Type key encoding

[backkem]

Summary

• Fix message type key encoding to use RFC 9000 varint instead of CBOR
• Fix hostname encoding to use URL-safe base64 (no padding) for DNS compatibility

Details

Message encoding (Issue #8): The spec requires message format [varint type_key][CBOR body], but we were encoding
as [CBOR array][CBOR u16 type][CBOR body]. Fixed by using quinn_varint for type keys.

Hostname encoding (Issue #9): Standard base64 contains +, /, = which are invalid in DNS labels and TLS SNI.
Changed to URL_SAFE_NO_PAD encoding. See w3c/openscreenprotocol#365.

[gemini-code-assist]

Summary of Changes

Hello @backkem, 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 refines the data encoding strategies within the OpenScreen Protocol implementation. It specifically addresses a compatibility concern by adopting URL-safe Base64 for certificate serial numbers, making them suitable for use in DNS labels. Furthermore, it overhauls the network message serialization by introducing RFC 9000 variable-length integer encoding for message type keys, aligning the protocol's message structure more closely with its specification and improving efficiency by removing redundant CBOR array wrappers.

Highlights

• URL-Safe Base64 for Serial Numbers: Switched certificate serial number encoding from standard Base64 to URL-safe Base64 without padding to ensure compatibility with DNS labels, resolving an identified specification issue.
• RFC 9000 Variable-Length Integer Encoding: Implemented RFC 9000 variable-length integer encoding for message type keys, replacing the previous CBOR array wrapper structure for network messages.
• Refactored Message Encoding/Decoding: Updated all authentication message types (AuthCapabilities, AuthSpake2Handshake, AuthSpake2Confirmation, AuthStatus) to use the new variable-length integer type key encoding, streamlining the message format.

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[gemini-code-assist]

Code Review

This pull request updates the message encoding to use a variable-length integer for the type key, prefixed to the CBOR message body, instead of a CBOR array. It also correctly switches to URL-safe Base64 encoding for certificate serial numbers to ensure they are valid in DNS labels. The changes are logical and well-motivated. I've found one critical issue that will prevent compilation due to an API change in a dependency, and one medium-severity issue related to code structure and efficiency. My detailed feedback is in the comments below.

[backkem]

PTAL @kaidokert

[kaidokert]

Thank you - looks like a good fix.

But that CBOR-array encoding is much more widespread, for all other app level messages too, i'll do a follow-up fix. I had completely misinterpreted the type key encoding

[backkem]

Fixed lint & did the application messages while at it.

[backkem]

Sorry for the back-n-forth. I realize I normally rely a bit too much on just running the CI a couple times 😅 Will do better.

[kaidokert]

/gemini review

[gemini-code-assist]

Code Review

This pull request correctly implements two important fixes: using RFC 9000 varint for message type key encoding and URL-safe base64 for hostnames. The changes are well-implemented, especially the refactoring of decode logic into decode_body functions, which fixes a bug in the previous implementation where the full buffer was passed for body decoding. The use of URL_SAFE_NO_PAD for base64 encoding is also correct for DNS compatibility. I have one suggestion to reduce code duplication for the new helper functions.

[kaidokert]

Thanks a lot ! This was very off spec