Bastien TEINTURIER [ARCHIVE] on Nostr: 📅 Original date posted:2021-12-08 📝 Original message: Hi again AJ and list, I ...

📅 Original date posted:2021-12-08
📝 Original message:
Hi again AJ and list,

I have slightly re-worked your proposal, and came up with the following
(I also added the musig2 nonces for completeness):

Alice -> Bob: commitment_proposed
channel id
adaptor sigs for PTLCs to Bob in Alice's next commitment
musig nonces for Alice to spend funding tx
musig nonces for Bob to spend funding tx

Bob -> Alice: commitment_proposed
channel id
adaptor sigs for PTLCs to Alice in Bob's next commitment
musig nonces for Alice to spend funding tx
musig nonces for Bob to spend funding tx

Bob -> Alice: commitment_signed
channel id
signature for Alice to spend funding tx
sigs for Alice to spend HTLCs and PTLCs from her next commitment

Alice -> Bob: revoke_and_ack
channel id
reveal previous commitment secret
next commitment point

Alice -> Bob: commitment_signed
channel id
signature for Bob to spend funding tx
sigs for Bob to spend HTLCs and PTLCs from his next commitment

Bob -> Alice: revoke_and_ack
channel id
reveal previous commitment secret
next commitment point

I believe it's exactly the same flow of data between peers as your
proposal, but I simply split the data into several messages. Let me
know if that's incorrect or if I missed a subtlety in your proposal.

This has some small advantages:

* commitment_signed and revoke_and_ack are mostly unchanged, we just
add a new message before the commit / revoke dance. The only change
happens in commitment_signed, where the signatures for PTLC-success
transactions will actually become adaptor signatures.
* the new adaptor signatures are in commitment_proposed instead of being
in commitment_signed, which ensures that we can still have 2*483
pending (H|P)TLCs: since the message size is limited to 65kB, we would
otherwise decrease our maximum to ~2*335 with your proposal (very rough
calculation)
* the messages are now symmetrical, which may be easier to reason about

One thing to note is that we reversed the order in which participants
sign new commitments. We previously had Alice sign first, whereas now
if Alice initiates, Bob will sign the updated commitment first. This is
why we add only 0.5 RTT instead of 1 RTT compared to the current protocol.
I don't think this is an issue, but if someone sees a way to maliciously
exploit this, please share it!

I updated my article [0], people jumping on the thread now may find it
helpful to better understand this discussion.

Thanks,
Bastien

[0] https://github.com/t-bast/lightning-docs/pull/16

Le mer. 8 déc. 2021 à 11:00, Bastien TEINTURIER <bastien at acinq.fr> a écrit :

