "change adress", but doesn't tell you about it. I'm not sure why exactly the developers implemented that like that. I think I read something about people would only get confused, but in my experience people get even more confused when they have adresses they don't even know about due to change.

[DannyHamilton]

Still there remains the discrepancy between blockchain and my wallet. My client is fully synced.

That's because blockchain is displaying the total value associated with an "address" not with a wallet.  A wallet can have hundreds of "addresses".

I guess the the total amount of 1.03 in my wallet is now made up of the 0.99 total of my address 17PmuT7rHcJh5DVenTRD7Nw5ZTjhjWXcyJ plus the 0.04 of the mysterious recipient 1KGnRoxAqffvszcffzf6xfJqKfzNwrdMVP (which is really a new address of mine now).

Bitcoin-Qt just doesn't tell me about the split that happened.

Correct?

Correct.  Your wallet now has at least two addresses.  The total value being tracked by the wallet is the total value of all the unspent outputs associated with the addresses in its wallet.dat file.  You can see these unspent outputs (their values and transactionIDs) if you go to "Help->Debug Window->Console" and enter:

Code:

listunspent

There are other clients/wallets that handle change differently.  You can use one of them if you don't like how Bitcoin-Qt handles it.

Some suggestions are:
Armory
Electrum
MultiBit
https://blockchain.info/wallet

[padre999]

Wow, you are of great help. Thank you all.