Subject: Re: [bitcoindev] Consensus Cleanup BIP draft

From: Sjors Provoost <[email-masked]>
Date: Fri, 28 Mar 2025 13:48:41 +0100

Hi Chris,

Thanks for that example.

I also realised that there indeed can exist 64 byte transactions that can't be malleated into a different size (see Stack Exchange link below). The example uses SIGHASH_SINGLE | SIGHASH_ANYONECANPAY, so as long as our hypothetical smart contracting system uses those flags for its burn-all / giveaway-all clauses, then if it produces a 64 byte transaction by mistake, it's recoverable.

But a SIGHASH_ALL could get stuck (can't be confirmed, can't be modified). And IIUC with OP_CTV even a SIGHASH_SINGLE | SIGHASH_ANYONECANPAY, once committed inside a CTV tree, can't be modified.

- Sjors

> Op 28 mrt 2025, om 12:02 heeft Chris Stewart <[email-masked]> het volgende geschreven:
> 
> Hi Sjors,
> 
> An example is any segwit transaction that 1 input 1 output that pays to a 2 byte witness program. Since witness data doesn't count towards the 64 byte limit, the witness could be of arbitrary size. This was pointed out <https://delvingbitcoin.org/t/great-consensus-cleanup-revival/710/74?u=chris_stewart_5> by garlonicon <https://delvingbitcoin.org/u/garlonicon> on delvingbitcoin. This type of witness program is used for pay-to-anchor outputs currently - although I don't believe anchor outputs make sense in a 1 input 1 output transaction? The author of pay to anchor outputs is aware of this issue <https://delvingbitcoin.org/t/great-consensus-cleanup-revival/710/78?u=chris_stewart_5>, but I haven't heard of any further updates since his post on delving.
> 
> -Chris
> 
> On Fri, Mar 28, 2025 at 4:25 AM Sjors Provoost <[email-masked] <mailto:[email-masked]>> wrote:
>> 
>> > Op 27 mrt 2025, om 21:45 heeft jeremy <[email-masked] <mailto:[email-masked]>> het volgende geschreven:
>> > 
>> > I'm also personally strongly against removing 64-byte transactions. It's a wart in how transactions work, and future upgrades (especially around tx programmability) might integrate very poorly with this kind of edge condition.
>> 
>> Do you have an example in mind where a 64-byte transaction could appear in such a system?
>> 
>> Given the limited space, in particular no room for a public key or signature, I could imagine a burn or anyone-can-spend clause. Those don't have to be exactly 64 bytes. Even if 64 byte transactions are generated by accident, I believe they can be tweaked after the fact (though that claim could use more scrutiny):
>> 
>> https://bitcoin.stackexchange.com/q/125971/4948
>> 

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