" C cannot unlock until 144 blocks, so D can delay its response for up to 143 blocks without any effect on its channels, this is *exactly* the griefing attack."- Is there any way to account for the amount of time elapsed in the redeem script so that when D goes on chain just before elapse of the locktime, a decision can be enforced like "do not pay the full amount to D because of the response delay "?
On Mon, Apr 13, 2020 at 9:13 AM ZmnSCPxj <zmnsc...@protonmail.com> wrote: > Good morning Subhra, > > > > Ok. But this is a worse situation where C pays money to D but bound to > keep its resource locked for a longer duration, unlike D not responding and > C being able to unlock after the elapse of lock time. > > It is exactly the griefing attack scenario: it is the lock time at D that > is the problem. > As such, it is not "worse", it is ***exactly*** the griefing attack > scenario. > > C cannot unlock until 144 blocks, so D can delay its response for up to > 143 blocks without any effect on its channels, this is *exactly* the > griefing attack. > > Regards, > ZmnSCPxj > > > > > On Mon, Apr 13, 2020, 08:21 ZmnSCPxj <zmnsc...@protonmail.com> wrote: > > > > > Good morning Subhra, > > > > > > > Hello, > > > > So based on what you have stated as possible scenario of > griefing attack, does delay in providing the preimage also counted as a > form of griefing in htlc? Like given the path A->B->C->D, what if C and D > has a lock time of 144 blocks and D responds after 142 block time elapses, > claiming the money locked with D? > > > > > > That ***is*** the griefing attack. > > > > > > Regards, > > > ZmnSCPxj > > > > > > > > > > > On Wed, Apr 1, 2020, 11:49 ZmnSCPxj via Lightning-dev < > lightning-dev@lists.linuxfoundation.org> wrote: > > > > > > > > > Introduction > > > > > ============ > > > > > > > > > > Given the fact that contracts on offchain protocols need to be > enforceable onchain as well, timelocks involved in multi-hop payments are > measured in blocks. > > > > > This is because the blockchain can only (third-party-verifiably) > enforce timeouts in units of entire blocks. > > > > > This leads to very long timeouts for payment delivery, thus > multi-hop offchain payment attempts can be, deliberately or accidentally, > be in a "pending" state up to the very large timeouts involved. > > > > > > > > > > Deliberately setting up a multi-hop payment such that it will be > in a "pending" state for long periods of time is colloquially known as a > "griefing attack". > > > > > In this article, we assess various proposed solutions to mitigate > the effects of griefing attacks, and propose a particular solution, > proof-of-closure, as well, that requires significant changes to the update > state machine. > > > > > > > > > > Digression: Why Grief? > > > > > ====================== > > > > > > > > > > Before embarking on our investigation for solutions to the > griefing problem, we should first wonder if griefing is, in fact, a problem. > > > > > > > > > > This brings up the question of: why would anybody grief at all? > > > > > > > > > > Humans, like cats and other less-sapient pieces of walking meat, > often find enjoyment in causing the suffering of others for no immediate > direct gain to themselves, as a public demonstration of dominance over > those they make suffer (aka "shits and giggles", which, if executed > correctly, can lead to eventual direct gains to themselves or their progeny > or relatives or allies, but such details are often outside the ken of the > very beings who execute such survival strategies: brains are pieces of meat > that have been hacked to act as action-reaction engines, but are not > sophisticated enough to execute as pure rationality engines at all times). > > > > > Fortunately, in the Bitcoin world, only purely rational beings of > pure selfishness can exist in the long run, thus we can neglect such > motivations as mere noise. > > > > > > > > > > First, let us investigate *how* griefing attacks can be performed. > > > > > > > > > > * An intermediate node in a multi-hop attempt can delay forwarding > or failing an incoming HTLC. > > > > > * A final node in a payment attempt can delay claiming an incoming > HTLC. > > > > > > > > > > Let us consider a purely rational intermediate node of pure > selfishness: > > > > > > > > > > * If it forwards as soon as possible, it can earn fees, and also > speed up the release of the HTLC-locked funds so that they can reuse those > funds as liquidity for further payment attempts. > > > > > * Thus, delaying an HTLC is not selfishly-rational for an > intermediate node. > > > > > > > > > > Thus, for an intermediate node, it seems there is no > selfishly-rational motivation to execute a griefing attack on an