On Sat, Jun 6, 2015 at 4:42 AM, Rusty Russell <ru...@rustcorp.com.au> wrote: > Title: Canonical Input and Output Ordering > Author: Rusty Russell <ru...@rustcorp.com.au> > Discussions-To: "Bitcoin Dev" <bitcoin-development@lists.sourceforge.net> > Status: Draft > Type: Standards Track > Created: 2015-06-06 > > Abstract > > This BIP provides a canonical ordering of inputs and outputs when > creating transactions. > > Motivation > > Most bitcoin wallet implementations randomize the outputs of > transactions they create to avoid trivial linkage analysis (especially > change outputs), however implementations have made mistakes in this area > in the past. > > Using a canonical ordering has the same effect, but is simpler, more > obvious if incorrect, and can eventually be enforced by IsStandard() and > even a soft-fork to enforce it. > > Specification > > Inputs should be ordered like so: > index (lower value first) > txid (little endian order, lower byte first) > > Outputs should be ordered like so: > amount (lower value first) > script (starting from first byte, lower byte first, shorter wins) > > Rationale > > Any single wallet is already free to implement this, but if other > wallets do not it would reduce privacy by making those transactions > stand out. Thus a BIP is appropriate, especially if this were to > become an IsStandard() rule once widely adopted. > > Because integers are fast to compare, they're sorted first, before the > lexographical ordering. > > The other input fields do not influence the sort order, as any valid > transactions cannot have two inputs with the same index and txid. > > Reference Implementation > > https://github.com/rustyrussell/bitcoin/tree/bip-in-out-ordering
Sorry I wasn't a part of the IRC conversation where this was first discussed, though I'm very happy to see a concrete implementation along with the proposal. I'm not a great fan of this proposal for two reasons: The first is that the strict ordering requirements is incompatible with future soft-forks that may expose additional ordering constraints. Today we have _SINGLE, which as noted this interacts with poorly, but there have been other constraints proposed that this would also interact with poorly. The second is that even absent consensus rules there may be invisible constraints in systems-- e.g. hardware wallets that sign top down, or future transaction covenants that have constraints about ordering, or proof systems that use (yuck) midstate compression for efficiency. Right now, with random ordering these applications are fairly indistinguishable from other random uses (since their imposed order could come about by chance) but if everyone else was ordered, even if wasn't enforced.. these would be highly distinguishable. Which would be unfortunate. Worse, if most transactions are ordered the few that aren't could be mishandled (which is, I assume, part of why you propose requiring the ordering-- but I think the soft fork constraints there hurt it more). [Sorry for the delay in stating my views here; I wanted to talk them over with a few other people to see if I was just being stupid and misunderstanding the proposal] I think perhaps the motivations here are understated. We have not seen any massive deployments of accidentally broken ordering that I'm aware of-- and an implementation that got this wrong in a harmful way would likely make far more fatal mistakes (e.g. non random private keys). As an alternative to this proposal the ordering can be privately derandomized in the same way DSA is, to avoid the need for an actual number source. If getting the randomness right were really the only motivation, I'd suggest we propose a simple derandomized randomization algorithm--- e.g. take the order from (H(input ids||client secret)). I think there is actually an unstated motivation also driving this (and the other) proposal related to collaborative transaction systems like coinjoins or micropayment channels; where multiple clients need to agree on the same ordering. Is this the case? If so we should probably talk through some of the requirements there and see if there isn't a better way to address it. ------------------------------------------------------------------------------ _______________________________________________ Bitcoin-development mailing list Bitcoin-development@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/bitcoin-development