> Hi AJ,
>
> I think the problem t-bast describes comes up here as well when you
>> collapse the fast-forwards (or, anytime you update the commitment
>> transaction even if you don't collapse them).
>
>
> Yes, exactly.
>
> I think doing a synchronous update of commitments to the channel state,
>> something like:
>
>
>
> Alice -> Bob: propose_new_commitment
>> channel id
>> adaptor sigs for PTLCs to Bob
>
>
>> Bob -> Alice: agree_new_commitment
>> channel id
>> adaptor sigs for PTLCs to Alice
>> sigs for Alice to spend HTLCs and PTLCs to Bob from her own
>> commitment tx
>> signature for Alice to spend funding tx
>>
>> Alice -> Bob: finish_new_commitment_1
>> channel id
>> sigs for Bob to spend HTLCs and PTLCs to Alice from his own
>> commitment tx
>> signature for Bob to spend funding tx
>> reveal old prior commitment secret
>> new commitment nonce
>>
>> Bob -> Alice: finish_new_commitment_2
>> reveal old prior commitment secret
>> new commitment nonce
>>
>> would work pretty well.
>
>
> I agree, this is better than my naive addition of a `remote_ptlcs_signed`
> message in both directions, and even though it changes the protocol
> messages
> it stays very close to the mechanisms we currently have.
>
> I'll spend some time specifying this in more details, to verify that we're
> not missing anything. What I really like about this proposal is that we
> can probably bundle that protocol change with `option_simplified_update`
> [0]
> without the adaptor sigs, and simply add the adaptor sigs as tlvs when we
> do PTLCs. That lets us deploy this new update protocol separately from
> PTLCs
> and ensure it also simplifies the state machine and makes other features
> such as splicing [1] and dynamic channel upgrades [2] easier.
>
> Thanks,
> Bastien
>
> [0] https://github.com/lightning/bolts/pull/867
> [1] https://github.com/lightning/bolts/pull/863
> [2] https://github.com/lightning/bolts/pull/868
>
> Le mer. 8 déc. 2021 à 10:29, Anthony Towns <aj at erisian.com.au> a écrit :
>
>> On Tue, Dec 07, 2021 at 11:52:04PM +0000, ZmnSCPxj via Lightning-dev
>> wrote:
>> > Alternately, fast-forwards, which avoid this because it does not change
>> commitment transactions on the payment-forwarding path.
>> > You only change commitment transactions once you have enough changes to
>> justify collapsing them.
>>
>> I think the problem t-bast describes comes up here as well when you
>> collapse the fast-forwards (or, anytime you update the commitment
>> transaction even if you don't collapse them).
>>
>> That is, if you have two PTLCs, one from A->B conditional on X, one
>> from B->A conditional on Y. Then if A wants to update the commitment tx,
>> she needs to
>>
>> 1) produce a signature to give to B to spend the funding tx
>> 2) produce an adaptor signature to authorise B to spend via X from his
>> commitment tx
>> 3) produce a signature to allow B to recover Y after timeout from his
>> commitment tx spending to an output she can claim if he cheats
>> 4) *receive* an adaptor signature from B to be able to spend the Y
>> output
>> if B posts his commitment tx using A's signature in (1)
>>
>> The problem is, she can't give B the result of (1) until she's received
>> (4) from B.
>>
>> It doesn't matter if the B->A PTLC conditional on Y is in the commitment
>> tx itself or within a fast-forward child-transaction -- any previous
>> adaptor sig will be invalidated because there's a new commitment
>> transaction, and if you allowed any way of spending without an adaptor
>> sig, B wouldn't be able to recover the secret and would lose funds.
>>
>> It also doesn't matter if the commitment transaction that A and B will
>> publish is the same or different, only that it's different from the
>> commitment tx that previous adaptor sigs committed to. (So ANYPREVOUT
>> would fix this if it were available)
>>
>> So I think this is still a relevant question, even if fast-forwards
>> make it a rare problem, that perhaps is only applicable to very heavily
>> used channels.
>>
>> (I said the following in email to t-bast already)
>>
>> I think doing a synchronous update of commitments to the channel state,
>> something like:
>>
>> Alice -> Bob: propose_new_commitment
>> channel id
>> adaptor sigs for PTLCs to Bob
>>
>> Bob -> Alice: agree_new_commitment
>> channel id
>> adaptor sigs for PTLCs to Alice
>> sigs for Alice to spend HTLCs and PTLCs to Bob from her own
>> commitment tx
>> signature for Alice to spend funding tx
>>
>> Alice -> Bob: finish_new_commitment_1
>> channel id
>> sigs for Bob to spend HTLCs and PTLCs to Alice from his own
>> commitment tx
>> signature for Bob to spend funding tx
>> reveal old prior commitment secret
>> new commitment nonce
>>
>> Bob -> Alice: finish_new_commitment_2
>> reveal old prior commitment secret
>> new commitment nonce
>>
>> would work pretty well.
>>
>> This adds half a round-trip compared to now:
>>
>> Alice -> Bob: commitment_signed
>> Bob -> Alice: revoke_and_ack, commitment_signed
>> Alice -> Bob: revoke_and_ack
>>
>> The timings change like so:
>>
>> Bob can use the new commitment after 1.5 round-trips (previously 0.5)
>>
>> Alice can be sure Bob won't use the old commitment after 2 round-trips
>> (previously 1)
>>
>> Alice can use the new commitment after 1 round-trip (unchanged)
>>
>> Bob can be sure Alice won't use the old commitment after 1.5 round-trips
>> (unchanged -- note: this is what's relevant for forwarding)
>>
>> Making the funding tx a musig setup would mean also supplying 64B
>> of musig2 nonces along with the "adaptor sigs" in one direction,
>> and providing the other side's 64B of musig2 nonces back along with the
>> (now partial) signature for spending the funding tx (a total of 256B of
>> nonce data, not 128B).
>>
>> Because it keeps both peers' commitments synchronised to a single channel
>> state, I think the same protocol should work fine with the revocable
>> signatures on a single tx approach too, though I haven't tried working
>> through the details.
>>
>> Fast forwards would then be reducing the 2 round-trip protocol to
>> update the state commitment to a 0.5 round-trip update, to reduce
>> latency when forwarding by the same amount as before (1.5 round-trips
>> to 0.5 round-trips).
>>
>> Cheers,
>> aj
>>
>>
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