Re: [quicwg/base-drafts] Authenticate connection IDs (#3499)

martinduke <> Mon, 11 May 2020 17:03 UTC

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Date: Mon, 11 May 2020 10:02:39 -0700
From: martinduke <>
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Subject: Re: [quicwg/base-drafts] Authenticate connection IDs (#3499)
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Is it not necessary to authenticate both client CIDs in a Retry scenario?

C->S: Initial SCID = C1 (attacker modifies to C2), DCID = S1
C<-S: Retry SCID = S2, DCID = C1
C->S: Initial SCID = C1, DCID = S2
C<-S: Initial SCID = S3, DCID = C1
C->S: Short DCID = S3
C<-S: Short DCID = C1

Admittedly, I don't really understand the threat model here, but this isn't authenticate unless the Initial SCID is in the Token. I don't think a TP is needed, as the server can reject the 2nd initial if there's a mismatch in SCID.

Relatedly, if it's written anywhere that the client MUST NOT change its CID during a Retry, it's not in a place that's obvious to me.

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