[bitcoin-dev] Simple tx ID malleability fix, opcode proposal: OP_TXHASHVERIFY

Rune K. Svendsen runesvend at gmail.com
Sat Sep 17 21:14:30 UTC 2016

I hadn't thought of that... There is a solution, I think, but it makes the
operation less simple.

If a transaction contains at least two OP_TXHASHVERIFY-protected inputs,
signed without ANYONECANPAY, their signatures would cover the other
input's OP_TXHASHVERIFY hash, right?


On Sat, Sep 17, 2016 at 10:56 PM, Matt Corallo <lf-lists at mattcorallo.com>

> (removing the list)
> Because the tx hash in your construction is not signed, someone wishing to
> maleate a transaction may do so by also updating the hash in the scriptSig.
> Matt
> On September 17, 2016 4:45:17 PM EDT, "Rune K. Svendsen via bitcoin-dev" <
> bitcoin-dev at lists.linuxfoundation.org> wrote:
>> I would really like to be able to create transactions that are immune to
>> transaction ID malleability now, so I have been thinking of the simplest
>> solution possible, in order to get a BIP through without too much trouble.
>> An opcode we could call OP_TXHASHVERIFY could be introduced. It would be
>> defined to work only if added to a scriptSig as the very first operation,
>> and would abort if the hash of the transaction **with all OP_TXHASHVERIFY
>> operations (including stack push) removed** does not match what has been
>> pushed on the stack.
>> So, in order to produce a transaction with one or more inputs protected
>> against tx ID malleability, one would:
>> 1. Calculate tx ID of the tx: TX_HASH
>> 2. For each input you wish to protect, add "0x32 $TX_HASH
>> OP_TXHASHVERIFY" to the beginning of the scriptSig
>> When evaluating OP_TXHASHVERIFY, we make a copy of the tx in question,
>> and remove the "0x32 <32 bytes> OP_TXHASHVERIFY" sequence from the
>> beginning of all scriptSigs (if present), and abort if the tx copy hash
>> does not match the top stack item.
>> This is a very simple solution that only adds 34 bytes per input, and
>> when something better becomes available (eg. Segwit), we will stop using
>> this. But in the meantime it's very valuable to be able to not worry about
>> tx ID malleability.
>> Please let me know what you think.
>>             /Rune
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