arbitrary > payment attempt. > > > > > We can then conclude that an intermediate that delays a payment > would do so, not of its own rational self-interest, but as an accident, > such as an unforeseen connectivity or power failure. > > > > > > > > > > However, things are different when we consider a non-arbitrary > payment. > > > > > Suppose a node were to make a payment attempt to itself, and > deliberately delay claiming this self-payment. > > > > > This lets any single node, *who happens to own large liquidity*, > to lock up the liquidity of other nodes. > > > > > > > > > > The motivation to lock up the liquidity of other nodes is to > *eliminate competition*. > > > > > Suppose we have a network as below: > > > > > > > > > > A -- B -- C > > > > > \ / > > > > > \ / > > > > > \ / > > > > > E > > > > > > > > > > When A and C want to transact with one another, they may choose to > route via either B or E. > > > > > B and E are therefore competitors in the business of forwarding > payments. > > > > > > > > > > But suppose E has much larger channels AE and CE than the channels > of AB and CB. > > > > > For example, suppose E has 100mBTC perfectly-balanced channels > while B has only 10mBTC perfectly-balanced channels, as all things should > be in simplified models of reality. > > > > > E can then "take out the competition" by making a 5mBTC > self-payment along E->A->B->C->E and a 5mBTC self-payment along > E->C->B->A->E, then refusing to claim the payment, tying up all the > liquidity of the channels of B. > > > > > By doing so, it can ensure that A and C will always fail to pay > via B, even if they wish to transact in amounts less than 5mBTC. > > > > > E thereby eliminates B as a competitor. > > > > > > > > > > This demonstrates that griefing attacks will be motivated, such > that such attacks will be performed by payers and payees *against > intermediate nodes*. > > > > > Intermediate nodes have no motivation to attack payers and payees > (those are their potential customers in the business of forwarding > payments, and attacking potential customers is bad business: such attacking > intermediate nodes will be removed economically in the long run). > > > > > However, payers and payees can become motivated to attack > intermediate nodes, if the "payer" and "payee" are actually competitor > intermediate nodes. > > > > > > > > > > (We can observe that this is always a possibility even outside of > Lightning: a service or product provider has no incentive to attack its > customers ("the customer is always right"), but have an incentive to > *pretend* to be a customer of a competitor and attack them.) > > > > > > > > > > We will keep this fact in mind: active griefing attacks are > attacks *on* intermediate nodes, not *by* intermediate nodes, because there > is no economic incentive for intermediate nodes to attack their customers. > > > > > > > > > > Previous Proposed Solutions > > > > > =========================== > > > > > > > > > > Time-Spent Reporting > > > > > -------------------- > > > > > > > > > > At each channel along the route, the time spent by a node to > handle its forwarding is recorded, and reported upstream in the route. > > > > > > > > > > Unfortunately, this solution protects payers from intermediate > nodes and payees: it does not protect intermediate nodes from colluding > payers and payees. > > > > > Even if an intermediate node knows that a particular node is > consistently slow via a previous time-spent report, it will not be able, > with our current onion routing, determine if an onion packet it just > received will or will not go through the known-slow node. > > > > > Thus, an intermediate node would not be able to defend against > distant payees that, with a colluding payer, will not claim a particular > payment. > > > > > > > > > > As we have established, an active griefing atttack will never be > deliberately performed by a selfishly-rational intermediate node. > > > > > Thus, this solution protects against the wrong thing: it protects > payers against slow/unreliable intermediate nodes, it does not protect > intermediate nodes against malicious payer/payee collusions. > > > > > It protects only against intermediate nodes that inadvertently go > offline during forwarding, but such nodes will inevitably lose out on the > forwarding market anyway, and will disappear in the long run. > > > > > > > > > > Up-Front Payment > > > > > ---------------- > > > > > > > > > > Payers pay for an attempt, not just the successful completion of > an attempt. > > > > > > > > > > A variation on this is that the payer (or payee) continuously pays > as long as the payment is pending. > > > > > Further variations include paying by other means, such as just > locking funds or paying with proof-of-work. > > > > > > > > > > While it certainly erects economic barriers against payer/payee > collusions attacking intermediate nodes, it *also* erects economic barriers > against normal, non-malicious payments. > > > > > > > > > > We can consider that economic barriers against non-malicious, > low-value, high-frequency payments ("micropayments") may be enough that > such payments become infeasible if we impose up-front payment for mere > attempts. > > > > > Thus, while this solution is certainly something we can consider, > we must be reluctant to use it due to its up-front, strict-evaluation > behavior. > > > > > > > > > > Proof-Of-Closure > > > > > ================ > > > > > > > > > > Observing the above, we want the properties for a "good" solution > to griefing attacks to be: > > > > > > > > > > * It should protect intermediate nodes against payer/payee > collusions. > > > > > * It should only come into play upon detection of an attack. > > > > > > > > > > We now present proof-of-closure, which (we hope) has the above > properties. > > > > > > > > > > We can consider instead a softer timeout, distinct from the HTLC > block-based timeout. > > > > > This softer timeout is measurable in fractions of a second, e.g. > units of 0.1 seconds. > > > > > > > > > > Each node on the network advertises, in addition to a block-based > `cltv_delta`, a `timeout_delta` in units of 0.1 seconds. > > > > > Further, each invoice contains, in addition to a block-based > `final_cltv`, a `final_timeout` in units of 0.1 seconds. > > > > > > > > > > Thus, there are two timeouts: > > > > > > > > > > * The current "hard" block-based timeout that is enforceable > onchain. > > > > > * A new "soft" sidereal-time-based timeout that is not onchain > enforceable. > > > > > > > > > > The soft timeout, as mentioned, is not enforceable onchain. > > > > > Instead, enforcement of the soft timeout *is* the act of putting > the channel state onchain. > > > > > > > > > > Now, for the current "hard" block-based timeout, we already have a > reaction. > > > > > If the HTLC "hard" timeout is approaching: > > > > > > > > > > * Drop the channel onchain and enforce the hard timeout onchain to > reclaim the funds in the HTLCs. > > > > > * Wait for the onchain action to be deeply resolved (either > timelock or hashlock branch is confirmed deeply) and report the result > (success or fail) upstream. > > > > > > > > > > What happens if the "soft" timeout is violated? > > > > > > > > > > * Drop the channel onchain. > > > > > * Report the channel closure upstream. > > > > > > > > > > The "hard" timeout is cancelled in any of these two conditions: > > > > > > > > > > * A success is reported via `update_fulfill_htlc`, OR, > > > > > * A failure is reported via `update_fail_htlc` AND the HTLC is > irrevocably removed from the latest commitments/state(s) of the channel. > > > > > > > > > > The "soft" timeout is cancelled in any of these three conditions, > the first two of which are the same as above: > > > > > > > > > > * A success is reported via `update_fulfill_htlc`, OR, > > > > > * A failure is reported via `update_fail_htlc` AND the HTLC is > irrevocably removed from the latest commitments/state(s) of the channel, OR > > > > > * A channel closure is reported. > > > > > > > > > > Let us fill this in more detail. > > > > > > > > > > Suppose we have a payment route A->B->C->E. > > > > > > > > > > Both the "hard" block timeouts and the "soft" second timeouts > decrement monotonically at each hop. > > > > > Thus, the payee E has the shortest "hard" and "soft" timeouts (as > normal). > > > > > > > > > > * Suppose E then delays claiming the payment and violates the > "soft" timeout. > > > > > * C then drops the CE channel onchain. > > > > > * C reports, before its own timeout (slightly larger than the > timeout imposed on E), the closing of the channel CE, to B. > > > > > * B validates this report, and if valid, propagates the report to > A. > > > > > * A validates this report, and if valid, accepts that the payment > will be "stuck" for up to the hard timeout it imposed on B. > > > > > > > > > > C has to report back to B in order to prevent B from closing the > BC channel, and B has to report back to A in order to prevent A from > closing the AB channel. > > > > > The decrementing seconds-unit timeouts are needed for each hop, > for the same reason that decrementing block-unit timeouts are needed. > > > > > > > > > > Since E is motivated to attack intermediate nodes because it wants > to redirect payment forwards through itself rather than its competitotrs, > having one of its channels closed (which prevents it from being used for > forwarding) is directly opposed to its end goal