[quicwg/base-drafts] Remove SHOULD for advice to pre-generate KeyUpdate (#3211)

ekr <notifications@github.com> Sun, 10 November 2019 22:07 UTC

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Subject: [quicwg/base-drafts] Remove SHOULD for advice to pre-generate KeyUpdate (#3211)
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> The process of creating new packet protection keys for receiving packets could
> reveal that a key update has occurred.  An endpoint MAY perform this process as
> part of packet processing, but this creates a timing signal that can be used by
> an attacker to learn when key updates happen and thus the value of the Key Phase
> bit in certain packets.  Endpoints SHOULD instead defer the creation of the next
> set of receive packet protection keys until some time after a key update
> completes, up to three times the PTO; see {{old-keys-recv}}.

I don't believe we had consensus on this -- and there was considerable debate in YUL. It just got pulled in as part of a big PR. 

This is a significantly more complicated mechanism than just generating as needed, and I haven't yet seen any real security analysis.

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