lightning-bolts/05-onchain.md

BOLT #5: Recommendations for On-chain Transaction Handling

Abstract

Lightning allows for two parties (A and B) to make transactions off-chain, by both holding a cross-signed commitment transaction, which describes the current state of the channel (basically the current balance). This commitment transaction is updated every time a new payment is made, and is spendable at all times.

There are three ways a channel can end:

1. The good way (mutual close): at some point A and B agree on closing the channel, they generate a closing transaction (which is similar to a commitment transaction without any pending payments), and publish it on the blockchain (see BOLT #2: Channel Close).
2. The bad way (unilateral close): something goes wrong, without necessarily any evil intent on either side (maybe one party crashed, for instance). Anyway, one side publishes its latest commitment transaction.
3. The ugly way (revoked transaction close): one of the parties deliberately tries to cheat by publishing an outdated version of its commitment transaction (presumably one that was more in her favor).

Because Lightning is designed to be trustless, there is no risk of loss of funds in any of these 3 cases, provided that the situation is properly handled. The goal of this document is to explain exactly how node A should react to seeing any of these on-chain.

Table of Contents

• General Nomenclature
• Commitment Transaction
• Failing A Channel
• Mutual Close Handling
• Unilateral Close Handling
• On-chain HTLC Output Handling: Our Offers
• On-chain HTLC Output Handling: Their Offers
• On-chain HTLC Transaction Handling
• Revoked Transaction Close Handling
  • Penalty Transactions Weight Calculation
• General Requirements
• Appendix A: Expected weights
  • Expected weight of the to_local penalty transaction witness
  • Expected weight of the received-htlc penalty transaction witness
• Authors

General Nomenclature

We consider any unspent output to be unresolved, and resolve them as detailed in this document. Usually this means spending it with another resolving transaction. Sometimes it simply means noting it for later wallet spending, in which case the transaction containing the output is considered to be its own resolving transaction.

Outputs which are resolved are considered irrevocably resolved once their resolving transaction is included in a block at least 100 deep on the most-work blockchain. 100 blocks is far greater than the longest known Bitcoin fork, and the same value used to wait for confirmations of miner's rewards[FIXME: ref].

Requirements

Once a node has broadcast a funding transaction or sent a commitment signature for a commitment transaction which contains an HTLC output, it MUST monitor the blockchain for transactions which spend any output which is not irrevocably resolved until all outputs are irrevocably resolved.

A node MUST resolve all outputs as specified below, and MUST be prepared to resolve them multiple times in case of blockchain reorganizations.

A node SHOULD fail the channel if it is not already closed when it sees the funding transaction spent. A node MAY send a descriptive error packet in this case.

Invalid transactions SHOULD be ignored.

Rationale

Once a node has had some money at stake, monitoring is required to ensure the other side does not close unilaterally.

Invalid transactions (eg. bad signatures) can be generated by anyone, (and will be ignored by the blockchain anyway), so they should not trigger any action.

Commitment Transaction

A and B each hold a commitment transaction, which has 4 types of outputs:

1. A's main output: Zero or one outputs which pay to A's commitment key.
2. B's main output: Zero or one outputs which pay to B's commitment key.
3. A's offered HTLCs: Zero or more pending payments (HTLCs) to pay B in return for a payment preimage.
4. B's offered HTLCs: Zero or more pending payments (HTLCs) to pay A in return for a payment preimage.

As an incentive for A and B to cooperate, an OP_CHECKSEQUENCEVERIFY relative timeout encumbers A's outputs in A's commitment transaction, and B's outputs in B's commitment transaction. If A publishes its commitment transaction, she won't be able to get her funds immediately but B will. As a consequence, A and B's commitment transactions are not identical, they are (usually) symmetrical.

See BOLT #3: Commitment Transaction for more details.

Failing A Channel

Various error cases involve closing a channel, and this can be done in several ways; the most efficient is preferred. Note that there are requirements around sending the error message to the peer in BOLT #1: The error message.