of getting more money, > thus, we can believe the action of closing a channel involved in a griefing > attack is sufficient disincentive. > > > > > > > > > > The major drawback is that enforcement of the soft timeout *is* a > channel closure, which is generally a negative for the network. > > > > > This is not a remote attack vector, since a node can only trigger > this closure if it is able to stall the fulfillment or failure of an HTLC > on a channel, which generally means the node triggering this closure can > only do so for its own channels (or it is able to, via a separate > mechanism, remotely crash a different node). > > > > > > > > > > Proving Channel Closes > > > > > ---------------------- > > > > > > > > > > What C *really* needs to prove is that: > > > > > > > > > > * It is *willing* to close a channel due to a violation of the > soft timeout. > > > > > * The channel it is willing to close was, in fact, involved in the > same payment attempt. > > > > > > > > > > With the above, B can believe that C was innocent of wrongdoing, > because: > > > > > > > > > > * C would only be wiling to close a channel in case of a protocol > violation, in this case, a violation of the soft timeout. > > > > > * The channel it closed was closed because of this payment > attempt, and not because of another payment attempt, or some other > unrelated channel being unilaterally closed. > > > > > > > > > > First, what C needs to prove is *NOT*, in fact, actual channel > closure: it needs to prove a *willingness* to close a channel. > > > > > Thus, it does not require the channel to actually be *closed* yet, > i.e. it does not have to wait for onchain activity that the channel closure > is in a mempool and is confirmed deeply onchain etc etc. > > > > > > > > > > Thus, to prove a *willingness to close* rather than an actual > close, C can provide the unilateral close of the channel CE. > > > > > The act of unilaterally closing a channel is the publication of > the transaction(s) making up the unilateral close. > > > > > Thus, if C is *willing* to close the channel, it is willing to > publish the transaction(s) involved, and thus, providing the unilateral > close to B and further upstream, shows a willingness to close the channel. > > > > > > > > > > B then validates the provided proof-of-closure by checking that > the unilateral close transaction is either onchain, in the mempool, or that > it spends a TXO that is not currently spent by another transaction. > > > > > In the case the unilateral close transaction is not confirmed and > in the mempool, B can speed up its propagation on the Bitcoin layer by > putting it in its own mempool as well --- after all, C is willing to close > the channel to exonerate itself and punish the actual culprit, and B > putting the unilateral close in its own mempool can only help C in what it > is willing to do. > > > > > > > > > > Secondly, C needs to prove that the channel it is willing to close > involves the payment attempt, and is not some other channel closure that it > is attempting to use to fulfill its own soft timeout. > > > > > Since the unilateral close transaction *is* the proof-of-closure, > B (and A) can inspect the transaction outputs and see (with some additional > data from C) that one of the outputs is to an HTLC that matches the payment > hash. > > > > > > > > > > Thus, B (and A) can believe that the proof-of-closure proves that > whoever is presenting it is free of wrongdoing, as whoever is actually > causing the delay has been punished (by someone being willing to close a > channel with the culprit), and that the proof-of-closure commits to this > particular payment attempt and no other (because it commits to a particular > payment hash). > > > > > > > > > > Further, if CE is closed by E dropping it onchain rather than C, C > will still be able to fulfill its own soft timeout by taking the closing > transaction from E, which should still contain the HTLC. > > > > > Indeed, neither A nor B will particularly care (nor need to know) > who dropped the channel onchain, or (for A) that the channel participants > are C and E. > > > > > > > > > > Update State Shenanigans > > > > > ------------------------ > > > > > > > > > > Bitcoin update mechanisms are complicated things, and it may be > possible for an attacking payee E to fool around with the update state > machine to make it difficult for C to report a willingness to close CE. > > > > > > > > > > In particular, I quote here the relevant passages from > `lightning-rfc`, `02-peer-protocol.md`, which is an implementation of the > Poon-Dryja update mechanism: > > > > > > > > > > > Thus each update traverses through the following states: > > > > > > > > > > > > 1. pending on the