Re: [quicwg/base-drafts] token-based greasing / initial packet protection (#3166)

Kazuho Oku <notifications@github.com> Wed, 30 October 2019 05:07 UTC

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Subject: Re: [quicwg/base-drafts] token-based greasing / initial packet protection (#3166)
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kazuho commented on this pull request.



> @@ -3940,6 +4007,19 @@ described in {{QUIC-TLS}}.  This protection does not provide confidentiality or
 integrity against on-path attackers, but provides some level of protection
 against off-path attackers.
 
+Additionally, the token is accompanied by a checksum.  This is because when a

We need Token Checksum so that the server can easily determine if there was a bit flip on the wire when it receives an invalid token.

The PR (up until of 7eded0a) requires the server to drop the packet if there was a bit flip on the wire. OTOH, if it was an unusable token (rather than a bit flip), the server responds with a VN. This distinction is necessary to prevent bit flips from being mis-detected as an downgrade attack (see https://github.com/quicwg/base-drafts/pull/3166#discussion_r340344021).

That said, I agree that this change is intrusive. Considering that we need to require certain properties for the token (see https://github.com/quicwg/base-drafts/pull/3166#discussion_r340396154), we might as well state that a NEW_TOKEN token should be generated in a way that the server can detect bit flips even it has lost the keys to decrypt the token. In practice, that can be easily achieved by attaching CRC32 (or zero-key GMAC).

But finally, this entire discussion is based on the assumption that we need a downgrade path, which @martinthomson disputes in https://github.com/quicwg/base-drafts/pull/3166#pullrequestreview-308948794.

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