[Bitcoin-development] [RFC] Canonical input and output ordering in transactions

Danny Thorpe danny.thorpe at gmail.com
Mon Jun 8 21:25:40 UTC 2015

FWIW, The Open Assets colored coin protocol (CoinPrism) places special
significance on the zeroth input and the position of the OP_RETURN colored
coin marker output to distinguish colored coin issuance outputs from
transfer outputs. Reordering the inputs or the outputs breaks the colored
coin representation.

Recommending sorting of the inputs and outputs as a best practice is fine
(and better than random, IMO), but not as part of IsStandard() or consensus
rules.  There are cases where the order of the inputs and outputs is


On Fri, Jun 5, 2015 at 9:42 PM, Rusty Russell <rusty at rustcorp.com.au> wrote:

> Title: Canonical Input and Output Ordering
> Author: Rusty Russell <rusty at rustcorp.com.au>
> Discussions-To: "Bitcoin Dev" <bitcoin-development at 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
> ------------------------------------------------------------------------------
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