Update trampoline to use official spec values #2819
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## master #2819 +/- ##
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+ Hits 15652 15694 +42
+ Misses 2576 2562 -14
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We add support for the official version of trampoline payments, as specified in lightning/bolts#836. We keep supporting trampoline payments that use the legacy protocol to allow a smooth transition. We hardcode the legacy feature bit 149 in a few places to make this work, which is a bit hacky but simple and should be removed 6 months after releasing the official version. We also keep supporting payments from trampoline wallets to nodes that don't support trampoline: this is bad from a privacy standpoint, but will be fixed when recipients start supporting Bolt 12.
We add support for trampoline payments to blinded recipients, where each node of the blinded path is used as trampoline node. This is particularly useful to include custom TLVs from the payer to the recipient.
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When returning trampoline failures for the payer (the creator of the trampoline onion), they must be encrypted using the sphinx shared secret of the trampoline onion. When relaying a trampoline payment, we re-wrap the (peeled) trampoline onion inside a payment onion: if we receive a failure for the outgoing payment, it can be either coming from before the next trampoline node or after them. If it's coming from before, we can decrypt that error using the shared secrets we created for the payment onion: depending on the error, we can then return our own error to the payer. If it's coming from after the next trampoline onion, it will be encrypted for the payer, so we cannot decrypt it. We must peel the shared secrets of our payment onion, and then re-encrypted with the shared secret of the incoming trampoline onion. This way only the payer will be able to decrypt the failure, which is relayed back through each intermediate trampoline node.
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Use the feature bit and tlvs defined in the official spec PR (lightning/bolts#836) and add support for:
We also add the official spec test vectors.
We keep backwards-compatibility for current wallet users relying on the legacy trampoline protocol, but will likely drop support at some point. We should notify the Electrum folks and give them enough time to migrate to the new protocol before dropping support for the legacy one.