lnwire+tlv+route: enforce TLV length validation and add tests

[erickcestari]

I was doing differential fuzzing on lightning P2P messages and noticed that LND doesn't verify
if the length of SCID TLV is correct or not causing it to parse messages with wrong TLV encoding.

This PR strengthens TLV decoding across multiple components by enforcing strict length checks
and adding corresponding tests.

lnwire

• ShortChannelID: decoder now validates l == 8.
• Fee: decoder now validates l == 8.
• Musig2Nonce: decoder now validates l == musig2.PubNonceSize.
• Added tests for invalid lengths in SCID, Fee, and Musig2Nonce TLVs.

tlv

• DBytes33: now enforces l == 33.
• Added a generic test suite ensuring all fixed-size primitive decoders reject incorrect lengths.

routing/route

• Vertex: decoder now validates l == VertexSize.
• Added tests for valid encode/decode and invalid-length rejection.

Note: 10 commits for this might be too much for these changes. Let me know if I should squash into a multi commit.

[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 significantly enhances the robustness of the system's TLV (Type-Length-Value) decoding mechanisms. By introducing strict length validation across several critical components, including ShortChannelID, Fee, Musig2Nonce, DBytes33, and Vertex, the changes prevent the parsing of malformed messages that could lead to unexpected behavior or vulnerabilities. The update also includes extensive new tests to confirm these validations are correctly applied, ensuring data integrity and protocol compliance.

Highlights

• Strict TLV Length Validation: Implemented strict length checks for ShortChannelID, Fee, and Musig2Nonce decoders within the lnwire package, ensuring that only correctly sized TLV values are parsed.
• tlv.DBytes33 Enforcement: The DBytes33 decoder in the tlv package now explicitly enforces a length of 33 bytes for [33]byte types.
• routing/route.Vertex Validation: The Vertex decoder in the routing/route package now validates its TLV length against VertexSize.
• Comprehensive Test Coverage: New tests have been added across lnwire, tlv, and routing/route packages to specifically verify that decoders correctly reject TLV values with invalid lengths, improving robustness against malformed messages.
• Generic Primitive Decoder Testing: A generic test suite was introduced in tlv/primitive_test.go to ensure all fixed-size primitive decoders fail gracefully when presented with incorrect TLV lengths.

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[gemini-code-assist]

Code Review

This pull request strengthens TLV decoding by enforcing strict length validation across several components, which is a great improvement for the robustness and security of the P2P message parsing. The changes are logical and well-implemented. You've also added comprehensive tests to cover these new validation rules. My feedback is minor and focuses on improving the clarity and cleanliness of the new test code, such as removing a debug print statement and correcting some copy-pasted comments and variable names, to better align with the repository's style guide.