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Re: [TLS] Epochs for ACKs

Hanno Becker <Hanno.Becker@arm.com> Mon, 20 April 2020 11:10 UTC

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From: Hanno Becker <Hanno.Becker@arm.com>
To: Eric Rescorla <ekr@rtfm.com>
CC: "tls@ietf.org" <tls@ietf.org>
Thread-Topic: [TLS] Epochs for ACKs
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Date: Mon, 20 Apr 2020 11:10:23 +0000
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Subject: Re: [TLS] Epochs for ACKs
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Hi Ekr,

Great, thanks, I left comments on that PR.

Cheers,
Hano
________________________________
From: Eric Rescorla <ekr@rtfm.com>
Sent: Sunday, April 19, 2020 10:39 PM
To: Hanno Becker <Hanno.Becker@arm.com>
Cc: tls@ietf.org <tls@ietf.org>
Subject: Re: [TLS] Epochs for ACKs

I have posted a PR to clarify this: https://github.com/tlswg/dtls13-spec/pull/142

On Tue, Apr 14, 2020 at 1:13 AM Hanno Becker <Hanno.Becker@arm.com<mailto:Hanno.Becker@arm.com>> wrote:
Hi all,

On ACK protection, DTLS 1.3 Draft 37 says in Section 7:

   ACK records MUST be sent with an epoch that is equal to or higher
   than the record which is being acknowledged.  Implementations SHOULD
   simply use the current key.

Since the update of incoming and outgoing keying material is
independent, I don't know how this can be enforced: After a
sequence of key updates, the incoming epoch might be 42 while
the outgoing epoch is 17.

What problems arise if one replaces the paragraph by the following:

   ACK records MUST be sent with the current key, irrespective
   of the epoch that is used to protect the record that is
   being acknowledged.

It appears that the paragraph is particularly relevant for the case
of ACKing a ServerHello, which as far as I understand shall happen
with epoch 1. Since 'current key' doesn't appear unambiguously
defined at the point of the client processing the ServerHello, it
might be worth spelling out this case explicitly.

Best,
Hanno
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