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Re: [quicwg/base-drafts] Forgery limits on packet protection (#3619)

Martin Thomson <notifications@github.com> Fri, 08 May 2020 01:37 UTC

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Date: Thu, 07 May 2020 18:37:09 -0700
From: Martin Thomson <notifications@github.com>
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Subject: Re: [quicwg/base-drafts] Forgery limits on packet protection (#3619)
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@chris-wood points out an error in my calculation.  I had based my calculations on direct mapping of `l[E]` in the CCM analysis to `l` in our calculations.  That is, `l` was the number of blocks in the message.  But the analysis by Jonsson defines this as:

![image](https://user-images.githubusercontent.com/67641/81360611-b252b580-911f-11ea-8d81-d6824c685d51.png)

The definition of \Beta is the CCM function, so this function really reduces to 2 times the length of the message (in blocks), plus 1 (to account for the additional encryption).  We can ignore the extra 1 as that is absorbed by the tag length.

I'm updating the numbers in the PR.  The result is another halving of the number of packets.  @fxguenther, @ad-l, it's pretty clear that I'm out of my depth here, so your input would be highly valued.

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