Subject: [bitcoin-dev] New BIP: Dealing with OP_IF and OP_NOTIF malleability in P2WSH

From: Johnson Lau <[email-masked]>
Date: Tue, 16 Aug 2016 13:53:08 -0400 (EDT)

A new BIP is prepared to deal with OP_IF and OP_NOTIF malleability in P2WSH:
https://github.com/jl2012/bips/blob/minimalif/bip-minimalif.mediawiki
https://github.com/bitcoin/bitcoin/pull/8526

   BIP: x
   Title: Dealing with OP_IF and OP_NOTIF malleability in P2WSH
   Author: Johnson Lau <[email-masked]>
   Status: Draft
   Type: Standards Track
   Created: 2016-08-17

Abstract

This document specifies proposed changes to the Bitcoin script validity rules in order to make transaction malleability related to OP_IF and OP_NOTIF impossible in pay-to-witness-script-hash (P2WSH) scripts.

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 True or False. However, this behaviour opens a source of malleability as a third party may replace a True (False) stack item with any other True (False) value without invalidating the transaction.

The proposed rules apply only to pay-to-witness-script-hash (P2WSH) scripts described in BIP141, which has not been activated on the Bitcoin mainnet as of writing. To ensure OP_IF and OP_NOTIF transactions created before the introduction of this BIP will still be accepted by the network, the new rules are not applied to non-segregated witness scripts.

Specification

In P2WSH, the argument for OP_IF and OP_NOTIF MUST be exactly an empty vector or 0x01, or the script evaluation fails immediately.

This is deployed using BIP9 after segregated witness (BIP141) is activated. Details TBD.

Compatibility

This is a softfork on top of BIP141. The rules are enforced as a relay policy by the reference client since the first release of BIP141 (v0.13.1). To avoid risks of fund loss, users MUST NOT create P2WSH scripts that are incompatible with this BIP. An OP_0NOTEQUAL may be used before OP_IF or OP_NOTIF to imitate the original behaviour (which may also re-enable the malleability vector depending on the exact script).

Implementation

https://github.com/bitcoin/bitcoin/pull/8526

Copyright

This work is placed in the public domain.