[bitcoin-dev] Congestion Control via OP_CHECKOUTPUTSHASHVERIFY proposal

ZmnSCPxj ZmnSCPxj at protonmail.com
Sat May 25 03:56:22 UTC 2019

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.


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‐‐‐‐‐‐‐ Original Message ‐‐‐‐‐‐‐
On Saturday, May 25, 2019 5:15 AM, Jeremy <jlrubin at 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?

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