Hi conduition, I think this would address the perverse incentive concern, but still fail in not disincentivizing mass migration.
It's important to consider that in any scenario where Bitcoin as we know it survives a CRQC, the vast majority of users would have had to migrate to an output type that includes an escape hatch long before we could have been reasonably certain that a CRQC would materialize. Therefore we should not assume that the vast majority of users strongly desire to migrate to an escape-hatch output type. (In fact, i think it would actually be reasonable to assume none have a strong desire to do so. This is because everyone has an incentive for everybody to migrate, but there is little incentive for each individual to migrate if nobody else does.) Therefore any additional barrier to encourage users to opt into an escape-hatch output type is working against CRQC risk mitigation. And i think the additional costs of using P2MR compared to P2TRv2 is one of them. Most likely all "long-tail" users, who would be the least likely to seek migration, use single-key spending policies. In fact, most Bitcoin users do: in the past 90 days of blocks, [93.6% of all transaction inputs are for single-key spending policies](https://mainnet.observer/charts/inputs-types-by-count/). For a typical 2-input 2-output transaction for a "single-key wallet", P2MR is about 15% more expensive to use than P2TRv2 [^0]. I understand that P2MR does not introduce this additional cost just for kicks, but i think "long-range" protection is a red-herring and not worth hindering individual migration to the escape-hatch output type, which is critical to the systemic migration of Bitcoin away from relying on EC cryptography. So your proposal does address one of my concerns with a P2MR + PQ opcode strategy. However, i still believe P2TRv2 to be a superior strategy for the reason outlined above. Best, Antoine [^0]: Using [57.5*2 + 43*2](https://bitcoin.stackexchange.com/a/75124/101498) = 201 vbytes as the base size. P2MR adds an additional 32 witness bytes in the control block for the PQ spend path, and an additional 35 witness bytes for the EC leaf script reveal. That's 33.5 more vbytes per input, which is 116.66% the size of the same transaction with P2TRv2. On Wednesday, June 3rd, 2026 at 7:26 PM, 'conduition' via Bitcoin Development Mailing List <[email protected]> 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 between1and 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 > > -- > You received this message because you are subscribed to the Google Groups > "Bitcoin Development Mailing List" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to [email protected]. > To view this discussion visit > https://groups.google.com/d/msgid/bitcoindev/Q8YTY1ArMzia7tRvcZ5v769WPxCMxAl_0rUy-byOBjBcTd4HXj7IB7Yt4gxLbKqk7EXFWy-PuGB6QtI28qRjE5Vob-l44UmBH6L8aXInoSk%3D%40proton.me. -- You received this message because you are subscribed to the Google Groups "Bitcoin Development Mailing List" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To view this discussion visit https://groups.google.com/d/msgid/bitcoindev/xDZdaNASwweJSIjFLVJidZYfsMsle8RuX0G74BWggmIBRZaMgUaPjJLtj5s6BRNr1XObrie1JV7dQ8w6h9h_hyBPOEnydBc2F9kCsEmDE80%3D%40protonmail.com.
