This week’s newsletter summarizes discussion about LN anchors and elements of the v3 transaction relay proposal and announces a research implementation of LN-Symmetry. Also included are our regular sections with the summary of Bitcoin Core PR Review Club meeting and the description of notable changes to popular Bitcoin infrastructure software.

News

  • Discussion about LN anchors and v3 transaction relay proposal: Antoine Poinsot posted to Delving Bitcoin to foster discussion about the proposals for v3 transaction relay policy and ephemeral anchors. The thread appears to have been motivated by Peter Todd posting a critique of v3 relay policy on his blog. We’ve arbitrarily divided the discussion into several parts:

    • Frequent use of exogenous fees may risk mining decentralization: an ideal version of the Bitcoin protocol would reward each miner proportionately to their hashrate. The implicit fees paid in transactions preserve that property: a miner with 10% of total hashrate has a 10% chance of capturing a next-block fee, whereas a miner with 1% of hashrate has a 1% chance. Fees paid outside of transactions and directly to miners, called out-of-band fees, violate that property: a system that pays miners who together control over 55% of hashrate has a 99% chance of getting a transaction confirmed within 6 blocks, likely resulting in little effort being made to pay small miners of 1% or less hashrate. If small miners are paid proportionately less than large miners, mining will naturally centralize, which will reduce the number of entities that need to be compromised in order to censor which transactions get confirmed.

      Actively used protocols such as LN-Penalty with anchors (LN-Anchors), DLCs, and client-side validation allow at least some of their onchain transactions to pay fees exogenously, meaning the fees paid by the core of the transaction can be augmented with fees paid using one or more independent UTXOs. For example, in LN-Anchors the commitment transaction includes one output for each party to fee bump using CPFP (the child transaction spending an extra UTXO) and the HTLC-Success and HTLC-Failure transactions (HTLC-X transactions) are partly signed using SIGHASH_SINGLE|SIGHASH_ANYONECANPAY so they can be aggregated into a single transaction with at least one extra input to pay fees (the extra input being a separate UTXO).

      Focusing on a thought-experiment version of LN that uses P2TR and proposed ephemeral anchors, Peter Todd argues that its dependency on exogenous fees significantly incentivizes paying out-of-band fees. In particular, the unilateral close of a channel with no pending payments (HTLCs) would allow a large miner accepting out-of-band fees to include twice as many close transactions in a block than could be included by a smaller miner who only accepted in-band fees paid for through CPFP fee bumping. The large miner could profitably encourage this by offering a moderate discount to users paying out of band. Peter Todd calls that a threat to decentralization.

      The post does suggest that some uses of exogenous fees in protocols is acceptable, so the concern may be about the frequency of their expected use and the relative size difference between using them and paying out of band. In other words, frequently occurring zero-pending unilateral closes with a 100% overhead would likely be considered more of a risk than potentially rarer unilateral closes with 20 pending HTLCs where the overhead is less than 10%.

    • Implications of exogenous fees on safety, scalability, and costs: Peter Todd’s post also noted that existing designs such as LN-Anchors and future designs that use ephemeral anchors require each user to keep an extra UTXO in their wallet to use for essential fee bumping. Because creating UTXOs costs block space, this reduces the maximum number of independent users of the protocol by half or more in theory. It also means a user can’t safely allocate their full wallet balance to their LN channels, which worsens the LN user experience. Finally, using CPFP fee bumping or attaching additional inputs to a transaction to pay fees exogenously requires using more block space and paying more transaction fees than paying fees directly from a transaction’s input value (endogenous fees), making it more expensive in theory even if the other problems were not a concern.

    • Ephemeral anchors introduce a new pinning attack: as described in last week’s newsletter, Peter Todd described a minor pinning attack against uses of ephemeral anchors. For a commitment transaction with no pending payments (HTLCs), an unprivileged attacker could create a situation when an honest user might have to pay 1.5x to 3.7x more in fees to obtain the feerate they intended. However, if the honest user chose to be patient instead of spending extra fees, the attacker would end up paying some or all of the honest user’s fees. Given that zero-pending commitment transactions don’t have any timelock-dependent urgency, many honest users might choose to be patient and get their transaction confirmed at the attacker’s expense. The attack also works when HTLCs are used, but it costs the honest user less to break free and can still result in the attacker losing money.

    • An alternative: use endogenous fees with presigned incremental RBF bumps: Peter Todd suggests and analyzes an alternative approach, signing multiple versions of each commitment transaction at different feerates. For example, he suggests signing 50 different versions of the LN-Penalty commitment transaction at feerates starting at 10 sats/vbyte and increasing in each version by 10% until a transaction paying 1,000 sats/vbyte is signed. For a commitment transaction with no pending payments (HTLCs), his analysis indicates the signing time would be about 5 milliseconds. However, for each HTLC attached to the commitment transaction, the number of signatures would increase by 50 and the signing time would increase by 5 milliseconds. Bastien Teinturier linked to a previous discussion he had started about a similar approach.

