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Re: [TLS] Epochs for ACKs

Eric Rescorla <ekr@rtfm.com> Sun, 19 April 2020 21:40 UTC

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From: Eric Rescorla <ekr@rtfm.com>
Date: Sun, 19 Apr 2020 14:39:27 -0700
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To: Hanno Becker <Hanno.Becker@arm.com>
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Subject: Re: [TLS] Epochs for ACKs
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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> 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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