On Tue, Feb 20, 2018 at 08:59:07AM +1000, Anthony Towns wrote:
> My understanding of lightning may be out of date, so please forgive
> (or at least correct :) any errors on my behalf.

> I'm not 100% sure how this approach works compared to the current one
> for the CSV/CLTV overlap problem. I think any case you could solve by
> obtaining a HTLC-Timeout or HTLC-Success transaction currently, you could
> solve in the above scenario by just updating the channel state to remove
> the HTLC.

So, I didn't understand the HTLC-Timeout/HTLC-Success transactions (you
don't have to obtain them separately, they're provided along with every
commitment tx), and the current setup works better than what I suggest
unless to_self_delay is very small.

It could be possible to make that a tradeoff: choose a small to_self_delay
because you're confident you'll monitor the chain and quickly penalise any
cheating, with the bonus that that makes monitoring cheaply outsourcable
even for very active channels; or choose a large to_self_delay and have
it cost a bit more to outsource monitoring.

Anyway.

You can redo all the current txes with Schnorr/muSig/scriptless-scripts
fine, I think:

 - funding tx is 2-of-2 muSig

 - the commitment tx I hold has outputs for:
      your balance - payable to A(i)
      my balance - payable to A(i)+R(B,i)
      each in-flight HTLC - payable to A(i)+R(B,i)+X(j)
   where
      A(i) is your pubkey for commitment i
      R(B,i) is my revocation hash for commitment i
      X(j) is a perturbation for the jth HTLC to make it hard to know
        which output is a HTLC and which isn't
   spends the funding tx
   locktime and sequence of the funding tx input encode i
   partially signed by you

 - the HTLC-Success/HTLC-Timeout txes need to have two phases, one that
   can immediately demonstrate the relevent condition has been met, and
   a second with a CSV delay to ensure cheating can be penalised.

   so:
     HTLC-Success: pays A(i)+R(B,i)+Y(j), partially signed by you
       with scriptless script requirement of revealing preimage for
       corresponding payment hash
     HTLC-Timeout: pays A(i)+R(B,i)+Y(j), partially signed by you
       with locktime set to enforce timeout

 - you also need a claim transaction for each output you can possibly
   spend:
     Balance-Claim: pays B(i), funded by my balance output, partially
       signed by you, with sequence set to enforce relative timelock of
       to_self_delay
     HTLC-Claim: pays B(i)+Z(j), funded by the j'th
       HTLC-Success/HTLC-Timeout transaction, partially signed by you,
       with sequence set to enforce relative timelock of to_self_delay

   where Y(j) and Z(j) are similar to X(j) and are just to make it hard
   for third parties to tell the relationship between outputs

Each of those partial signatures require me to have sent you a unique ECC
point J, for which I know the corresponding secret. I guess you'd just
need to include those in the revoke_and_ack and update_add_htlc messages.

The drawback with this approach is that to outsource claiming funds
(without covenants or SIGHASH_NOINPUT), you'd need to send signatures
for 2+2N outputs for every channel update, rather than just 1, and the
claiming transactions would be a lot larger.

This retains the advantage that you don't have to store any info about
outdated HTLCs if you're monitoring for misbehaviour yourself; you just
need to send an extra two signatures for every in-flight HTLC for every
channel update if you're outsourcing channel monitoring.

Posting a penalty transaction in this scheme isn't as cheap as just
being 1-in-1-out, but if you're doing it yourself, it's still cheaper
than trying to claim the funds while misbehaving: you can do it all in a
single transaction, and if cross-input signature aggregation is supported,
you can do it all with a single signature; while they will need to supply
at least two separate transactions, and 1+2N signatures.

> If your channel updates 100 times a second for an entire year, that's
> 200GB of data, which seems pretty feasible.

If you update the channel immediately whenever a new HTLC starts or
ends, that's 50 HTLCs per second on average; if they last for 20 seconds
on average, it's 1000 HTLCs at any one time on average, so trustless
outsourcing would require storing about 2000 signatures per update,
which at 64B per signature, is 13MB/second, or about a terabyte per
day. Not so feasible by comparison.

The channel update rate is contributing quadratically to that calculation
though, so reducing the rate of incoming HTLCs to 2 per second on average,
but capping channel updates at 1 per second, gives an average of 40
HTLCs at any one time and 81 signatures per update, for 450MB per day
or 163GB per year, which isn't too bad.

(I guess if you want the privacy preserving features of WatchTower
monitoring you'd have to roughly double that space requirement? Not
real sure)

Cheers,
aj

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