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Re: [TLS] Should we require implementations to send alerts?

Dave Garrett <davemgarrett@gmail.com> Thu, 17 September 2015 22:44 UTC

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From: Dave Garrett <davemgarrett@gmail.com>
To: Brian Smith <brian@briansmith.org>
Date: Thu, 17 Sep 2015 18:44:54 -0400
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Subject: Re: [TLS] Should we require implementations to send alerts?
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On Thursday, September 17, 2015 06:37:29 pm Brian Smith wrote:
> A fatal alert during the handshake is never authenticated, because (a) the
> alert record is the last record sent (i.e. there is no Finished message
> sent afterward to authenticate it), and (b) The handshake hashes only cover
> Handshake records, not Alert records.
[...]
> The alert is unauthenticated, full stop.

Yes, this is correct. I replied to quickly. I sent a followup message to correct myself and note that SCSV is needed here.

It might be nice if this got included in the session hash.

> A conformant TLS 1.3 implementation will not be version intolerant. If the
> client does insecure version fallback in response to an alert or connection
> close by a conformant TLS 1.3 implementation then it is guaranteed to be
> doing the wrong thing.

The client initially has no way of telling apart a conformant TLS 1.3 server, a TLS 1.2 server, a TLS 1.0 server full of bugs, or a potato.


Dave

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