[bitcoin-dev] Statechain coinswap: assigning blame for failure in a two-stage transfer protocol.

[Tom Trevethan]

Hi ZmnSCPxj,

Thanks for the reply.

> Okay, I suppose this is much too high-level a view, and I have no idea
what you mean by "statecoin" exactly.

Sorry, most of the protocol details are in the links, but terminology
should be made clearer. A "statecoin" is a UTXO that is a 2-of-2 between
the owner and SE (the tr*sted signing server) i.e. can be transferred
off-chain.

Also, should have been clear that `addr1` is the 'statecoin address' which
is different from the on-chain address (the shared public key the bitcoin
is paid to). The on-chain address does not change, whereas
the 'statecoin address' changes with each new owner and is used to
authenticate owners to the SE and act as proof of ownership on
the statechain - it is not related to the onchain address/pubkey and
controlled by the owner only.

> So it seems to me that this requires tr\*st that the coordinator is not
going to collude with other participants.

This is correct. The SE also must be trusted to not actively defraud users.
The main advantage of this scheme is that assuming the SE can be trusted,
it is strictly non-custodial.

> This is strictly worse than say Wasabi, where the coordinator colluding
with other participants only allows the coordinator to break privacy, not
outright steal funds.
> It seems to me that the trust-minimized CoinSwap plan by belcher_ is
superior to this, with reduced scope for theft.

This is true if the overriding aim is trust minimisation, but not if the
aim is speed and cost while staying non-custodial. Off-chain SE
transactions are near instant and orders of magnitude cheaper than
on-chain. Probably best thought of as a non-custodial centralised mixer.

Tom

On Wed, Sep 16, 2020 at 2:04 AM ZmnSCPxj <ZmnSCPxj at protonmail.com> wrote:

> Good morning Tom,
>
>
> > Here is a high-level description of how this blinding can operate - with
> the aim that the conductor does learn how the ownership of individual coins
> has changed.
> > For example, imagine 4 individuals (A,B,C and D) who own equal value
> statecoins utxo1, utxo2, utxo3 and utxo4 respectively. They want to swap
> ownership privately, trusting the conductor/SCE to enforce atomicity. In
> other words, the conductor will randomly assign each statecoin to one of
> the owners (the mix), but will not be able to gain knowledge of that
> assignment.
> > 1. A,B,C and D signal their participation by signing the swap_token
> (which has details of the swap) with the proof-key of their input coin. (A
> statecoin address is formed of a concatenation of the proof key and backup
> address).
> > 2. Each of A,B,C and D then generate a new statecoin address (where they
> what to receive the swapped coin), which they blind (encrypt) and sign with
> the proof key of their input coin: add1, add2, add3 and add4 and send to
> the conductor.
> > 3. The conductor authenticates each signature and then signs each
> payload (i.e. the blinded destination addresses) with a blinded signature
> scheme and returns these signatures to A,B,C and D.
> > 4. Each of A,B,C and D then reconnects over TOR with a new identity.
> > 5. Each of A,B,C and D then send their unblinded destination address
> with the conductor signature to the conductor (the conductor now knows that
> these 4 addresses belong to A,B,C and D, but not which ones map to each
> input.)
> > 6. The conductor randomly assigns each address to one of utxo1, utxo2,
> utxo3 and utxo4 (e.g. utxo1:add3, utxo2:add1, utxo3:add4 and utxo4:add2)
> and requests each participant to initiate the transfer to the given address.
> > 7. Each participant then finalises each transfer - if any transfer fails
> (due to a participant disappearing or acting maliciously) then all
> transfers are reverted - here atomicity is guaranteed by the SCE.
>
> Okay, I suppose this is much too high-level a view, and I have no idea
> what you mean by "statecoin" exactly.
>
> Let me try to fill in the details and correct me if I am wrong okay?
>
> I imagine that the `add1` etc. are implemented as 2-of-2 between the
> purported owner and the tr\*sted signing module.
> The owner of that address can easily create this knowing only the pubkey
> of the tr\*sted signing module.
>
> The initial `utxo1`... are also in similar 2-of-2s.
>
> (they cannot be unilateral control, since then a participant can broadcast
> a replacement transaction, even without RBF, almost directly to miners.)
>
> So when the coordinator talks to Alice, who owns `utxo1` and destination
> `addr1`, it provides partially-signed transactions of `utxo#:addr#`.
> Alice then checks that its `addr1` is on one of those transactions, with
> the correct amount, then provides a signature for the `utxo1:addr#`
> transaction.
>
> However, then the coordinator, who happens to be in cahoots with Bob,
> Charlie, and Dave, simply broadcasts that transaction without soliciting
> the `utxo#:addr1` transaction.
>
> So it seems to me that this requires tr\*st that the coordinator is not
> going to collude with other participants.
> This is strictly worse than say Wasabi, where the coordinator colluding
> with other participants only allows the coordinator to break privacy, not
> outright steal funds.
>
> It seems to me that the trust-minimized CoinSwap plan by belcher_ is
> superior to this, with reduced scope for theft.
> The plan by belcher_ is potentially compatible with using watchtowers that
> can be used for both CoinSwap and Lightning as well (if we design it well)
> with the watchtower potentially not even learning whether it is watching a
> CoinSwap or a Lightning channel.
>
> Though of course I could be misunderstanding the scheme itself.
> Is my understanding correct?
>
> Regards,
> ZmnSCPxj
>
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