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Re: [TLS] Working Group Last Call for draft-ietf-tls-downgrade-scsv-00

Bodo Moeller <bmoeller@acm.org> Thu, 16 October 2014 13:54 UTC

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From: Bodo Moeller <bmoeller@acm.org>
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Archived-At: http://mailarchive.ietf.org/arch/msg/tls/eoG8eswopYlwm18bLpIwWBHme_Y
Subject: Re: [TLS] Working Group Last Call for draft-ietf-tls-downgrade-scsv-00
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Florian Weimer <fweimer@redhat.com>:

 > One more issue: The draft is silent on how to respond to
>> an inappropriate_fallback alert.
>>
>

> No, it is expressly a fatal alert. What else do you think would need to
>> be said?
>>
>

> So far I've come up with the following questions:
>

> Should the client continue to retry with lower TLS versions once it
> receives an inappropriate_fallback alert?
>
> Should it retry without the SCSV?  I guess you mean "no way!", but this is
> only implicit.


I see. The current I-D indeed doesn't say that you can't continue to retry
with lower versions in response to inappropriate_fallback, but I see no
need for normative language on that (it seems stupid, but not harmful). It
still can't hurt to mention that point, I guess.

Retry without SCSV in response to inappropriate_fallback? Here the same
SHOULD NOT applies as for the previous attempt, and I fail to see why
anyone would decide that doing the additional retry without SCSV could be
acceptable if they understand the full implications and carefully weigh the
case, so again I see no need to change the normative language. But then if
you honestly believe that the current language invites misguided
implementors to retry without SCSV, I wouldn't be opposed to adding an
explicit MUST NOT for that case.

Bodo

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