Default address as change address about bitcoin-wallet

[schildbach]

What usecase do you have in mind?

I know the whole address handling is broken. In future, we will migrate to BIP32 deterministic wallets. This will mean a new change address is generated each time one is needed. The ability to manually specify a change address is however not planned.

[janderholm]

I am uncomfortable using the same address all the time for two reasons. First reason is the security aspect. The more transactions sent from an address the more probable it is someone might be able to exploit any implementation problem.

The other is privacy. If I use just one address it is much much easier to pinpoint me in this giant graph of address nodes. The ability to split the node being me into several smaller nodes will greatly strengthen my pseudonymity.

A third reason is quite simply that the current situation is not very intuitive. Why have a concept of default addresses, or addresses at all, if all transactions still end up at an arbitrary address in the end?

My proposed change is that instead of picking an arbitrary (oldest) address in my wallet as change address, pick the one marked as default. I understand BIP32 wallets is a better fit but to be honest that might take quite a long time. It has been looming for well over a year and is still not finished. My understanding is that BIP32 is a long term goal and is no where near implementation.

[schildbach]

I agree HD wallets may take another half a year to arrive. I also agree the current workaround of just picking the first (non-vulnerable) key is not very safe in terms of privacy. However, I think picking the selected address is not any better.

How is this. The change address is always one of the input addresses. If the default address is amongst them, it is preferred. Otherwise, the oldest address is preferred. If the tx does not have any input addresses (e.g. because the outputs pay to a pubkey), the default address is picked. Of course, all vulnerable addresses are excluded.

[janderholm]

I think that would be very nice. Of course I would like to be able to choose change address entirely myself but I don't think that would be fitting for this particular wallet. There's plenty of other wallets with very advanced features for such things already!

[schildbach]

Since version 4.11, change addresses are now used only once.