[bitcoin-dev] Dealing with OP_IF and OP_NOTIF malleability

Tier Nolan tier.nolan at gmail.com
Fri Nov 6 09:27:24 UTC 2015

One and zero should be defined as arrays of length one.  Otherwise, it is
still possible to mutate the transaction by changing the length of the

They should also be minimally encoded but that is covered by previous rules.

On Fri, Nov 6, 2015 at 8:13 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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