Re: [bitcoin-dev] Congestion Control via OP_CHECKOUTPUTSHASHVERIFY proposal

[ZmnSCPxj <ZmnSCPxj@protonmail.com>]

Good morning Jeremy,

I believe I have caught the general point.
Indeed, I agree that this is useful, but it is *not* useful for these cases:

1.  CoinJoin - the initial funding transaction must be signed by the participants anyway after checking that the output is correct.
    Further any spend that is not a signature spend is going to defeat the purpose of CoinJoin trying to be private by imitating "typical" spends: if `OP_CHECKOUTPUTSHASHVERIFY` path is used, you have just lost the CoinJoin privacy by reducing anonymity set.
2.  Channel Factories - the initial funding transaction must be signed by the participants anyway after each initial sub-channel initial commitment / initial update+state transaction is signed.

In both above cases, the issue of users dropping out during the step of signing the initial funding transaction is unavoidable even with `OP_CHECKOUTPUTSHASHVERIFY`.

For congestion control, and for general "I promise to set this up later" as in c*stodial-service-directly-to-channel etc., I already agree this is useful.

My objection lies *only* with the above two cases, wherein `OP_CHECKOUTPUTSHASHVERIFY` does not really improve things, as you *still* need to coordinate multiple signers anyway.

You have convinced me already that the other cases are good example usages of this opcode.

Regards,
ZmnSCPxj




Sent with ProtonMail Secure Email.

‐‐‐‐‐‐‐ Original Message ‐‐‐‐‐‐‐
On Saturday, May 25, 2019 5:15 AM, Jeremy <jlrubin@mit.edu> wrote:

> ZmnSCIPxj,
>
> I think you're missing the general point, so I'm just going to respond to one point to see if that helps your understanding of why OP_COSHV is better than just pre-signed.
>
> The reason why MuSig and other distributed signing solutions are not acceptable for this case is they all require interaction for guarantee of payout.
>
> In contrast, I can use a OP_COSHV Taproot key to request a withdrawal from an exchange which some time later pays out to a lot of people, rather than having to withdraw multiple times and then pay. The exchange doesn't have to know this is what I did. They also don't have to tell me the exact inputs they'll spend to me or if I'm batched or not (batching largely incompatible with pre-signing unless anyprevout)
>
> The exchange can take my withdrawal request and aggregate it to other payees into a tree as well, without requiring permission from the recipients.
>
> They can also -- without my permission -- make the payment not directly into me, but into a payment channel between me and the exchange, allowing me to undo the withdrawal by routing money back to the exchange over lightning.
>
> The exchange can take some inbound payments to their hot wallet and move them into cold storage with pre-set spending paths. They don't need to use ephemeral keys (how was that entropy created?) nor do they need to bring on their cold storage keys to pre-sign the spending paths.
>
> None of this really works well with just pre-signing because you need to ask for permission first in order to do these operations, but with OP_COSHV you can, just as the payer without talking to anyone else, or just as the recipient commit your funds to a complex txn structure.
>
> Lastly, think about this in terms of DoS. You have a set of N users who request a payment. You build the tree, collect signatures, and then at the LAST step of building the tree, one user drops out. You restart, excluding that user. Then a different user drops. Meanwhile you've had to keep your funds locked up to guarantee those inputs for the txn when it finalizes.
>
> In contrast, once you receive the requests with OP_COSHV, there's nothing else to do. You just issue the transaction and move on.
>
> Does that make sense as to why a user would prefer this, even if there is an emulation with pre-signed txns?