Hi Rene, Thanks for disclosing this vulnerability,
I think this blackmail scenario holds but sadly there is a lower scenario. Both "Flood & Loot" and your blackmail attack rely on `update_fee` mechanism and unbounded commitment transaction size inflation. Though the first to provoke block congestion and yours to lockdown in-flight fees as funds hostage situation. > 1. The current solution is to just not use up the max value of htlc's. Eclaire and c-lightning by default only use up to 30 htlcs. As of today, yes I would recommend capping commitment size both for ensuring competitive propagation/block selection and limiting HTLC exposure. > 2. Probably the best fix (not sure if I understand the consequences correctly) is coming from this PR to bitcoin core (c.f. https://github.com/bitcoin/bitcoin/pull/15681 by @TheBlueMatt . If I get it correctly with that we could always have low fees and ask the person who want to claim their outputs to pay fees. This excludes overpayment and could happen at a later stage when fees are not spiked. Still the victim who offered the htlcs would have to spend those outputs at some time. It's a bit more complex, carve-out output, even combined with anchor output support on the LN-side won't protect against different flavors of pinning. I invite you to go through logs of past 2 LN dev meetings. > 3. Don't overpay fees in commitment transactions. We can't foresee the future anyway Once 2. is well-addressed we may deprecate `update_fee`. > 4. Don't add htlcs for which the on chain fee is higher than the HTLCs value (like we do with sub dust amounts and sub satoshi amounts. This would at least make the attack expensive as the attacker would have to bind a lot of liquidity. Ideally we want dust_limit to be dynamic, dust cap should be based on HTLC economic value, feerate of its output, feerate of HTLC-transaction, feerate estimation of any CPFP to bump it. I think that's kind of worthy to do once we solved 3. and 4 > 5. Somehow be able to aggregate htlc's. In a world where we use payment points instead of preimages we might be able to do so. It would be really cool if separate HTLC's could be combined to 1 single output. I played around a little bit but I have not come up with a scheme that is more compact in all cases. Thus I just threw in the idea. Yes we may encode all HTLC in some Taproot tree in the future. There are some wrinkles but for a high-level theoretical construction see my post on CoinPool. > 6. Split onchain fees differently (now the attacker would also lose fees by conducting this attack) - No I don't want to start yet another fee bikeshadding debate. (In particular I believe that a different split of fees might make the Flood & Loot attack economically more viable which relies on the same principle) Likely a bit more of fee bikeshedding is something we have to do to make LN secure... Switching fee from pre-committed ones to a single-party, dynamic one. > Independently I think we should have a hint in our readme file about where and how people can disclose attacks and vulnerabilities. Implementations have this but the BOLTs do not. I 100% agree, that's exactly https://github.com/lightningnetwork/lightning-rfc/pull/772, waiting for your feedback :) Cheers, Antoine Le mer. 17 juin 2020 à 09:41, ZmnSCPxj via Lightning-dev < lightning-dev@lists.linuxfoundation.org> a écrit : > > Good morning all, > > > > > Fee futures could help against this. > > I remember writing about this some time ago but cannot find where (not > sure if it was in lightning-dev or bitcoin-dev). > > `harding` found it: > https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2020-January/017601.html > > Regards, > ZmnSCPxj > _______________________________________________ > Lightning-dev mailing list > Lightning-dev@lists.linuxfoundation.org > https://lists.linuxfoundation.org/mailman/listinfo/lightning-dev >
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