Re: [Curdle] State of draft-ietf-curdle-ssh-kex-sha2?

Ron Frederick <ronf@timeheart.net> Sun, 12 July 2020 16:17 UTC

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From: Ron Frederick <ronf@timeheart.net>
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Date: Sun, 12 Jul 2020 09:17:05 -0700
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Cc: "Mark D. Baushke" <mdb@juniper.net>, curdle <curdle@ietf.org>, curdle-chairs <curdle-chairs@ietf.org>
To: denis bider <denisbider.ietf@gmail.com>
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Subject: Re: [Curdle] State of draft-ietf-curdle-ssh-kex-sha2?
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On Jul 12, 2020, at 5:05 AM, denis bider <denisbider.ietf@gmail.com> wrote:
> It looks like I found the key thing I missed: Google did not show me RFC 8268 when I searched for "diffie-hellman-group14-sha256". It finds it now when I search for "rfc diffie-hellman-group14-sha256".
> 
> I could have found it in the IANA registry but I trusted the lack of Google results in my first search. Alas.
> 
> That resolves the key concern for me: that names like "diffie-hellman-group14-sha256" are authoritatively defined. I was looking for this reference for the SSH QUIC draft.
> 
> With regard to MUST / MAY / SHOULD / etc assignments, I agree with all of these:
> 
> > curve25519-sha256                     SHOULD
> > curve448-sha512                       MAY
> > diffie-hellman-group-exchange-sha1    SHOULD NOT
> > diffie-hellman-group-exchange-sha256  MAY
> > diffie-hellman-group1-sha1            SHOULD NOT
> > diffie-hellman-group14-sha1           SHOULD NOT
> > diffie-hellman-group14-sha256         SHOULD
> > diffie-hellman-group15-sha256         MAY
> 
> I do not agree with this one:
> 
> > diffie-hellman-group16-sha512   MUST
> 
> I find this too computationally expensive to justify "MUST" for servers. Last time I checked, this costs about 100 ms in server CPU time, more on weaker CPUs, and makes it trivial to DoS a resource-constrained server - no DDoS needed.

I’m good with group16-sha512 remaining a “MAY” along with group17-sha512 and group18-sha512, What about group15-sha256, though? Should that perhaps be a “SHOULD”, and also group14-sha256 changed to a “MUST”? I think we want at least one DH algorithm to be listed as a “MUST”.



> I agree with all of these:
> 
> > diffie-hellman-group17-sha512  MAY
> > diffie-hellman-group18-sha512  MAY
> > ecdh-sha2-*                    MAY
> > ecdh-sha2-nistp256             SHOULD
> > ecdh-sha2-nistp384             SHOULD
> > ecmqv-sha2                     MAY
> > ext-info-c                     SHOULD
> > ext-info-s                     SHOULD
> > gss-*                          MAY
> 
> I conditionally agree on these:
> 
> > gss-curve25519-sha256-*  SHOULD
> > gss-curve448-sha512-*    MAY
> > gss-gex-sha1-*           SHOULD NOT
> > gss-group1-sha1-*        SHOULD NOT
> > gss-group14-sha256-*     SHOULD
> > gss-group15-sha512-*     MAY
> > gss-group16-sha512-*     SHOULD
> > gss-group17-sha512-*     MAY
> > gss-group18-sha512-*     MAY
> > gss-nistp256-sha256-*    SHOULD
> > gss-nistp384-sha384-*    MAY
> > gss-nistp521-sha512-*    MAY
> 
> The condition is that the reader understands that "SHOULD" for these methods applies only if you implement any GSS-API key exchange methods at all. In other words, the whole category of GSS-API needs to be "MAY", but within the category the above SHOULDs are appropriate as long as the decision is made to implement any of the GSS-API key exchange methods in the first place.

With that condition in mind, I think all of these should match their non-GSS equivalents in terms of MAY/SHOULD/MUST. So, if changes are made to group14/15/16, they should be reflected here as well.


> I agree with these:
> 
> > rsa1024-sha1             MUST NOT
> > rsa2048-sha256           MAY
> 
> In summary - my main complaint is that "diffie-hellman-group16-sha512" is too expensive to be a MUST. If that leaves no key exchange method that is a "MUST", I'm fine with it.
> 
> If there needs to be a method that's a "MUST", I vote for either "curve25519-sha256" or "diffie-hellman-group14-sha256" as meeting security requirements while being the most widely compatible.

I like having curve25519-sha256 as a strongly encouraged SHOULD, but I think it might be better to stick with group14-sha256 as the MUST.
-- 
Ron Frederick
ronf@timeheart.net