Update trampoline to use the official spec version #688
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The v4 serialization format was released in July 2023 with better support for backwards-compatibility: the v2 and v3 formats used kotlin's serialization, which prevents us from removing legacy classes and requires additional code whenever we change something in the channel state. Users who haven't migrated their data to the v4 format will not be able to deserialize their channel data: they will need to use a previous version of the app to read their old channel data, write it to the v4 format and then upgrade to the latest version. We also remove a bunch of fields/options that were deprecated a while ago: - legacy channel funding states - unused legacy features - legacy private key generation - legacy node events
We update the trampoline feature to match the official specification from lightning/bolts#836. We remove support for the previous version of trampoline, which means that when paying nodes that use the experimental version, we will use the trampoline-to-non-trampoline flow instead. Similarly, when older nodes pay updated nodes, they won't understand the new trampoline feature bit and will use the trampoline-to-non-trampoline flow. We update the trampoline-to-non-trampoline flow to remove the unused trampoline payload in the onion, which saves some space. Note that we don't want to officially specify this scenario, as it leaks some data about the recipient to the trampoline node. We rather wait for nodes to either support trampoline or blinded paths, which fixes this issue.
We update our trampoline payments to blinded paths to match the official specification from lightning/bolts#836. The blinded paths and recipient features are included in the trampoline onion, which potentially allows using multiple trampoline hops. That was already what we were doing with experimental TLVs, so we simply update the TLV values to match the spec values.
We add the ability to pay recipients that support trampoline *and* blinded paths. We include the blinded path data in the trampoline payloads for each node inside the blinded path. This doesn't reveal unnecessary information to the trampoline node: this is specified in details in lightning/bolts#836.
Add support for the trampoline failure messages added to the BOLTs. We also add support for encrypting failures e2e using the trampoline shared secrets on top of the outer onion shared secrets.
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When the recipient supports blinded trampoline payments and uses the same introduction node as our trampoline node, we mustn't add a hop from our trampoline node to itself: we can directly use them as the blinded path's trampoline introduction node. This is what payments between mobile wallets using the same trampoline node will look like.
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We update our trampoline code to use the official values from the BOLTs. Note that we don't maintain backwards-compatibility with the experimental trampoline feature bit and types, which means that:
I think this is a good trade-off to avoid the technical debt and risks associated with maintaining two versions of trampoline in parallel.
Builds on top of #685