Requirements

• If no local commitment transaction ever contained a to_local or HTLC output, the node MAY simply forget the channel.
• Otherwise, if the current commitment transaction does not contain to_local or HTLC outputs, a node MAY simply wait and rely on the other node to close, but MUST not forget the channel.
• Otherwise, if the node has received a valid closing_signed message with high enough fee level, it SHOULD use that to perform a mutual close.
• Otherwise, it MUST use the last commitment transaction for which it has a signature to perform unilateral close.

Rationale

Since dust_limit_satoshis is supposed to prevent uneconomic output creation (which would be left unspent forever in the blockchain), we insist on spending the commitment transaction outputs.

In the early stages of a channel, it's common for one side to have little or no money in the channel; with nothing to lose, there's no reason to consume resources monitoring the channel state.

There's a bias towards using mutual close over unilateral because outputs are unencumbered by delay, directly spendable by wallets, and because fees tend to be less exaggerated than commitment transactions: thus the only reason not to use the signature from closing_signed would be if the fee offered was too small for it to be processed.

Mutual Close Handling

A mutual close transaction resolves the funding transaction output.

A node doesn't need to do anything else as it has already agreed to the output, which is sent to its specified scriptpubkey (see BOLT #2: Closing initiation: shutdown).

Unilateral Close Handling

There are two cases to consider here: in the first case, node A sees its own commitment transaction, in the second, it sees the node B's unrevoked commitment transaction.

Either transaction resolves the funding transaction output.

Requirements

When node A sees its own commitment transaction:

1. A's main output: A node SHOULD spend this output to a convenient address. A node MUST wait until the OP_CHECKSEQUENCEVERIFY delay has passed (as specified by the other node's to_self_delay field) before spending the output. If the output is spent (as recommended), the output is resolved by the spending transaction, otherwise it is considered resolved by the commitment transaction itself.
2. B's main output: No action required, this output is considered resolved by the commitment transaction itself.
3. A's offered HTLCs: See "On-chain HTLC Output Handling: Our Offers" below.
4. B's offered HTLCs: See "On-chain HTLC Output Handling: Their Offers" below.

Similarly, when node A sees a commitment transaction from B:

1. A's main output: No action is required; this is a simple P2WPKH output. This output is considered resolved by the commitment transaction itself.
2. B's main output: No action required, this output is considered resolved by the commitment transaction.
3. A's offered HTLCs: See "On-chain HTLC Output Handling: Our Offers" below.
4. B's offered HTLCs: See "On-chain HTLC Output Handling: Their Offers" below.

A node MUST handle the broadcast if any valid commitment transaction from B in this way.

Rationale

Spending the to_local output avoids having to remember the complicated witness script associated with that particular channel for later spending.

Note that there can be more than one valid, unrevoked commitment transaction after a signature has been received via commitment_signed and before the corresponding revoke_and_ack. Either commitment can serve as B's commitment transaction, hence the requirement to handle both.

On-chain HTLC Output Handling: Our Offers

Each HTLC output can only be spent by us after it's timed out, or them if they have the payment preimage.

The HTLC has timed out once the depth of the latest block is equal or greater than the HTLC cltv_expiry.

The method by which we time out the HTLC output differs depending on whether it's our own commitment transaction, or theirs.

Requirements

If the HTLC output is spent using the payment preimage, the HTLC output is considered irrevocably resolved, and the node MUST extract the payment preimage from the transaction input witness.

If the HTLC output has timed out and not been resolved, the node MUST resolve the output. If the transaction is the node's own commitment transaction, it MUST resolve the output by spending it using the HTLC-timeout transaction, and the HTLC-timeout transaction output MUST be resolved as described in "On-chain HTLC Transaction Handling". Otherwise it MUST resolve the output by spending it to a convenient address.

Rationale

If the commitment transaction is theirs, the only way to spend the HTLC output using a payment preimage is for them to use the HTLC-success transaction. If the commitment transaction is ours, they could create any transaction using the preimage.

The payment preimage either serves to prove payment (if this node originated the payment), or to redeem the corresponding incoming HTLC from another peer. Note that we don't care about the fate of the HTLC-spending transaction itself once we've extracted the payment preimage; the knowledge is not revocable.

Note that in cases where both resolutions are possible (payment success seen after timeout, for example), either interpretation is acceptable; it is the responsibility of the other node spend it before this occurs.

