Anonymous Bitcoin Transactions with Coinjoin

[maaku]

So will combining multiple transactions also help with blockchain bloat / sustainablity? Or would the size of the coinjoined transactions be much the same as the separate ones?

This probably increases both traffic and the size of the unspent transaction output set.

[drawingthesun]

So will combining multiple transactions also help with blockchain bloat / sustainablity? Or would the size of the coinjoined transactions be much the same as the separate ones?

No not really, because if it was not for the intention of making this transaction to mix the coins then this transaction would not happen. So its not really cutting down on bloat.

On the plus side, this would be a normal sized transaction so its not going to do any harm.

[Peter Todd]

So will combining multiple transactions also help with blockchain bloat / sustainablity? Or would the size of the coinjoined transactions be much the same as the separate ones?

This probably increases both traffic and the size of the unspent transaction output set.

CoinJoin does increase network traffic a bit, but it can be implemented in a way that decreases UTXO and blockchain size.

For instance in addition to everything proposed already, you can make CoinJoin automatically join together requests multiple requests to pay the same address. For instance if Alice, Bob, and Charlie all want to donate 1BTC to Wikileaks's public address their CoinJoin implementation can take note of that and generate a single 3BTC txout with that address. You could even extend the payment protocol with support so that, for instance, BitPay could have all customers in a given 10 second interval join their payments together for totally unrelated items into a single txout.

[600watt]

impressive!

[ChuckSteinmetz]

Would it be possible for each party to a coinjoin transaction to give multiple output addresses so that if arbitrary input amounts are used, the output amounts can be randomly split across several outputs to prevent inferring an input to output relationship by amount?

[coinft]

Is it possible to unravel the signed transaction to see where 1a wanted their money to end up?
The network must know the path otherwise how would they get from A to B.
The intention for the money to end up in a certain place is the weak link right?

No. The whole transaction  ("this group of things to that group of things") is the intention the transaction coveys, and it cannot be separated when using sighash all (the default sighash type).  Thats the point. There are possible side channels— how different clients encode signatures, for example, but the transaction style itself doesn't leak any information about the mapping when used correctly.

Of course you like you have said in that thread all the inputs must be the same denomination right?

There is no fundamental requirement in the Bitcoin protocol for them to be the same, but if they are different sizes you likely leak some information about the input to output mapping: If I put in 5 and you put in 50... and addr X gets out 50 and Y gets out 5 ... how do you think they map?

idk ... what if I to payed you 45 BTC? Using this scheme for ordinary payment transactions would add a great deal of uncertainty to the whole blockchain.

I'd love to see this added to standard clients as standard payment mechanism if anyone can work out a reasonable protocol to automate collecting the sigs. Maybe with a payment request already including the merchant's input to the tx.

[P_Shep]

Could payment processors, web shops etc. use this to combine payments (within a few minute window or so) to the same or small number of addresses?
Guess that'd have to be done carefully to maintain anonymity.