How the hell was I able to receive BTCs via an address I do not own?

A few minutes ago I received 0.00000001 BTC. I don't know who sent them or why.

But what makes it crazy is, this sum was sent to an address I do not own and which the client I'm running (the standard Bitcoin-QT, versione 0.8.5) had never heard of before. I know this for sure because I only created 2 (two) addresses using this client, and the receiving address of this transaction is not one of them.

How is this possible at all?
Is it a hash collision, or what else?

The address is 1Cz2AmLe8U52KR549d13JQkqsqkV64RchB and the transaction ID is b9591410ff030e40deb3eda9d2fe30a29642ede443f354bb5f4ed5bb55f0d36d.

According to this page, the address seems to have received money a little time before from 1HTawQNCS4axPXft6L6fkzrv5ya7TguqwD, which actually is one of my addresses; the last two transactions are me sending all my BTCs to a new address, just to be sure. This worked, so it looks like I'm actually the owner of that address, even if my client doesn't list it as one of mine.

What's exactly going on here?

Massimo

Posted 2013-10-15T09:27:20.257

Reputation: 978

Satoshi was clever. Change addresses add an extra layer of anonymity. The bitcoin-qt wallet GUI doesn't have any indication that they exist, however. This might be for simplicity's sake. – bvpx – 2013-10-15T15:40:09.320

Yes, Satoshi was clever, but you mean "anonymity" with quotes. It's possible to trace money in Bitcoin with a reasonable confidence level. – goodguys_activate – 2013-10-15T15:51:56.533

Are you involved in a mining pool? You may have received a proportion for your work – Patrick – 2013-10-15T10:33:59.053

I actually am, but the pool doesn't make such small payouts. And still the main question remains... I don't own that address! – Massimo – 2013-10-15T10:35:53.333

Answers

As Murch says one possibility is that this address is in your keypool. Another is that this is a change address the client created. If you're willing to share the relevant transaction we may have more to say.

Owning an address means "being in unique possession of a corresponding private key". It's difficult to know if you're the only owner, but verifying you have a key is easy - ask bitcoin-qt to sign a message with this address.

Meni Rosenfeld

Posted 2013-10-15T09:27:20.257

Reputation: 18 542

Question updated. And yes, it looks like that address actually belongs to me... – Massimo – 2013-10-15T11:03:39.010

Ok, I've looked up change addresses (https://en.bitcoin.it/wiki/Change), and it looks like it's one of them, and it's normal to not see it in the client GUI. But why did I suddenly receive 0.00000001 BTC on it? Who sent them and why?

– Massimo – 2013-10-15T12:03:48.180

@Massimo: This is indeed a mystery, but once this address received change it is publicly visible and anyone can choose to send a satoshi to it, for whatever reason. Bulk sending of satoshis to random addresses has been observed before. – Meni Rosenfeld – 2013-10-15T15:01:17.383

@Massimo: Looking at the transaction, it seems to have come from an address associated with "LetsDice.com". Have you used that service? Perhaps you sent them funds from the change address, and they returned coins to it to carry some information (they could also send payment for winnings to that address, but 1 satoshi doesn't seem to be it). – Meni Rosenfeld – 2013-10-15T15:03:24.177

1Never heard of that service before... and the change address was created only a couple of hours before receiving that satoshi. Also, I received ANOTHER satoshi from the same address on the new address I started using soon after. Maybe someone is sending satoshis to any new address it sees on the blockchain?!? I'm puzzled. – Massimo – 2013-10-15T16:15:14.317

1@Massimo: Some people do weird things. – Meni Rosenfeld – 2013-10-16T05:34:49.187

3@Massimo: Actually, this tx is probably an advertisement. Note that there's also a public note on blockchain.info advertising their site. – Meni Rosenfeld – 2013-10-16T05:37:26.653

Bitcoin-qt has always at least 100 unused addresses in store. You probably received the payment to one of those.

If it were a hash collision that would very disconcerting, it might indicate that there is also trouble with the randomness in the standard libraries used by bitcoin-qt.

Murch

Posted 2013-10-15T09:27:20.257

Reputation: 41 609

But even if this is the case, I don't know them and never told them to anyone... – Massimo – 2013-10-15T11:03:04.757

Check if that address appeared on the blockchain before that mystery payment. If it does, they could have gotten it from there. Also if you used it as a change address for a payment to someone, maybe you overpaid and they are refunding your overpayment (possibly in an automated process)

RentFree

Posted 2013-10-15T09:27:20.257

Reputation: 2 391