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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