SIGHASH

It would be useful if multiple payments to the same address could be summed together. For instance with CoinJoin if Alice, Bob and Charlie wish to, say, pay 1BTC to Wikileaks they can create a single 3BTC txout with Wikileak's public address. However they have to do that interactively.

So I'm proposing SIGHASH_SUM, a new SIGHASH bit that would allow a given txin signature to specify that at least nBTC of value should be added to a txout signed for. Thus if three txin signatures each specify that 1BTC should be added to the txout with Wikileak's public address, the transaction is valid if there exists a single 3BTC txout. Similarly any company accepting multiple payments at once could combine all their customers payments together into a single transaction, with a single txout, thus saving blockchain and UTXO space as well as improving privacy.

Of course this isn't too useful by itself, but the idea can be combined with more sophisticated SIGHASH bits that say that a particular set of txin's and txout must exist, rather than the more restrictive modes available today. But my lunch break is over so I'll write more about that later.