Date: Tue, 12 May 2020 15:05:57 +0000
To: Ruben Somsen <rsomsen@gmail.com>
From: ZmnSCPxj <ZmnSCPxj@protonmail.com>
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Cc: Bitcoin Protocol Discussion <bitcoin-dev@lists.linuxfoundation.org>
Subject: Re: [bitcoin-dev] SAS: Succinct Atomic Swap
Precedence: list

Good morning Ruben,

> >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 inte=
rest wait until a moment where two timelocks become valid, so maybe it is n=
ot 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 do=
ubts 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, b=
ut 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 ensur=
e that the more expensive chain only receives one transaction.

If the shortened refund transaction exists (labeled "refund transaction #1"=
 in the SVG) then the same issue still occurs: after 1 day it is possible f=
or either success or refund#1 to be broadcasted, leading to revelation of b=
oth secrets, leading to the same failure mode you described.

Without the refund#1 in your proposal, Bob refusing cooperation after Alice=
 puts the BTC into lock for 3 days and 2 further onchain transactions (with=
 the refund#2 transaction being relative-locked, meaning it cannot be used =
to CPFP the revoke transaction; my formulation allows any of the result tra=
nsactions to be CPFP directly by their beneficiaries).

It seems to me that there is still an onlineness requirement in case Bob do=
es not complete the protocol: once the revoke tx becomes valid an online Bo=
b can cheat an offline Alice by broadcasting the revoke tx (which, if my un=
derstanding of the protocol is correct, the signatures are shared to both A=
lice and Bob).
So Alice needs to be online starting at 2 days to 3 days in order to ensure=
 it reclaims it funds.

I have not seen the 2-tx variant video yet, as I prefer to read than listen=
, but I will also check it if I can find an opportunity.

Regardless, the overall protocol of using 3 clauses in the swap, and reusin=
g the privkey as the payment secret demanded by the pointlocks, is still a =
significant innovation.


In the context of CoinSwap, a proposal is that a CoinSwap server would prov=
ide swapping service to incoming clients.
Using my counterproposal, the Bob position can be taken by the server and t=
he Alice position taken by the client.
In this context, the L1 can be made reasonably close in the future and L2 f=
ar in the future, in which case Alice the client can be "weakly offline" mo=
st of the time until L2, and even in a protocol abort would be able to reco=
ver its funds.
If the protocol completes, the server Bob can claim its funds before L1, an=
d (with knowledge of Alice[0]) can immediately put it in a new funding tx f=
or a new incoming client before L1, which is a fine tradeoff for server Bob=
 since presumably Bob is always online.

Regards,
ZmnSCPxj