[bitcoindev] Re: Aligning privacy incentives in P2MR

["'conduition' via Bitcoin Development Mailing List" <bitcoindev@googlegroups.com>]

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I have just submitted a PR into BIP360 which implements the suggestion 
described in OP <https://github.com/bitcoin/bips/pull/2198>.

regards,
conduition





On Wednesday, June 3, 2026 at 7:26:39 PM UTC-4 conduition wrote:

> Hi list. I'm following up on an idea I sketched in this thread debating 
> P2MR vs P2TRv2 <https://groups.google.com/g/bitcoindev/c/Qy4gwAGTK2w>. 
>
> The premise is this: What if we modified this line of BIP360:
>
> The last stack element is called the control block c, and must have length 
> 1 + 32 * m, for a value of m that is an integer between 0 and 128, 
> inclusive. Fail if it does not have such a length.
>
>
> To this:
>
> The last stack element is called the control block c, and must have length 
> 1 + 32 * m, for a value of m that is an integer between *1* and 128, 
> inclusive. Fail if it does not have such a length.
>
>
> The key change is that the control block must now include at least one 
> merkle authentication path. This effectively bans depth-zero script trees 
> in P2MR, and requires every P2MR address to always pay the cost of having 
> at least two spending conditions.
>
> This seems like a reduction in P2MR's features and efficiency. Why would 
> we want to ban depth-zero script trees? Well these seem to be the source 
> of a perverse incentive, as pointed out by Matt Corallo and Antoine Poinsot 
> in prior threads <https://groups.google.com/g/bitcoindev/c/JA3kDl8AmQg>. 
> Bitcoin script users are economically and practically incentivized by P2MR 
> to use depth-zero script trees because their witnesses are *smaller* than 
> if the same script were used in a taproot script spend.
>
> Furthermore, depending on the exact scripts and the developers' resources, 
> devs using P2MR for a multi-party scripting protocol may not be 
> sufficiently incentivized to add a cooperative spending path, even if their 
> protocol might allow it. Doing so would require a depth-1 tree, which 
> increases the witness size of the non-cooperative script spend path by 32 
> bytes. For some short scripts, e.g <height> CLTV <pubkey> CHECKSIG​, the 
> devs may actually have incentive *not* to add a cooperative spending 
> path, because the cooperative path would actually be *less-efficient* 
> than just using the non-cooperative path as the single leaf of a depth-zero 
> tree. This leads to easy fingerprinting of scripting protocols, less 
> privacy for everyone else, and undoes a key design goal of P2TR.
>
> In Matt's words 
> <https://groups.google.com/g/bitcoindev/c/JA3kDl8AmQg/m/EWS5ZxqZAAAJ>:
>
> The value of taproot is that its design natively adds [a cooperative 
> spending path] *for free* to every contracting protocol, and not only adds 
> it for free but *forces* every contracting protocol to carry at least some 
> of the cost if they decline to do this. This results in a massive net 
> privacy win across the entire Bitcoin ecosystem...
>
>
> a goal of taproot is *privacy* while offering efficiency for the common 
> (all-sign) case. That is generally true across contracting protocols and 
> makes things net-cheaper with a taproot-style system where you hit the 
> common case often. This is another reason why P2MR is quite a loss
>
>
> By eliminating depth-zero script trees, we'd force all P2MR users to pay 
> for the cost of having *at least* two spending conditions. We'd eliminate 
> the incentive for script devs to use P2MR over P2TR because both are 
> equally efficient, and incentivize P2MR users to add a cooperative spending 
> path using BIP340, since those users are already paying for the cost of 
> having a depth-1 tree anyway.
>
> This change to BIP360 wouldn't affect the recommended standard use-cases 
> for single-signer and multisig P2MR: hybrid addresses with one Schnorr leaf 
> and one PQ leaf. If anything, this change would encourage the proper use of 
> PQ fallback scripts, because the incentive logic can be applied to someone 
> who tries to use P2MR with only a single EC Schnorr leaf: This user must 
> pay for the cost of a second leaf script anyway, so why not follow 
> best-practices and add a PQ leaf?
>
> AFAICT this change only affects use cases which would otherwise degrade 
> the fungibility of the UTXO set according to BIP360 critics, and encourages 
> those use-cases to adopt a cooperative spending path which (assuming all 
> other factors equal) will be indistinguishable from a regular single-signer 
> P2MR wallet with a PQ fallback leaf.
>
> I've already chatted about this off-list with some BIP360 proponents and 
> they seem receptive, but I'm really curious about the opinions of those who 
> are leaning towards P2TRv2. Would this change to BIP360 address your 
> concerns surrounding P2MR's privacy incentives? If not, why?
>
> regards,
> conduition
>
>
>
>

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