      Although the idea may work in some situations, Peter Todd’s post noted that endogenous fees with presigned incremental fee bumps were not a satisfactory replacement for exogenous fees in all cases. When the delays required for presigning commitment transactions containing multiple HTLCs are multiplied by the several hops on a typical payment path, the delay can easily become more than a second and, at least in theory, extend to delays of more than a minute. Peter Todd notes that the delay could be reduced to roughly constant time if the proposed SIGHASH_ANYPREVOUT opcode (APO) were available.

      Even if the delay was a constant 5 milliseconds, it’s possible that could lead to forwarding nodes using endogenous fees earning less forwarding fees than nodes using exogenous fees due to anticipated effects of LN payers eventually making redundant overpayments that will economically reward faster forwarding over slower forwarding, even when the difference is on the order of milliseconds.

      An additional challenge would be using the same endogenous fees for the presigned HTLC-Success and HTLC-Timeout transactions (HTLC-X transactions). Even with APO, that would naively imply creating n2 signatures, although Peter Todd notes that the number of signatures could be reduced by assuming the HTLC-X transactions would pay a similar feerate to the commitment transaction.

      There was an unresolved debate about whether endogenous fees would result in an excessive amount of capital being reserved for fees. For example, if Alice signs fee variants from 10 s/vb to 1,000 s/vb, she must make decisions based on the possibility that her counterparty Bob will put the 1,000 s/vb variant onchain, even if she wouldn’t pay that feerate herself. That means she can’t accept payments from Bob where he spends the money he would need for the 1,000 s/vb variant. For example, a commitment transaction with 20 HTLCs would make 1 million sats temporarily unspendable ($450 USD at the time of writing). If endogenous fees were also used for the HTLC-X transactions, the temporarily unspendable amount for 20 HTLCs would be closer to 4.5 million sats ($2,050 USD). By comparison, if Bob was expected to pay his fees exogenously, then Alice wouldn’t need to reduce the capacity of the channel for her safety.

    • Overall conclusions: discussion was ongoing at the time of writing. Peter Todd concluded that “existing usage of anchor outputs should be phased out due to the above-mentioned miner decentralization risks; new anchor output support should not be added to new protocols or Bitcoin Core.” LN developer Rusty Russell posted about using a more efficient form of exogenous fees in new protocols to minimize the concern about out-of-band fees. In the Delving Bitcoin thread, other developers working on LN, v3 transactions, and ephemeral anchors defended the usefulness of anchors and it seemed likely that they would continue working on v3-related protocols. We will provide updates in future newsletters if anything significant changes.

  • LN-Symmetry research implementation Gregory Sanders posted to Delving Bitcoin about a proof-of-concept implementation he made of the LN-Symmetry protocol (originally called eltoo) using a software fork of Core Lightning. LN-Symmetry provides bi-directional payment channels that guarantee the ability to publish the latest channel state onchain without a need for penalty transactions. However, they require allowing a child transaction to spend from any possible version of a parent transaction, which is only possible with a soft fork protocol change such as SIGHASH_ANYPREVOUT. Sanders offers several highlights from his work:

    • Simplicity: LN-Symmetry is a much simpler protocol than the currently used LN-Penalty/LN-Anchors protocol.

    • Pinning:Pinning is super hard to avoid.” Sander’s work on this concern gave him insight and inspiration that has led to his contributions to package relay and his widely praised proposal for ephemeral anchors.

    • CTV:CTV (through emulation) […] allowed for ‘fast forwards’ that are extremely simple and would likely reduce payment times if widely adopted.”

    • Penalties: Penalties truly did not seem necessary. This was the hope for LN-Symmetry, but some people thought that a penalty protocol would still be necessary to deter malicious counterparties from attempting theft. Support for penalties significantly increases protocol complexity and requires reserving some channel funds to pay the penalties, so it is preferable to avoid supporting them if they are not necessary for safety.

    • Expiry deltas: LN-Symmetry requires longer HTLC expiry deltas than expected. When Alice forwards an HTLC to Bob, she gives him a certain number of blocks to claim its funds with a preimage; after that time expires, she can take back the funds. When Bob further forwards the HTLC to Carol, he gives her a lower number of blocks during which she must reveal the preimage. The delta between those two expires is the HTLC expiry delta. Sanders found that the delta needed to be long enough to prevent the counterparty from benefiting if they aborted the protocol midway through a commitment round.

