t-bast

When should we store an interactive-tx?

When using the interactive-tx protocol, we currently store the channel state only after we've sent tx_signatures. This makes sense from a funds safety point of view: the commitment transactions can only be broadcast once we've given our peer the signatures they needed to broadcast the funding transaction first.

But in some cases, this creates an issue where one side has stored the channel state but not the other side. See the following scenario for example (where -->X indicates a message that wasn't received):

  Alice                          Bob
 | |

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/**
 * @param cmd             command to send the HTLC for this payment.
 * @param outgoingChannel channel to send the HTLC to.
 * @param sharedSecrets   shared secrets (used to decrypt the error in case of payment failure).
 */
case class OutgoingPaymentPacket(cmd: CMD_ADD_HTLC, outgoingChannel: ShortChannelId, sharedSecrets: Seq[(ByteVector32, PublicKey)])

/** Helpers to create outgoing payment packets. */
object OutgoingPaymentPacket {

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Sample lightning transactions using package RBF

This document contains sample fee-bumping scenarios used by lightning nodes to replace a malicious commitment transaction. Commitments transactions in these test cases don't pay any fees. The fee-bumping child (the anchor transaction) spends from one or more commitment transactions and an unrelated wallet transaction (used to add funds).

The actual tests implementing this can be found here.

Basic commitment RBF

t-bast

A different approach to solve RBF for off-chain contracts

Key observation: on-chain fees for off-chain application settlement should be endogenous, not exogenous. In more plain english, there is no good reason to bring funds from "outside" of an off-chain contract to pay the fees necessary to settle that contract on-chain. Any party in the contract is ready to burn at most their contract balance in on-chain fees, but it wouldn't make sense to burn more, even for a scorched earth strategy. This is something I brought up a while ago in lightning/bolts#845

We could achieve this with an extension to SIGHASH_ANYPREVOUT, which I'm calling SIGHASH_ANYAMOUNT. This could be a flag to complement a SIGHASH_ANYPREVOUT / SIGHASH_ANYPREVOUTANYSCRIPT and add the following behavior:

t-bast

Economics of making on-chain vs off-chain payments

The fees for on-chain payments are independent of the payment amount, while the fees for off-chain payments are proportional to the amount sent. It is trivial to compute the amount at which it makes more sense to make an on-chain payment.

Parameters used to compute the following numbers:

• the weight of a 2 inputs, 2 outputs on-chain transaction using p2wpkh is 836 vbytes
• a route in the lightning network has roughly a median 1000ppm proportional fee

t-bast

   Mempool #1
+------------------------------------------------------------------------------+
|                                                                              |
|      1 sat/byte                  1 sat/byte                  1 sat/byte      |
|   +-------------+             +-------------+             +-------------+    |
|   |     tx1     |------------>|     tx2     |------------>|     tx3     |    |
|   +-------------+             +-------------+             +-------------+    |
|                                                                              |

t-bast

Combining BIP32 keys for fun and profit

We only consider non-hardened children here.

Private parent key → private child key

CKDpriv((kpar, cpar), i) → (ki, ci) computes a child extended private key from the parent extended private key:

• I = HMAC-SHA512(cpar, serP(point(kpar)) || ser32(i))
• IL = I[0:32]

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Pinning discussion

t-bast

Asymmetrical features

Let's explore the following scenario. There is an asymmetrical feature F, with a clear client side and provider side. The client side of the protocol is activated by feature X. The provider side of the protocol is activated by feature Y.

Some nodes want to be a client, but not a provider, and want to connect only to nodes that are providers of this feature. They obviously turn on feature X. Since they are only clients of the feature, they cannot turn on feature Y, so how do they require their peers to support feature Y?

t-bast

Trampoline error encryption

Trampoline nodes apply the same error-wrapping mechanism that's used for normal onions at the trampoline onion level. That means there's simply two layers of error wrapping.

Notations

We note ss(Alice, Bob) the shared secret between Alice and Bob for the outer onion encryption, and tss(Alice, Bob) the shared secret between Alice and Bob