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Re: [TLS] RFC 7919 on Negotiated Finite Field Diffie-Hellman Ephemeral Parameters for Transport Layer Security (TLS)

Benjamin Kaduk <bkaduk@akamai.com> Tue, 16 August 2016 20:34 UTC

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To: Peter Gutmann <pgut001@cs.auckland.ac.nz>, "tls@ietf.org" <tls@ietf.org>
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From: Benjamin Kaduk <bkaduk@akamai.com>
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Date: Tue, 16 Aug 2016 15:34:36 -0500
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Subject: Re: [TLS] RFC 7919 on Negotiated Finite Field Diffie-Hellman Ephemeral Parameters for Transport Layer Security (TLS)
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On 08/16/2016 05:44 AM, Peter Gutmann wrote:
> As far as I can see what this text is saying is that if the client can't guess
> in advance which PFS suite/group the server knows about, the server must
> disable use of PFS.  In other words instead of saying "give me a PFS suite,
> preferably with this group", it's saying "give me a PFS suite with exactly
> this group and if you can't do that, don't do PFS".  This seems like a pretty
> awful way to handle things.
>

Recall that the "perfect" part depends on both sides doing what they're
supposed to.  And if the server wants to behave badly and is doing a
not-named group, it can send something that is not prime, or has small
subgroups, etc. -- not all clients will check, so the client is
definitely not guaranteed forward secrecy.

-Ben

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