[bitcoin-dev] SAS: Succinct Atomic Swap

[Ruben Somsen]

Hi Lloyd,

>In my opinion, this protocol is theoretical breakthrough as well as a
practical protocol. Well done!

Thanks for the kind praise, and for providing a summary of what you think
makes the protocol useful. Your different perspective is undoubtedly useful
for others who are trying to understand it.

>We might call this a "Forced Refund *TLC"

Good description, I like it.

>The advantages that Ruben's two tx protocol has over this is that
timelocks and monitoring is only needed on one of the chains.

Well put, and I agree with your point that the traditional 4 tx protocol
can also be turned into 2 tx with an online requirement. One minor thing to
add is that this would make the 4 tx protocol more clunky in the
non-cooperative case (a 4 tx timeout). In the SAS protocol it comes at no
cost.

Cheers,
Ruben

On Tue, May 12, 2020 at 1:30 PM Ruben Somsen <rsomsen at gmail.com> wrote:

> Hi ZmnSCPxj,
>
> >Would this not work?
>
> I considered and rejected that model for the following reason: there are
> moments where both Alice and Bob can claim the BTC. If they both attempt to
> do so, it also reveals both secrets, causing the LTC to also be claimable
> by both parties. This chaotic scenario is a failure mode that did not seem
> acceptable to me. The revoke transaction was specifically added to mitigate
> that issue (invalidating any attempt of Bob to claim the coins and reveal
> his secret). That said, it doesn't particularly seem in either party's
> interest wait until a moment where two timelocks become valid, so maybe it
> is not quite as bad as I thought. However, it still means that the
> incompetence/malevolence of one party can lead to losses for both parties.
> I have my doubts a gain in privacy in the uncooperative case is worth that
> risk.
>
> Of course it also reverts the protocol to 3 transactions, instead of 2,
> but regardless, not having to watch the chain is probably more practical in
> many cases. As an aside, if both chains support timelocks then we can
> ensure that the more expensive chain only receives one transaction.
>
> >if relative locktimes are used as often as absolute locktimes for
> block-sniping-prevention and a decent Scriptless Script system, then all
> protocol aborts should be doable with no information leaks
>
> I see your point, interesting observation.
>
> >A sidenote as well, that if Alice typically uses an HD wallet, the UTXO
> on the LTC side would not be in that HD, and if Alice wants to cold-store
> the LTC, it should move the money as well into an HD pubkey.
>
> Agreed, I had that listed as one of the disadvantages: "Access to money is
> contingent on remembering secrets (backup complexity)"
>
> Cheers,
> Ruben
>
>
> On Tue, May 12, 2020 at 8:50 AM Lloyd Fournier <lloyd.fourn at gmail.com>
> wrote:
>
>> A quick correction to my post:
>>
>>>
>>> Here's where the truly novel part comes in. Ruben solves this by
>>> extending the standard *TLC contract:
>>> 1. Bob redeem with secret
>>> 2. Alice refund after T1
>>> 3. Bob redeem without secret after T2
>>>
>>> This is actually:
>>
>> 1. Bob redeem with redeem secret
>> 2. Alice refund after T1 with refund secret
>> 3. Bob redeem without secret after T2
>>
>> The fact that Alice reveals a secret when she refunds is crucial.
>>
>> LL
>>
>
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