Re: [bitcoin-dev] [Opt-in full-RBF] Zero-conf apps in immediate danger

[Greg Sanders <gsanders87@gmail.com>]

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Another downside is that the sender may not opt into a non-pinnable future
format like "V3 transactions", making CPFP difficult. They may spend a lot
of fees to do this however, so maybe we're really reaching here.

On Wed, Oct 19, 2022 at 12:07 PM Sergej Kotliar via bitcoin-dev <
bitcoin-dev@lists.linuxfoundation.org> wrote:

> It's an interesting idea, presumably it would work w the new package relay.
> Scorched earth bidding war is definitely fine to deter this type of abuse.
> Need to consider it more thoroughly from all sides tho. CPFP on the server
> side generally has a couple of downsides:
> * Requires a hot wallet to receive bitcoin
> * an entity that is reliably known to do CPFP can be abused by people
> looking to consolidate utxos, which can be quite costly. Might be solvable
> with a set of conditionals, and bad UX for abusers is less of a concern :)
>
> Will follow up after more deliberation, thanks!
>
>
> On Wed, 19 Oct 2022 at 17:43, Jeremy Rubin <jeremy.l.rubin@gmail.com>
> wrote:
>
>> If they do this to you, and the delta is substantial, can't you sweep all
>> such abusers with a cpfp transaction replacing their package and giving you
>> the original txn?
>>
>> On Wed, Oct 19, 2022, 7:33 AM Sergej Kotliar via bitcoin-dev <
>> bitcoin-dev@lists.linuxfoundation.org> wrote:
>>
>>> Hi all,
>>>
>>> Chiming in on this thread as I feel like the real dangers of RBF as
>>> default policy aren't sufficiently elaborated here. It's not only about the
>>> zero-conf (I'll get to that) but there is an even bigger danger called the
>>> american call option, which risks endangering the entirety of BIP21 "Scan
>>> this QR code with your wallet to buy this product" model that I believe
>>> we've all come to appreciate. Specifically, in a scenario with high
>>> volatility and many transactions in the mempools (which is where RBF would
>>> come in handy), a user can make a low-fee transaction and then wait for
>>> hours, days or even longer, and see whether BTCUSD moves. If BTCUSD moves
>>> up, user can cancel his transaction and make a new - cheaper one. The
>>> biggest risk in accepting bitcoin payments is in fact not zeroconf risk
>>> (it's actually quite easily managed), it's FX risk as the merchant must
>>> commit to a certain BTCUSD rate ahead of time for a purchase. Over time
>>> some transactions lose money to FX and others earn money - that evens out
>>> in the end. But if there is an _easily accessible in the wallet_ feature to
>>> "cancel transaction" that means it will eventually get systematically
>>> abused. A risk of X% loss on many payments that's easy to systematically
>>> abuse is more scary than a rare risk of losing 100% of one occasional
>>> payment. It's already possible to execute this form of abuse with opt-in
>>> RBF, which may lead to us at some point refusing those payments (even with
>>> confirmation) or cumbersome UX to work around it, such as crediting the
>>> bitcoin to a custodial account.
>>>
>>> To compare zeroconf risk with FX risk: I think we've had one incident in
>>> 8 years of operation where a user successfully fooled our server to accept
>>> a payment that in the end didn't confirm. To successfully fool (non-RBF)
>>> zeroconf one needs to have access to mining infrastructure and probability
>>> of success is the % of hash rate controlled. This is simply due to the fact
>>> that the network currently won't propagage the replacement transaction to
>>> the miner, which is what's being discussed here. American call option risk
>>> would however be available to 100% of all users, needs nothing beyond the
>>> wallet app, and has no cost to the user - only upside.
>>>
>>> Bitrefill currently processes 1500-2000 onchain payments every day. For
>>> us, a world where bitcoin becomes de facto RBF by default, means that we
>>> would likely turn off the BIP21 model for onchain payments, instruct
>>> Bitcoin users to use Lightning or deposit onchain BTC to a custodial
>>> account that we have.
>>> This option is however not available for your typical
>>> BTCPayServer/CoinGate/Bitpay/IBEX/OpenNode et al. Would be great to hear
>>> from other merchants or payment providers how they see this new behavior
>>> and how they would counteract it.
>>>
>>> Currently Lightning is somewhere around 15% of our total bitcoin
>>> payments. This is very much not nothing, and all of us here want Lightning
>>> to grow, but I think it warrants a serious discussion on whether we want
>>> Lightning adoption to go to 100% by means of disabling on-chain commerce.
>>> For me personally it would be an easier discussion to have when Lightning
