Default to padding blinded paths #4213
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TheBlueMatt
commented
Nov 10, 2025
TheBlueMatt
commented
Because they end up both being used to validate a `Bolt12Invoice`, we ended up with a single `OffersContext` both for inclusion in a `Refund` and an `InvoiceRequest`. However, this is ambiguous, and while it doesn't seem like an issue, it also seems like a nice property to only use a given `OffersContext` in one place. Further, in the next commit, we use `OffersContext` to figure out what we're building a blinded path for and changing behavior based on it, so its nice to be unambiguous. Thus, we split the single existing context into `OutboundPaymentInRefund` and `OutboundPaymentInInvReq`.
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I've assigned @valentinewallace as a reviewer! |
After much discussion in lightningdevkit#3246 we mostly decided to allow downstream developers to override whatever decisions the `DefaultMessageRouter` makes regarding blinded path selection by providing easy overrides for the selected `OnionMessageRouter`. We did not, however, actually select good defaults for `DefaultMessageRouter`. Here we add those defaults, taking advantage of the `MessageContext` we're given to detect why we're building a blinded path and selecting blinding and compaction parameters based on it. Specifically, if the blinded path is not being built for an offers context, we always use a non-compact blinded path and always pad it to four hops (including the recipient). However, if the blinded path is being built for an `Offers` context which implies it might need to fit in a QR code (or, worse, a payment onion), we reduce our padding and try to build a compact blinded path if possible. We retain the `NodeIdMessageRouter` to disable compact blinded path creation but use the same path-padding heuristic as for `DefaultMessageRouter`.
If we're building a blinded message path with extra dummy hops, we have to ensure we at least hide the length of the data in pre-final hops as otherwise the dummy hops are trivially obvious. Here we do so, taking an extra `bool` parameter to `BlindedMessagePath` constructors to decide whether to pad every hop to the existing `MESSAGE_PADDING_ROUND_OFF` or whether to only ensure that each non-final hop has an identical hop data length. In cases where the `DefaultMessageRouter` opts to use compact paths, it now also selects compact padding, whether short channel IDs are available or not.
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Codecov Report❌ Patch coverage is Additional details and impacted files@@ Coverage Diff @@
## main #4213 +/- ##
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+ Coverage 89.33% 89.34% +0.01%
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Files 180 180
Lines 138055 138086 +31
Branches 138055 138086 +31
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+ Hits 123326 123368 +42
+ Misses 12122 12116 -6
+ Partials 2607 2602 -5
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