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Re: [TLS] The TLS_FALLBACK_SCSV time bomb (was: Re: Working Group Last Call for draft-ietf-tls-downgrade-scsv-00)

Bodo Moeller <bmoeller@acm.org> Fri, 17 October 2014 13:51 UTC

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Date: Fri, 17 Oct 2014 15:51:06 +0200
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From: Bodo Moeller <bmoeller@acm.org>
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Subject: Re: [TLS] The TLS_FALLBACK_SCSV time bomb (was: Re: Working Group Last Call for draft-ietf-tls-downgrade-scsv-00)
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Hubert Kario <hkario@redhat.com>:

> (Arguably the OpenSSL API is too fragile, and SSL_MODE_SEND_FALLBACK_SCSV
> should be ignored in this case. So clarifying that in the spec is not the
> *only* thing we can do to prevent this.
>
>

> NSS API does allow this too.
>
> Thing is, we want to have a way to test if the servers are tolerant to
> future clients
> that claim they had to downgrade. So having the ability to set TL1.2 with
> this
> SCSV is useful, just like is having an easy to use, widely distributed CLI
> tool
> (openssl command) that can do this should help with testing and
> interoperability,
> IMHO.
>

I know. This is exactly why OpenSSL does honor SSL_MODE_SEND_FALLBACK_SCSV
even when the highest supported protocol version is enabled. (So the best
way to make the API less fragile might be to create an additional setting
that makes SSL_MODE_SEND_FALLBACK_SCSV behavior unconditional, such as
SSL_MODE_SEND_FALLBACK_SCSV_FOR_TESTING.)

Bodo

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