>>> is at 80%+ of all bitcoin transactions. Currently far too many bitcoin
>>> users simply don't have access to Lightning, and of those that do and hold
>>> their own keys Muun is the biggest wallet per our data, not least due to
>>> their ease-of-use which is under threat per the OP. It's hard to assess how
>>> many users would switch to Lightning in such a scenario, the communication
>>> around it would be hard. My intuition says that the majority of the current
>>> 85% of bitcoin users that pay onchain would just not use bitcoin anymore,
>>> probably shift to an alt. The benefits of Lightning are many and obvious,
>>> we don't need to limit onchain to make Lightning more appealing. As an
>>> anecdote, we did experiment with defaulting to bech32 addresses some years
>>> back. The result was that simply users of the wallets that weren't able to
>>> pay to bech32 didn't complete the purchase, no support ticket or anything,
>>> just "it didn't work 🤷‍♂️" and user moved on. We rolled it back, and later
>>> implemented a wallet selector to allow modern wallets to pay to bech32
>>> while other wallets can pay to P2SH. This type of thing  is clunky, and
>>> requires a certain level of scale to be able to do, we certainly wouldn't
>>> have had the manpower for that when we were starting out. This why I'm
>>> cautious about introducing more such clunkiness vectors as they are
>>> centralizing factors.
>>>
>>> I'm well aware of the reason for this policy being suggested and the
>>> potential pinning attack vector for LN and other smart contracts, but I
>>> think these two risks/costs need to be weighed against eachother first and
>>> thoroughly discussed because the costs are non-trivial on both sides.
>>>
>>> Sidenote: On the efficacy of RBF to "unstuck" stuck transactions
>>> After interacting with users during high-fee periods I've come to not
>>> appreciate RBF as a solution to that issue. Most users (80% or so) simply
>>> don't have access to that functionality, because their wallet doesn't
>>> support it, or they use a custodial (exchange) wallet etc. Of those that
>>> have the feature - only the power users understand how RBF works, and
>>> explaining how to do RBF to a non-power-user is just too complex, for the
>>> same reason why it's complex for wallets to make sensible non-power-user UI
>>> around it. Current equilibrium is that mostly only power users have access
>>> to RBF and they know how to handle it, so things are somewhat working. But
>>> rolling this out to the broad market is something else and would likely
>>> cause more confusion.
>>> CPFP is somewhat more viable but also not perfect as it would require
>>> lots of edge case code to handle abuse vectors: What if users abuse a
>>> generous CPFP policy to unstuck past transactions or consolidate large
>>> wallets. Best is for CPFP to be done on the wallet side, not the merchant
>>> side, but there too are the same UX issues as with RBF.
>>> In the end a risk-based approach to decide on which payments are
>>> non-trivial to reverse is the easiest, taking account user experience and
>>> such. Remember that in the fiat world card payments have up to 5%
>>> chargebacks, whereas we in zero-conf bitcoin land we deal with "fewer than
>>> 1 in a million" accepted transactions successfully reversed. These days we
>>> have very few support issues related to bitcoin payments. The few that do
>>> come in are due to accidental RBF users venting frustration about waiting
>>> for their tx to confirm.
>>> "In theory, theory and practice are the same. In practice, they are not"
>>>
>>> All the best,
>>> Sergej Kotliar
>>> CEO Bitrefill.com
>>>
>>>
>>> --
>>>
>>> Sergej Kotliar
>>>
>>> CEO
>>>
>>>
>>> Twitter: @ziggamon <https://twitter.com/ziggamon>
>>>
>>>
>>> www.bitrefill.com
>>>
>>> Twitter <https://www.twitter.com/bitrefill> | Blog
>>> <https://www.bitrefill.com/blog/> | Angellist
>>> <https://angel.co/bitrefill>
>>>
>>>
>>> --
>>>
>>> Sergej Kotliar
>>>
>>> CEO
>>>
>>>
>>> Twitter: @ziggamon <https://twitter.com/ziggamon>
>>>
>>>
>>> www.bitrefill.com
>>>
>>> Twitter <https://www.twitter.com/bitrefill> | Blog
>>> <https://www.bitrefill.com/blog/> | Angellist
>>> <https://angel.co/bitrefill>
>>> _______________________________________________
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>>> bitcoin-dev@lists.linuxfoundation.org
>>> https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev
>>>
>>
>
> --
>
> Sergej Kotliar
>
> CEO
>
>
> Twitter: @ziggamon <https://twitter.com/ziggamon>
>
>
> www.bitrefill.com
>
> Twitter <https://www.twitter.com/bitrefill> | Blog
> <https://www.bitrefill.com/blog/> | Angellist <https://angel.co/bitrefill>
> _______________________________________________
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