Topic: relatively big transaction from me to unknown address. Error? (Read 1401 times)

Unless you are playing with building your own raw transactions the Bitcoin-Qt wallet is designed to not re-use a "change" address.  So when you receive the payment from the bitcoin mixer, that payment should be sent to a bitcoin address that you provide to the mixer (If the mixer tries to guess where to send the payment based on addresses that it received the bitcoins from, well that wouldn't be very anonymous at all would it?)  Change addresses are created by the wallet and NOT shown to you, so unless you go looking in the blockchain and choose to provide one of your change addresses to the mixing service, you shouldn't be able to provide a change address for receipt of funds.

Change addresses actually help increase anonymity.

If I have 5 bitcoins, and I create a transaction sending 2 bitcoins to a service with the remaining 3 bitcoins coming right back to my original address, then anyone who knows my address (anyone I've every sent bitcoins to or received bitcoins from) could look at the blockchain and know that I have exactly 3 bitcoins remaining and that exactly 2 bitcoins were paid to a service.

If I have 5 bitcoins, and I create a transaction sending 2 bitcoins to a service with the remaining 3 bitcoins going to a brand new address never before seen on the blockchain, then anyone who knows my original address won't be able to tell if I sent 2 with 3 change, or if I sent 3 with 2 change, or if I sent 5 and split the payment between 2 recipients.

Now here is a situation where you could run into a loss of anonymity. . .

Lets say your wallet has 6 bitcoins all in one address, ADDR_A, that was received in 2 payments (1 BTC and 5 BTC).
Now you decide to send 4 bitcoins off to a mixing service, and create a brand new address, ADDR_B, in that same wallet to receive the anonymous bitcoins.
Lets say the wallet chose to send the 5 BTC output and therefore sent the 1 BTC change to a brand new address, ADDR_C.
The wallet now has 1 bitcoin in an ADDR_A, 1 bitcoin in ADDR_C and 4 bitcoins in ADDR_B.

Now lets say you decide to send 2 BTC somewhere.

Possible outcomes:

• Wallet uses 1 BTC from ADDR_A and 4 BTC FROM ADDR_B, sending the 3 BTC change to a new address.  Anonymity is lost because ADDR_B is now linked to ADDR_A (your non-anonymous receiving address)
• Wallet uses 1 BTC from ADDR_A and 4 BTC FROM ADDR_C, sending the 3 BTC change to a new address.  Anonymity is lost because ADDR_B is now linked to ADDR_C (an address that is only 1 transaction from your receiving address
• Wallet uses 4 BTC from ADDR_C, sending the 1 BTC change to a new address, ADDR_D. Anonymity appears to be maintained, but the 1 BTC in ADDR_D could be linked to ADDR_A or ADDR_C in the future via either of the 2 previous scenarios.

Yes it can, there are some threads about that.

I agree so much...

Thanks to you both.

Does this behaviour pose any problem if I want to anonymize bitcoins? I haven't started researching that field of knowledge yet, but it sounds like it could be a risk, say I send 0.5 BTC to a randomizer and another 1.0 happens to end up on the same address I receive the anonymized BTC on?



I hate when programs assumes the users doesn't want to know what's going on "under the hood". Sure, most people don't care and maybe find it confusing, but they could at least put in something like a "show advanced settings and information" checkbox...

Read more about it here:
https://en.bitcoin.it/wiki/Change

Your wallet hides these messy technical details from you by generating the new address and then keeping it hidden from you.  The address is still part of your wallet, and with some technical know how you can see the information in your wallet, but for the average user the wallet simply hides the address from you since it doesn't think you need to know it.

The "unknown address" is also one of yours. Bitcoin-Qt pregenerates a set of 100 addresses (and only shows you the first one). Whenever you send a transaction, it takes an entire output (the term for some amount of incoming coin from a single transaction), sends whatever you specified to the recipient address and the rest to a change address, for which Bitcoin-Qt picks one of the pregenerated addresses.

This change address behavior can be quite confusing if you're new and it's not at all obvious what is going on without searching external sources for info (like this forum or the Bitcoin wiki).

I recently started Bitcoin-Qt to receive my first payout from Slush's Pool on just over 0.1 BTC. To test how things work, I sent a small amount to one of my own addresses. I had to pay a 0.0005 fee too, and the only thing I could see in Bitcoin-Qt was "transfer to yourself" and the negative value of the fee. The balance was the same, minus the fee.

Then I went to Blockchain.info to see from what address it was sent, as I could not see that in Qt. I searched for the transaction number, and found my small transaction to another one of my addresses, but also a ~0.099 transaction to an address I do not recognize.

As I said, the balance in Qt is correct, but I don't own the unknown address, as far as I know.

This is probably a stupid question, I guess everything is as it should, but what happened?