receiver > > > > > > 2. in the receiver's latest commitment transaction > > > > > > 3. ... and the receiver's previous commitment transaction has > been revoked, > > > > > > and the update is pending on the sender > > > > > > 4. ... and in the sender's latest commitment transaction > > > > > > 5. ... and the sender's previous commitment transaction has been > revoked > > > > > > > > > > The payee E is the "receiver" in this context. > > > > > > > > > > In this case, once the update has reached step 2, then E has a > commitment transaction that it can put onchain, that contains an HTLC it > can claim. > > > > > From this step onward, C cannot send a failure (i.e. it cannot > send back an `update_fail_htlc`) back to B, because E could drop its latest > commitment onchain and claim the HTLC onchain. > > > > > > > > > > However, until step 4, C does not have a unilateral close > containing the HTLC, and thus cannot provide a proof-of-closure that > contains an HTLC that refers to the payment. > > > > > > > > > > Thus, between steps 2 to 4, C cannot safely respond to its own > soft timeout. > > > > > C cannot respond with a failure, as E could then drop its latest > commitment transaction onchain and claim the payment from C, and extract > money from C that way. > > > > > C also cannot respond with a proof-of-closure, as it does not have > a transaction that it can use to provide this proof. > > > > > > > > > > The best that C can do would be to impose an even shorter timeout > between steps 2 and 4 above, and to drop its current commitment transaction > (which does not contain the HTLC yet and thus does not constitute a valid > proof-of-closure) onchain. > > > > > In between the time it drops the commitment transaction and its > own incoming soft timeout, there is a chance, however small, that this > transaction will be confirmed, and the channel will (with high probability) > settle in a state where the HTLC is not instantiated, thus C can safely > fail its incoming HTLC (not show a proof-of-closure, since that is not > possible for C to do) without risk of loss, just prior to its own soft > timeout. > > > > > > > > > > Of course, C is still at risk here: E could collude with miners > via a side-channel fee offer to confirm its commitment transaction with the > HTLC present, and ensure that C is liable for the HTLC value. > > > > > > > > > > With Decker-Russell-Osuntokun, we can remove this risk by > requiring a ritual as follows: > > > > > > > > > > 1. C requests exclusive access to update their single shared > state. > > > > > * This can be done via a variety of sub-protocols, including a > fair coin toss in case of near-simultaneous requests for exclusive locks on > both sides. > > > > > 2. C provides the details of the new HTLC to E. > > > > > 3. C and E generate the new state transaction and exchange > signatures for it. > > > > > 4. C and E generate (without signing) the new update transaction. > > > > > 5. E provides the signature (or share of signature, if MuSig) for > the new update transaction to C. > > > > > 6. C provides the signature for the new update transaction to E, > which releases the exclusive lock on the shared state atomically with the > finalization of the new update transaction. > > > > > > > > > > Prior to step 5, C can simply fail the incoming HTLC from B in > case its own soft timeout is near. > > > > > Even if E performs step 5 after C has already failed the incoming > HTLC from B, C can simply not perform step 6 and drop the channel onchain > with the previous update and state transactions. > > > > > > > > > > With Poon-Dryja, we will have to rearrange the order in which we > perform things, effectively adding an extra communications turnaround > between the participants. > > > > > Specifically, the order would have to be revised to: > > > > > > > > > > > 1. pending on the sender > > > > > > 2. in the sender's latest commitment transaction > > > > > > 3. ... and the sender's previous commitment transaction has been > revoked, > > > > > > and the update is pending on the receiver > > > > > > 4. ... and in the receiver's latest commitment transaction > > > > > > 5. ... and the receiver's previous commitment transaction has > been revoked > > > > > > > > > > This allows the sender (C in our context) to provide a > proof-of-closure after step 2, and before step 2, C can safely return a > failure with `update_fail_htlc` (and refuse to proceed beyond step 2, thus > ensuring it can still use the previous commitment that still has no HTLC). > > > > > > > > > > Of course, this change will require redesigning the update state > machine, increasing the number of communication turnarounds, and creating a > subtle incompatbility when transitioning