[bitcoin-dev] Dealing with OP_IF and OP_NOTIF malleability

Nick ODell nickodell at gmail.com
Fri Nov 6 09:22:27 UTC 2015

Your suggested modification seems sound.

Though, a script author could do something similar right now by
prefacing his IF with this:

OP_NOTIF OP_RETURN OP_ENDIF [actual OP_IF goes here]

That checks whether the input is 0 or 1, and runs OP_RETURN if not.
Your way is cleaner, though.

On Fri, Nov 6, 2015 at 1:13 AM, jl2012 via bitcoin-dev
<bitcoin-dev at lists.linuxfoundation.org> wrote:
> I have a new BIP draft for fixing OP_IF and OP_NOTIF malleability. Please
> comment:
> https://github.com/jl2012/bips/blob/master/opifmalleability.mediawiki
> Copied below:
> BIP: x
>   Title: Dealing with OP_IF and OP_NOTIF malleability
>   Author: jl2012 <jl2012 at xbt.hk>
>   Status: Draft
>   Type: Standards Track
>   Created: 2015-11-06
> Abstract
> As an supplement to BIP62, this document specifies proposed changes to the
> Bitcoin transaction validity rules in order to make malleability of
> transactions with OP_IF and OP_NOTIF impossible.
> Motivation
> OP_IF and OP_NOTIF are flow control codes in the Bitcoin script system. The
> programme flow is decided by whether the top stake value is 0 or not.
> However, this behavior opens a source of malleability as a third party may
> alter a non-zero flow control value to any other non-zero value without
> invalidating the transaction.
> As of November 2015, OP_IF and OP_NOTIF are not commonly used in the
> blockchain. However, as more sophisticated functions such as
> OP_CHECKLOCKTIMEVERITY are being introduced, OP_IF and OP_NOTIF will become
> more popular and the related malleability should be fixed. This proposal
> serves as a supplement to BIP62 and should be implemented with other
> malleability fixes together.
> Specification
> If the transaction version is 3 or above, the flow control value for OP_IF
> and OP_NOTIF must be either 0 or 1, or the transaction fails.
> This is to be implemented with BIP62.
> Compatibility
> This is a softfork. To ensure OP_IF and OP_NOTIF transactions created before
> the introduction of this BIP will still be accpeted by the network, the new
> rules only apply to transactions of version 3 or above.
> For people who want to preserve the original behaviour of OP_IF and
> OP_NOTIF, an OP_0NOTEQUAL could be  used before the flow control code to
> transform any non-zero value to 1.
> Reference
> BIP62: https://github.com/bitcoin/bips/blob/master/bip-0062.mediawiki
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