Fix hop payload bounds

[erickcestari]

unwrapPacket passes the full 2600-byte hopInfo buffer to DecodeHopPayload, but only the first 1300 bytes contain routing info. The remaining 1300 bytes are zero-padding used for XOR decryption.

A malformed packet with a large payload size field can cause the decoder to read beyond the routing info into the padding area. This creates parsing inconsistencies with other implementations that enforce the 1300-byte boundary.

Found through differential fuzzing (bitcoinfuzz) where Core Lightning and rust-lightning rejected the malformed onion packet.

[gemini-code-assist]

Summary of Changes

Hello @erickcestari, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request introduces a critical fix to how onion packet payloads are processed, specifically preventing DecodeHopPayload from reading beyond the intended routing information boundary. Previously, a malformed packet could exploit this by specifying an oversized payload, causing the decoder to read into the zero-padding area. The change ensures that only the valid routing information portion of the hopInfo buffer is processed, enhancing parsing consistency and robustness against malformed inputs. Additionally, new tests have been added to validate this boundary enforcement, alongside minor test function name corrections.

Highlights

• Payload Boundary Enforcement: The unwrapPacket function now explicitly limits the hopInfo buffer passed to DecodeHopPayload to routingInfoLen (1300 bytes). This prevents the decoder from reading into the zero-padding area of the buffer, addressing a potential parsing inconsistency with malformed packets.
• New Test Case for Malformed Packets: A new test, TestUnwrapPacketBeyondRoutingInfoBoundary, has been added to verify that unwrapPacket correctly handles malformed onion packets where the payload size attempts to exceed the routing information boundary, ensuring it fails as expected.
• Test Function Renames: Several test functions in sphinx_test.go were renamed to correct a typo, changing 'Relpay' to 'Replay' (e.g., TestSphinxNodeRelpay is now TestSphinxNodeReplay).

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[gemini-code-assist]

Code Review

This pull request addresses a potential vulnerability in unwrapPacket where a malformed packet could cause the decoder to read beyond the intended routing information boundary. The fix correctly restricts the data passed to DecodeHopPayload by slicing the buffer to the appropriate length. This is a solid and important correction. The addition of a specific test case, TestUnwrapPacketBeyondRoutingInfoBoundary, is excellent as it directly verifies the fix against the described vulnerability. I've also noted the correction of typos in other test function names. My review includes a couple of minor suggestions for the new test code to align with the repository's style guide concerning line length.

[morehouse]

Maybe we should also change DecodeHopPayload to enforce the correct size, rather than UINT16_MAX.