a payment from a hop that knows > only the old update state machine to a hop that knows the new update state > machine. > > > > > > > > > > Purely Falsified Proof-Of-Closure > > > > > --------------------------------- > > > > > > > > > > Of course, the attacking node E might want to create a false > proof-of-closure. > > > > > E can do this by simulating a Lightning channel: lock an amount of > funds in a 2-of-2 (where E controls both keys), then spend it in a set of > transactions mimicking the unilateral close. > > > > > > > > > > We observe, however, that the overhead of simulating a Lightning > channel is the same as the overhead of actually creating and closing a > Lightning channel. > > > > > Since the punishment of proof-of-closure is to force attackers to > have their channels closed, we can consider that this simulation of a > channel open and close is sufficient as well. > > > > > > > > > > Future-Proofing > > > > > --------------- > > > > > > > > > > This sketch of proof-of-closure can be used for any update > mechanism: > > > > > > > > > > * With Poon-Dryja, C can use its own commitment transaction as the > proof-of-closure. > > > > > * With Decker-Wattenhofer, C can give all the offchain > transactions up to the last stage in the multi-stage > decrementing-`nSequence` mechanism. > > > > > * With Deckker-Russell-Osuntokun, C can give the latest update and > state trnsaction. > > > > > > > > > > Basically, we expect that for now, and in the future, any update > mechanism worth consideration will have a concept of "unilateral close" > where a channel can be dropped onchain, using data that only one of the > channel participants holds. > > > > > > > > > > Such a unilateral close will be a sequence of one or more valid > transactions, terminating in a transaction containing an HTLC-like contract > in one of its outputs. > > > > > > > > > > Thus, to validate the unilateral close, it is only required to > validate all the transactions contained in the proof-of-closure, and see > that the last transaction has an HTLC output. > > > > > > > > > > The limitations are thus: > > > > > > > > > > * The acceptable forms of HTLC would need to be agreed-upon by the > entire network. > > > > > * Implementations would need to be able to assess, in a > Bitcoin-consensus-compatible way, whether a transaction is valid or not. > > > > > > > > > > Payment Decorrelation and Payment Points > > > > > ---------------------------------------- > > > > > > > > > > Of course, having a single payment hash for the entire payment > attempt is a privacy loss, which we intend to fix in the near future by > using payment points, and adding a blinding scalar at each hop, aka. > payment decorrelation. > > > > > > > > > > Thus, in the future, there will not be any HTLC, but instead a > PTLC. > > > > > Further, the payment point at each hop will be changed at each > hop, in order to prevent decorrelation. > > > > > > > > > > Thus, C needs to provide proofs: > > > > > > > > > > * That an apparent singlesig on the unilateral close output is in > fact a PTLC. > > > > > C needs to provide: > > > > > * A target point P. > > > > > * A partial signature that would spend that singlesig for a > particular sighash. > > > > > * An adaptor signature which, with knowledge of the completed > signature, adaptor signature, and sighash message, would have revealed the > scalar behind P. > > > > > * That the PTLC belongs to the same payment attempt as what B > offered to C. > > > > > C needs to provide: > > > > > * The C-only blinding factor that is the difference between the > payment point of the B-to-C PTLC and the C-to-E PTLC on the unilateral > close. > > > > > > > > > > Then, when B needs to propagate the proof-of-closure back to A, B > simply adds its own blinding factor to the reported blinding factor, in > order to convince A that this is the same payment attempt. > > > > > > > > > > As we have brought up privacy, we observe that, when this > mechanism triggers, there is a mild privacy loss, in that intermediate > nodes now know some channel closure that is related to this payment, and > can thus determine the exact path that the payment attempt went through, at > least until the channel being closed. > > > > > However, proof-of-closure is only propagated in case of violation > of the soft timeout, so for normal non-malicious payments, proof-of-closure > does not cause any privacy loss. > > > > > _______________________________________________ > > > > > Lightning-dev mailing list > > > > > Lightning-dev@lists.linuxfoundation.org > > > > > https://lists.linuxfoundation.org/mailman/listinfo/lightning-dev > > > -- Yours sincerely, Subhra Mazumdar.
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