Re: [bitcoindev] Algorithm Agility for Bitcoin to maintain security in the face of quantum and classic breaks in the signature algorithms

Ethan Heilman
2026-02-11T18:43:25Z

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> i see so a minimal change to taproot would be a new version: “script
spend only” or similar.

BIP 360's P2MR (Pay-to-Merkle-Root) is P2TR but script spend only via a
simple and minimal change.

Thanks for working on commit-reveal -- also thanks to Tadge for lifeboat, I
think this line of research is worthwhile and helpful. Correct me if you
disagree, I think of commit-reveal as useful for the following case: Q-day
happens AND we have P2MR AND OP_TXHASH, but not OP_SLH_CHECKSIG,
commit-reveal might be the best option to migrate to a PQ signature scheme.

I am pursuing the P2MR + OP_SLH_CHECKSIG approach this approach is the
least disruptive to how people use Bitcoin today and would be simpler for
wallets to support (SLH only requires signing a single transaction vs. two
transactions and having to handle griefing/reset).

In both cases we want BIP 360's P2MR. I'd also like OP_TXHASH and
OP_SLH_CHECKSIG as well.

I'm using OP_SLH_CHECKSIG as a placeholder here, if you like
OP_SHRINCS_CHECKSIG better, you could also do P2MR + OP_SHRINCS_CHECKSIG

On Wed, Feb 11, 2026 at 2:25 AM Erik Aronesty <erik@q32.com> wrote:

> i see so a minimal change to taproot would be a new version: “script spend
> only” or similar.
>
> On Tue, Feb 10, 2026 at 6:41 PM Ethan Heilman <eth3rs@gmail.com> wrote:
>
>> >> You'd still need BIP 360 P2MR (or P2TRD) since OP_TXHASH needs
>> tapscript
>> > false, covenant based multistep secret-reveal spending paths don't
>> rely on signatures at all
>>
>> P2TR has a public key baked into it via the key spend path. This key path
>> spend bypasses any covenant or script constraints. The attacker does not
>> need to see a value spend to break the key path spend. This means a quantum
>> attacker can break the nums point in key spend path of the initial output
>> and the AnchorPublishTx output and then just taking all the coins [0].
>>
>> > agreed. they have to spend resources to attack your private key and
>> the only thing they can do is "grief" using a timing attack with the
>> results, rather than steal outright. a massive incentive difference.
>>
>> Ok, so a core assumption you are making here is that a CRQC isn't
>> powerful enough for recovering signing keys to be effectively free. This is
>> likely to be true at the early stages of CRQC, but this assumption may not
>> hold forever. If ECC is mathematically broken via a classical attack this
>> assumption might not hold at all. I'm attempting to address both quantum
>> and classical breaks.
>>
>> > TX_HASH is simple and generally useful and there is no guarantee that
>> q-day will even come
>>
>> TX_HASH is great!
>>
>> [0]: As originally noted here:
>> https://delvingbitcoin.org/t/a-quantum-resistance-script-only-using-op-ctv-op-txhash-and-no-new-signatures/2168/4
>>
>> On Tue, Feb 10, 2026 at 7:19 PM Erik Aronesty <erik@q32.com> wrote:
>>
>>>
>>>>
>>>> You'd still need BIP 360 P2MR (or P2TRD) since OP_TXHASH needs
>>>> tapscript, and the only available tapscript supporting output type, P2TR,
>>>> isn't quantum safe.
>>>>
>>>
>>> false, covenant based multistep secret-reveal spending paths don't rely
>>> on signatures at all
>>>
>>>
>>>>
>>>> I'm going to assume:
>>>> - you mean to use this commit-reveal for migrating between signature
>>>> algorithms, not for everyday use,
>>>>
>>>
>>> it can be used if "q day" happens. otherwise ignored.
>>>
>>>
>>>> - TXHASH is being used because you are waiting for the commitment to be
>>>> confirmed on-chain rather than lifeboat's out-of-band commitment system
>>>>
>>>
>>> it's used so you can commit to a spending constraint without committing
>>> to the final "as yet to be determined" quantum-safe destination:
>>> https://delvingbitcoin.org/t/a-quantum-resistance-script-only-using-op-ctv-op-txhash-and-no-new-signatures/2168
>>>
>>>
>>>
>>>> Once you post your commit-txn, but before it confirms, other parties
>>>> can post competing commit-txns that double spend your output. If one of
>>>> malicious transactions confirm, you must now wait for a timelock to expire
>>>> and then try to post your transaction.
>>>>
>>>
>>> agreed. they have to spend resources to attack your private key and the
>>> only thing they can do is "grief" using a timing attack with the results,
>>> rather than steal outright. a massive incentive difference.
>>>
>>>
>>>> They can block you again. Each time they burn some of you coins in
>>>> fees. Miners get the fees, so they might be incentivized to do this. Thus,
>>>> we must trust miners not to do this. Lifeboat doesn't have this issue since
>>>> it uses out-of-band commitments, but out-of-band commitments have their own
>>>> issues.
>>>>
>>>
>>> each time you use the reset-path, they have to re-attack a new key.
>>> very expensive just to grief a small amount of fees spread across all
>>> miners. sounds like science-fiction levels of compute.
>>>
>>>
>>> plus.... TX_HASH is simple and generally useful and there is no
>>> guarantee that q-day will even come
>>>
>>

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