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Re: [tcpm] tcp-auth-opt issue: replay protection

Eric Rescorla <ekr@networkresonance.com> Wed, 06 August 2008 13:37 UTC

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Date: Wed, 06 Aug 2008 06:37:34 -0700
From: Eric Rescorla <ekr@networkresonance.com>
To: Lars Eggert <lars.eggert@nokia.com>
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Cc: Adam Langley <agl@imperialviolet.org>, tcpm@ietf.org, ext Joe Touch <touch@isi.edu>
Subject: Re: [tcpm] tcp-auth-opt issue: replay protection
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At Wed, 6 Aug 2008 14:32:02 +0300,
Lars Eggert wrote:
> 
> On 2008-8-1, at 21:44, ext Joe Touch wrote:
> > IMO: retransmitted packets would use the currently active keyID  
> > (even if
> > different from what was previously transmitted).
> 
> I agree. TCP retransmits previous data, not previous segments.

I agree it should be permissible to use the new key-id, but
should it be required? I don't know a lot about
TCP stack implementations, but do any of them currently save
existing packets and just retransmit them? That's legal, right?
If so, this would break that.

I would argue that if we are to use internal key rollover to handle
seqno rollover that you then use the old key even when you retransmit. [0]
Otherwise, you end up with ambiguity about which key is in play.

-Ekr

[0] Incidentally, this is an argument for ESNs, I believe. If you
have a key-id transition at the same time as a seqno rollover, you
might have ambiguity about whether the key had rolled over in
between in the face of packet loss.



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