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I believe that qpack(like hpack) still has some sharp edges. As in...low entropy headers are still vulnerable to leakage via a CRIME[0] style attack on the HTTP/3 header compression.

In practice, high entropy headers aren't vulnerable, as an attacker has to match the entire header:value line in order to see a difference in the compression ratio[1].

[0]: https://en.wikipedia.org/wiki/CRIME [1]: https://www.ietf.org/archive/id/draft-ietf-quic-qpack-20.htm...

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Given that Qpack has a dedicated stream for dynamic table updates, can atleast limit which header:value combinations are inserted.

And there are bit flags in the encoder instructions that carry header values that signal they should never be inserted into a dynamic table.

> And there are bit flags in the encoder instructions that carry header values that signal they should never be inserted into a dynamic table.

My understanding is that these header values have to be known to the Qpack implementation beforehand. As a result, it isn't possible for an HTTP/3 client to signal to Qpack to treat a particular custom header as a never-indexed literal.

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