If the commitment transaction is theirs, our signature alone is enough to spend the HTLC output (see BOLT #3), but we need to do so, otherwise they could fulfill the HTLC after the timeout. If the commitment transaction is ours, we need to use the HTLC-timeout transaction.

The fulfillment of an on-chain HTLC delivers the payment_preimage required to fulfill the incoming HTLC (if it, too, is on-chain) or use in the update_fulfill_htlc message for the incoming HTLC. Otherwise, it needs to send the update_fail_htlc (presumably with reason permanent_channel_failure) as detailed in BOLT 02.

On-chain HTLC Output Handling: Their Offers

Each HTLC output can only be spent by us if we have the payment preimage, or them if it has timed out.

Requirements

If the node receives (or already knows) a payment preimage for an unresolved HTLC output it was offered, it MUST resolve the output by spending it. If the transaction is the nodes' own commitment transaction, then the it MUST use the HTLC-success transaction, and the HTLC-success transaction output MUST be resolved as described in "On-chain HTLC Transaction Handling". Otherwise, it MUST resolve the output by spending it to a convenient address.

Otherwise, if the HTLC output has expired, it is considered irrevocably resolved.

Rationale

If this is our commitment transaction, we can only use a payment preimage with the HTLC-success transaction (which preserves the to_self_delay requirement). Otherwise we can create any transaction we want to resolve it.

We don't care about expired offers: we should have ensured that the HTLC can only expire long it is needed.

On-chain HTLC Transaction Handling

Because to-self payments have to be delayed (to allow time for a penalty transaction), HTLC outputs can only be spent by the node which broadcast the commitment transaction using the HTLC-timeout or the HTLC-success transaction, which include that delay.

Requirements

A node SHOULD resolve its own HTLC transaction output by spending it to a convenient address. A node MUST wait until the OP_CHECKSEQUENCEVERIFY delay has passed (as specified by the other node's open_channel to_self_delay field) before spending the output.

If the output is spent (as recommended), the output is resolved by the spending transaction, otherwise it is considered resolved by the commitment transaction itself.

Rationale

Spending the to_local output avoids having to remember the complicated witness script associated with that particular channel for later spending.

Revoked Transaction Close Handling

If a node tries to broadcast old state, we can use the revocation key to claim all the funds.

Requirements

A node MUST NOT broadcast a commitment transaction for which it has exposed the revocation key.

If a node sees a commitment transaction for which it has a revocation key, that resolves the funding transaction output.

A node MUST resolve all unresolved outputs as follows:

1. A's main output: No action is required; this is a simple P2WPKH output. This output is considered resolved by the commitment transaction.
2. B's main output: The node MUST resolve this by spending using the revocation key.
3. A's offered HTLCs: The node MUST resolve this in one of three ways by spending:

• the commitment tx using the payment revocation
• the commitment tx using the payment preimage if known
• the HTLC-timeout tx if B publishes them

4. B's offered HTLCs: The node MUST resolve this in one of two ways by spending:

• the commitment tx using the payment revocation
• the commitment tx once the HTLC timeout has passed.

5. B's HTLC-timeout transaction: The node MUST resolve this by spending using the revocation key.
6. B's HTLC-success transaction: The node MUST resolve this by spending using the revocation key. The node SHOULD extract the payment preimage from the transaction input witness if not already known.

The node MAY use a single transaction to resolve all the outputs, but MUST handle its transactions being invalidated by HTLC transactions.

Rationale

A single transaction which resolves all the outputs will be under the standard size limit thanks to the 511 HTLC-per-party limit (see BOLT #2).

Note that if a single transaction is used, it may be invalidated as B broadcasts HTLC-timeout and HTLC-success transactions, but the requirement that we persist until all outputs are irrevocably resolved should cover this. [FIXME: May have to divide and conquer here, since they may be able to delay us long enough to avoid successful penalty spend? ]

Penalty Transactions Weight Calculation

There are three different scripts for penalty transactions, with the following witnesses weight (details of the computation in Appendix A):

to_local_penalty_witness: 160 bytes
offered_htlc_penalty_witness: 243 bytes
accepted_htlc_penalty_witness: 249 bytes

The penalty txinput itself takes 41 bytes, thus has a weight of 164, making the weight of each input:

to_local_penalty_input_weight: 324 bytes
offered_htlc_penalty_input_weight: 407 bytes
accepted_htlc_penalty_input_weight: 413 bytes

The rest of the penalty transaction takes 4+1+1+8+1+34+4=53 bytes of non-witness data, assuming it has a pay-to-witness-script-hash (the largest standard output script), in addition to a 2 byte witness header.

