Does Botan support enabling certificate compression in TLS?

[shilohshi]

Hi, I would like to inquire whether Botan supports enabling certificate compression in TLS (for example, as described in https://datatracker.ietf.org/doc/html/rfc8879#name-compression-algorithms). If it does support this feature, is there a parameter available during server deployment to control it? Thank you!

[randombit]

No, this is not supported currently

[shilohshi]

Thank you for your response.

[reneme]

RFC 8879 essentially adds two extensions, one to negotiate certificate compression and another one to encapsulate the encoded (and compressed) "Certificate" message. As per the RFC 8879, compression may be performed using zlib, zstd or brotli. Note that this is explicitly supported in TLS 1.3 only.

Given that Botan already has an integration with zlib, it should be relatively straight-forward to add support for RFC 8879, at least using zlib for the compression. The "CompressedCertificate" message takes the place of the conventional (uncompressed) "Certifcate" message. This requires a change in TLS 1.3's state machine, which would likely be the most critical modification required.

Further, one would need to add a few customization points, along these lines:

• std::vector<TLS::CompressionAlgorithm> TLS::Policy::supported_certificate_compression_algorithms()
• std::optional<TLS::CompressionAlgorithm> TLS::Policy::choose_certificate_compression_algorithm(std::span<const TLS::CompressionAlgorithm> from_peer)

And (if we wanted to support user-defined compression algorithms), the actual compression should be wrapped into callbacks:

• std::vector<uint8_t> TLS::Callbacks::tls_compress_certificate_message(TLS::CompressionAlgorithm algorithm, std::span<const uint8_t> encoded_certificate_message)
• std::vector<uint8_t> TLS::Callbacks::tls_decompress_certificate_message(TLS::CompressionAlgorithm algorithm, std::span<const uint8_t> compressed_certificate_message, size_t expected_uncompressed_size)

Support should be implemented for both the client and server side.

Note that the boringssl test suite also provides tests for certificate compression that should be enabled (and passed, obviously).