    Sanders is currently working on making improvements to Bitcoin Core’s mempool and relay policy that will make it easier to deploy LN-Symmetry and other protocols in the future.

Bitcoin Core PR Review Club

In this monthly section, we summarize a recent Bitcoin Core PR Review Club meeting, highlighting some of the important questions and answers. Click on a question below to see a summary of the answer from the meeting.

Nuke adjusted time (attempt 2) is a PR by Niklas Gögge that modifies a block validity check related to the block’s timestamp. Roughly, if a block’s timestamp (contained in its header) is too far in the past or the future, the node rejects the block as invalid. Note that if the block is invalid because its timestamp is too far in the future, it can become valid later (although the chain may have moved on).

  • Is it necessary for block headers to have a timestamp? If so, why?

    Yes, the timestamp is used in difficulty adjustment and to validate transaction timelocks. 

  • What is the difference between Median Time Past (MTP) and network-adjusted time? Which of these are relevant to the PR?

    MTP is the median time of the last 11 blocks, and is the lower bound of block timestamp validity. Network-adjusted time is calculated as our own node’s time plus the median of the offsets between our time and that of a random selection of 199 of our outbound peers. (This median can be negative.) The network-adjusted time plus 2 hours is the maximum valid block timestamp. Only network-adjusted time is relevant to this PR. 

  • Why are these times conceptually very different?

    MTP is uniquely defined for all nodes synced to the same chain; there is consensus on time. Network-adjusted time can vary across nodes. 

  • Why don’t we just use MTP for everything and scrap network-adjusted time?

    MTP is used as the lower bound of block timestamp validity, but it can’t be used as an upper bound because future block timestamps are unknown. 

  • Why are limits enforced on how far “off” a block header’s timestamp is allowed to be from the node’s internal clock? And since we don’t require exact agreement on time, can these limits be made more strict?

    The block timestamp range is restricted so that malicious nodes’ ability to manipulate difficulty adjustments and locktimes is limited. These kinds of attacks are called timewarp attacks. The valid range can be made more strict to an extent, but making it too strict could lead to temporary chain splits since some nodes may reject blocks that other nodes accept. Block timestamps don’t need to be exactly correct, but they need to track actual time over the long run. 

  • Before this PR, why would an attacker try to manipulate a node’s network-adjusted time?

    If the node is a miner, to get its mined blocks rejected by the network or to get it to not accept a valid block so it keeps wasting hashrate on an old tip (both of those would give an advantage to a competing miner); to get the attacked node to follow the wrong chain; to cause a time-locked transaction to not be mined when it should be; to perform a time dilation attack on the Lightning Network. 

  • Prior to this PR, how could an attacker try to manipulate a node’s network-adjusted time? Which network message(s) would they use?

    An attacker would need to send us version messages with manipulated timestamps from multiple peers that they control. They would need us to make more than 50% of our outbound connections to their nodes, which is hard but much easier than completely eclipsing the node. 

  • This PR uses the node’s local clock as the upper-bound block validation time, rather than network-adjusted time. Can we be sure that this reduces esoteric attack surfaces, rather than increasing them?

    Discussion ensued with no clear resolution as to whether it’s easier for an attacker to affect a node’s peer set or its internal clock (using malware or NTP fakery, for example), but most participants agreed that the PR is an improvement. 

  • Does this PR change consensus behavior? If so, is this a soft fork, a hard fork, or neither? Why?

    Because consensus rules can’t consider data from outside of the block chain (such as each node’s own clock), this PR can’t be considered a consensus change; it’s just a network acceptance policy change. But that doesn’t mean it’s optional; having some policy rule limiting how far a block’s timestamp can be in the future is essential to the security of the network. 

  • Which operations were relying on network-adjusted time prior to this PR?

    TestBlockValidity, CreateNewBlock (used by miners to build block templates), and CanDirectFetch (used in the P2P layer). The variety of these uses shows that the PR doesn’t just affect block validity, but there are other implications, which we need to validate. 

Notable code and documentation changes

Notable recent changes in Bitcoin Core, Core Lightning, Eclair, LDK, LND, libsecp256k1, Hardware Wallet Interface (HWI), Rust Bitcoin, BTCPay Server, BDK, Bitcoin Improvement Proposals (BIPs), Lightning BOLTs, and Bitcoin Inquisition.

  • LND #8308 raises the min_final_cltv_expiry_delta from 9 to 18 as recommended by BOLT 02 for terminal payments. This value affects external invoices that don’t supply the min_final_cltv_expiry parameter. The change remedies the interoperability issue discovered after CLN stopped including the parameter when the default of 18 was used, as mentioned in last week’s newsletter.