Re: [TLS] Problem with DTLS 1.2 handshake

Eric Rescorla <ekr@rtfm.com> Mon, 26 March 2018 14:15 UTC

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From: Eric Rescorla <ekr@rtfm.com>
Date: Mon, 26 Mar 2018 07:14:44 -0700
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To: Jim Schaad <ietf@augustcellars.com>
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Subject: Re: [TLS] Problem with DTLS 1.2 handshake
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On Mon, Mar 26, 2018 at 6:48 AM, Jim Schaad <ietf@augustcellars.com>; wrote:

> Yes, I am talking about the TLS record MAC.
>
>
>
> In my case this was happening because of a misconfiguration on the PSK.
> When we finally figured out that this was a MAC error on the record, then
> we immediately looked at the values of the PSK knowing that this was the
> most likely failure.
>

Well, you'll note that TLS 1.3 will not have this problem because the
binder check will fail.



> The fact that in the main handshake this leads to a large amount of
> retries by the client I am not sure that this is just a slower failure
> detection.  I agree that this is less of an issue for a simple rotate the
> keys problem.  I am slightly worried about having the same problem on a
> re-negotiation though.
>

How would that happen unless you had an implementation defect.

-Ekr





>
>
> Jim
>
>
>
>
>
> *From:* Eric Rescorla <ekr@rtfm.com>;
> *Sent:* Monday, March 26, 2018 6:24 AM
> *To:* Jim Schaad <ietf@augustcellars.com>;
> *Cc:* <tls@ietf.org>; <tls@ietf.org>;
> *Subject:* Re: [TLS] Problem with DTLS 1.2 handshake
>
>
>
> First, just for clarification, you mean the TLS record MAC on the Finished
>
> rather than the TLS Finished MAC, right?
>
>
>
> Assuming that is correct, then I believe this is reasonable behavior. It
>
> makes the protocol somewhat more resistant to damaged bits on the wire.
>
> Note that QUIC takes this position even further: every packet is integrity
>
> protected (the initial packets use a key based on the CID). The primary
>
> consequence of this is slower failure detection, but the kind of case
> you're
>
> talking about primarily happens when there is an implementation error,
>
> so not much in the field anyway.
>
>
>
> -Ekr
>
>
>
>
>
> On Mon, Mar 26, 2018 at 6:09 AM, Jim Schaad <ietf@augustcellars.com>;
> wrote:
>
> I appear to have run across an implementation that does not appear to
> violate the specification, but which in my opinion is just plain wrong.
>
> I am doing a handshake with PSK.  On the second flight from the client it
> sends
>
> [ChangeCipherSpec]
> Finished
>
> The server sees that the ChangeCipherSpec occurs and moves to use the keys.
> It then attempts to validate the MAC on the Finished message and silently
> ignores the Finish message because the MAC is incorrect and the text says
> that it is legal to ignore packets which have a bad MAC.  This means that
> my
> client re-sends the same flight to the server on and on because it never
> gets a response and assumes that the packet must be getting lost in
> transit.
>
>
> The document does not say that ignoring of bad MACs does not apply until
> the
> Finished message is received and processed.  I am not sure, but I believe
> the document needs to say that one cannot ignore a failed MAC on the first
> block of data in any epoch and must error on those messages.
>
> I have not looked to see if this is an issue for DTLS 1.3, but it could
> easily be.
>
> Jim
>
>
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