In addition to outputs being swept under as penalty, the node MAY also sweep the to_remote output of the commitment transaction, e.g. to reduce the total amount paid in fees. Doing so requires the inclusion of a p2wpkh witness and additional txinput, resulting in an additional 108 + 164 = 272 bytes.

In a worst case scenario, we have only incoming HTLCs and the HTLC-timeout transactions are not published, forcing us to spend from the commitment transaction.

With a maximum standard weight of 400000, the maximum number of HTLCs that can be swept in a single transaction:

max_num_htlcs = (400000 - 324 - 272 - 4*53 - 2) / 413 = 966

Thus we could allow 483 HTLCs in each direction (with both to_local and to_remote outputs) and still resolve it with a single penalty transaction. Note that even if the to_remote output is not swept, the resulting max_num_htlcs is 967, which yields the same unidirectional limit of 483 HTLCs.

General Requirements

A node SHOULD report an error to the operator if it sees a transaction spend the funding transaction output which does not fall into one of these categories (mutual close, unilateral close, or revoked transaction close). Such a transaction implies its private key has leaked, and funds may be lost.

A node MAY simply watch the contents of the most-work chain for transactions, or MAY watch for (valid) broadcast transactions a.k.a mempool. Considering mempool transactions should cause lower latency for HTLC redemption, but on-chain HTLCs should be such an unusual case that speed cannot be considered critical.

Appendix A: Expected weights

Expected weight of the to_local penalty transaction witness

As described in BOLT #3, the witness for this transaction is:

<sig> 1 { OP_IF <key> OP_ELSE to_self_delay OP_CSV OP_DROP <key> OP_ENDIF OP_CHECKSIG }

The expected weight is calculated as follows:

to_local_script: 83 bytes
    - OP_IF: 1 byte
        - OP_DATA: 1 byte (revocationkey length)
        - revocationkey: 33 bytes
        - OP_CHECKSIG: 1 byte
    - OP_ELSE: 1 byte
        - OP_DATA: 1 byte (localkey length)
        - localkey: 33 bytes
        - OP_CHECKSIG_VERIFY: 1 byte
        - OP_DATA: 1 byte (delay length)
        - delay: 8 bytes
        - OP_CHECKSEQUENCEVERIFY: 1 byte
    - OP_ENDIF: 1 byte

to_local_penalty_witness: 160 bytes
    - number_of_witness_elements: 1 byte
    - revocation_sig_length: 1 byte
    - revocation_sig: 73 bytes
    - one_length: 1 byte
    - witness_script_length: 1 byte
    - witness_script (to_local_script)

Expected weight of the offered-htlc penalty transaction witness

The expected weight is calculated as follows (some calculations have already been made in BOLT #3):

offered_htlc_script: 133 bytes
	
offered_htlc_penalty_witness: 243 bytes
    - number_of_witness_elements: 1 byte
    - revocation_sig_length: 1 byte
    - revocation_sig: 73 bytes
    - revocation_key_length: 1 byte
    - revocation_key: 33 bytes
    - witness_script_length: 1 byte
    - witness_script (offered_htlc_script)

Expected weight of the received-htlc penalty transaction witness

The expected weight is calculated as follows (some calculations have already been made in BOLT #3):

accepted_htlc_script: 139 bytes
	
accepted_htlc_penalty_witness: 249 bytes
    - number_of_witness_elements: 1 byte
    - revocation_sig_length: 1 byte
    - revocation_sig: 73 bytes
    - revocation_key_length: 1 byte
    - revocation_key: 33 bytes
    - witness_script_length: 1 byte
    - witness_script (accepted_htlc_script)

Authors

FIXME

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