{"type":"rich","version":"1.0","author_name":"npub1e46n428mcyfwznl7nlsf6d3s7rhlwm9x3cmkuqzt3emmdpadmkaqqjxmcu","author_url":"https://nostr.ae/npub1e46n428mcyfwznl7nlsf6d3s7rhlwm9x3cmkuqzt3emmdpadmkaqqjxmcu","provider_name":"njump","provider_url":"https://nostr.ae","html":"📅 Original date posted:2020-04-22\n📝 Original message:\nA few replies inline.\n\nOn 4/22/20 12:13 AM, Olaoluwa Osuntokun wrote:\n\u003e Hi Matt,\n\u003e \n\u003e \n\u003e\u003e While this is somewhat unintuitive, there are any number of good anti-DoS\n\u003e\u003e reasons for this, eg:\n\u003e \n\u003e None of these really strikes me as \"good\" reasons for this limitation, which\n\u003e is at the root of this issue, and will also plague any more complex Bitcoin\n\u003e contracts which rely on nested trees of transaction to confirm (CTV, Duplex,\n\u003e channel factories, etc). Regarding the various (seemingly arbitrary) package\n\u003e limits it's likely the case that any issues w.r.t computational complexity\n\u003e that may arise when trying to calculate evictions can be ameliorated with\n\u003e better choice of internal data structures.\n\u003e \n\u003e In the end, the simplest heuristic (accept the higher fee rate package) side\n\u003e steps all these issues and is also the most economically rationale from a\n\u003e miner's perspective. Why would one prefer a higher absolute fee package\n\u003e (which could be very large) over another package with a higher total _fee\n\u003e rate_?\n\nThis seems like a somewhat unnecessary drive-by insult of a project you don't contribute to, but feel free to start with\na concrete suggestion here :).\n\n\u003e\u003e You'll note that B would be just fine if they had a way to safely monitor the\n\u003e\u003e global mempool, and while this seems like a prudent mitigation for\n\u003e\u003e lightning implementations to deploy today, it is itself a quagmire of\n\u003e\u003e complexity\n\u003e \n\u003e Is it really all that complex? Assuming we're talking about just watching\n\u003e for a certain script template (the HTLC scipt) in the mempool to be able to\n\u003e pull a pre-image as soon as possible. Early versions of lnd used the mempool\n\u003e for commitment broadcast detection (which turned out to be a bad idea so we\n\u003e removed it), but at a glance I don't see why watching the mempool is so\n\u003e complex.\n\nBecause watching your own mempool is not guaranteed to work, and during upgrade cycles that include changes to the\npolicy rules an attacker could exploit your upgraded/non-upgraded status to perform the same attack.\n\n\u003e\u003e Further, this is a really obnoxious assumption to hoist onto lightning\n\u003e\u003e nodes - having an active full node with an in-sync mempool is a lot more\n\u003e\u003e CPU, bandwidth, and complexity than most lightning users were expecting to\n\u003e\u003e face.\n\u003e \n\u003e This would only be a requirement for Lightning nodes that seek to be a part\n\u003e of the public routing network with a desire to _forward_ HTLCs. This isn't\n\u003e doesn't affect laptops or mobile phones which likely mostly have private\n\u003e channels and don't participate in HTLC forwarding. I think it's pretty\n\u003e reasonable to expect a \"proper\" routing node on the network to be backed by\n\u003e a full-node. The bandwidth concern is valid, but we'd need concrete numbers\n\u003e that compare the bandwidth over head of mempool awareness (assuming the\n\u003e latest and greatest mempool syncing) compared with the overhead of the\n\u003e channel update gossip and gossip queries over head which LN nodes face today\n\u003e as is to see how much worse off they really would be.\n\nIf mempool-watching were practical, maybe, though there are a number of folks who are talking about designing\npartially-offline local lightning hubs which would be rendered impractical.\n\n\u003e As detailed a bit below, if nodes watch the mempool, then this class of\n\u003e attack assuming the anchor output format as described in the open\n\u003e lightning-rfc PR is mitigated. At a glance, watching the mempool seems like\n\u003e a far less involved process compared to modifying the state machine as its\n\u003e defined today. By watching the mempool and implementing the changes in\n\u003e #lightning-rfc/688, then this issue can be mitigated _today_. lnd 0.10\n\u003e doesn't yet watch the mempool (but does include anchors [1]), but unless I'm\n\u003e missing something it should be pretty straight forward to add which mor or less\n\u003e resolves this issue all together.\n\u003e \n\u003e\u003e not fixing this issue seems to render the whole exercise somewhat useless\n\u003e \n\u003e Depends on if one considers watching the mempool a fix. But even with that a\n\u003e base version of anchors still resolves a number of issues including:\n\u003e eliminating the commitment fee guessing game, allowing users to pay less on\n\u003e force close, being able to coalesce 2nd level HTLC transactions with the\n\u003e same CLTV expiry, and actually being able to reliably enforce multi-hop HTLC\n\u003e resolution.\n\u003e \n\u003e\u003e Instead of making the HTLC output spending more free-form with\n\u003e\u003e SIGHASH_ANYONECAN_PAY|SIGHASH_SINGLE, we clearly need to go the other\n\u003e\u003e direction - all HTLC output spends need to be pre-signed.\n\u003e \n\u003e I'm not sure this is actually immediately workable (need to think about it\n\u003e more). To see why, remember that the commit_sig message includes HTLC\n\u003e signatures for the _remote_ party's commitment transaction, so they can\n\u003e spend the HTLCs if they broadcast their version of the commitment (force\n\u003e close). If we don't somehow also _gain_ signatures (our new HTLC signatures)\n\u003e allowing us to spend HTLCs on _their_ version of the commitment, then if\n\u003e they broadcast that commitment (without revoking), then we're unable to\n\u003e redeem any of those HTLCs at all, possibly losing money.\n\nHmm, maybe the proposal wasn't clear. The idea isn't to add signatures to braodcasted transactions, but instead to CPFP\na maybe-broadcasted transaction by sending a transaction which spends it and seeing if it is accepted. You only need to\nknow the transaction's exact format (ie txid, which we do, since we sent a signature for it long ago) to do this, you\ndon't have to actually *have* the fully-signed transaction (and you don't).\n\n\u003e In an attempt to counteract this, we might say ok, the revoke message also\n\u003e now includes HTLC signatures for their new commitment allowing us to spend\n\u003e our HTLCs. This resolves things in a weaker security model, but doesn't\n\u003e address the issue generally, as after they receive the commit_sig, they can\n\u003e broadcast immediately, again leaving us without a way to redeem our HTLCs.\n\u003e \n\u003e I'd need to think about it more, but it seems that following this path would\n\u003e require an overhaul in the channel state machine to make presenting a new\n\u003e commitment actually take at least _two phases_ (at least a full round trip).\n\u003e The first phase would tender the commitment, but render them unable to\n\u003e broadcast it. The second phase would then \u003cinsert something something\n\u003e scriptless scripts here\u003e enter a new sub-protocol which upon conclusion,\n\u003e gives the commitment proposer valid HTLC signatures, and gives the responder\n\u003e what they need to be able to broadcast their commitment and claim their\n\u003e HTCLs in an atomic manner.\n\u003e \n\u003e -- Laolu\n\u003e \n\u003e [1]: https://github.com/lightningnetwork/lnd/pull/3821"}