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

denis bider <> Sun, 12 July 2020 12:05 UTC

Return-Path: <>
Received: from localhost (localhost []) by (Postfix) with ESMTP id 7C70D3A077A; Sun, 12 Jul 2020 05:05:26 -0700 (PDT)
X-Virus-Scanned: amavisd-new at
X-Spam-Flag: NO
X-Spam-Score: -2.097
X-Spam-Status: No, score=-2.097 tagged_above=-999 required=5 tests=[BAYES_00=-1.9, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, FREEMAIL_FROM=0.001, HTML_MESSAGE=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001, URIBL_BLOCKED=0.001] autolearn=ham autolearn_force=no
Authentication-Results: (amavisd-new); dkim=pass (2048-bit key)
Received: from ([]) by localhost ( []) (amavisd-new, port 10024) with ESMTP id 4hkhNh8YPsAQ; Sun, 12 Jul 2020 05:05:24 -0700 (PDT)
Received: from ( [IPv6:2607:f8b0:4864:20::332]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by (Postfix) with ESMTPS id D50823A0778; Sun, 12 Jul 2020 05:05:23 -0700 (PDT)
Received: by with SMTP id 72so7527415otc.3; Sun, 12 Jul 2020 05:05:23 -0700 (PDT)
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed;; s=20161025; h=mime-version:references:in-reply-to:from:date:message-id:subject:to :cc; bh=DxIaY11QSCguSoJN465k/s5LWm+9qMIeZjmHaUWHqIY=; b=c2OY2PrLMNtdFMIogNb4jYY1CToCfjHOSiKEb1NOJqQ6l6Kylq+pdXCXKn0W1ulM0o qrh4vgWNMSji6WJljiDtZiyPvva4cUoqE69bNbNQrsRpCnR/O0NN/46p2gE0ZwWuxpYe vqBpAUyJsxNdPTmUn+lldQzdbxfOdhe5jp9Y06Yz5IGHqtgW5OLTWrQvA3Ml1ZR3Rt7a x6R9J0H4cujGjbS7WFfBIEU2iVMywp36/KivoFtu9LrUdRkJNBNKZ36+RhCfT7tsjOpO +d6C9L2X47H578b8F6xmnWjAxB8mOfrgU8gpSeheB+nfXGydRRVceIuU+yGWpuZKDSrg th0g==
X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed;; s=20161025; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=DxIaY11QSCguSoJN465k/s5LWm+9qMIeZjmHaUWHqIY=; b=jAldF3fl1jwQzSX5WtSS8UW9JfMIsY3tc3UaoAJzdHfuHkul82deuwa0WomxJ3j1N3 x4e3+5SJ40JaYXBzJF+13olq1+UsE+0ZGTTrXOpiRThQz9wtw7HxWrW5p736KU2zU39y rvxVeO+fOs05HrpUtUBry3z3EzDOoQbkRtC6VSi0NLXZlVWoAJPWmnVYm441QGadgVbZ XRMivrDK/1tUjizGqLV6BX8gaEyHLJrp3/mRwO0BmS67Zz4WFaonx7MtF8Nnc1zivM15 EjLYUIFPWdsTCewlkqFQE0UVEAgiWlgr1bWtsZLDTEM7I5Es0GQ4Vv6W8nvRiZDHmlNm 63bg==
X-Gm-Message-State: AOAM5316xQOZU1G8FPNvGaG7OcIbnInhvNGHo4Hex9TUs9G+bdmOtcHV hrTLew+qTOAKBWO042knprJiCigFfMvxpM7DJ40=
X-Google-Smtp-Source: ABdhPJy3csiutEua7FdplnVbiL49myQj9uDt81qP3U3qkbKmTxORdJSVxuSNINjpytDZlEvR8iDWqbWEDHq7R6/8UOs=
X-Received: by 2002:a05:6830:cd:: with SMTP id x13mr30668791oto.220.1594555522991; Sun, 12 Jul 2020 05:05:22 -0700 (PDT)
MIME-Version: 1.0
References: <> <>
In-Reply-To: <>
From: denis bider <>
Date: Sun, 12 Jul 2020 07:05:10 -0500
Message-ID: <>
To: "Mark D. Baushke" <>
Cc: curdle <>, curdle-chairs <>
Content-Type: multipart/alternative; boundary="00000000000085c75605aa3d63e4"
Archived-At: <>
Subject: Re: [Curdle] State of draft-ietf-curdle-ssh-kex-sha2?
X-Mailman-Version: 2.1.29
Precedence: list
List-Id: "List for discussion of potential new security area wg." <>
List-Unsubscribe: <>, <>
List-Archive: <>
List-Post: <>
List-Help: <>
List-Subscribe: <>, <>
X-List-Received-Date: Sun, 12 Jul 2020 12:05:27 -0000

Hey Mark!

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
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

> 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 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.

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.

Now that I've found RFC 8268, I'm also fine if we can't get consensus on
this draft, though it would be a nice to have.


On Sun, Jul 12, 2020 at 4:38 AM Mark D. Baushke <> wrote:

> Hi denis,
> denis bider <> writes:
> > Hey everyone,
> >
> > I notice the following draft has not moved forward:
> >
> >
> >
> > This seems to be an important draft which would standardize the
> > current use of key exchange algorithms in SSH. However, it looks like
> > no changes have been made in 2.5 years?
> Correct.
> > Did I miss some event where this draft morphed into something else so
> > that I'm not seeing the right information about progress?
> I have not progressed the draft, mostly due to private email received
> over two years ago...
> A number of people told me to not move it forward until after all of the
> RFCs for draft-ietf-curdle-ssh-curves (now RFC 8731) and
> draft-ietf-curdle-gss-keyex-sha2 (now RFC 8732) were adopted. Also, many
> people were unhappy with the characterizations of the existing
> algorithms and my scoring of MUST, SHOULD, and MAY
> In addition, there was a general dislike for the references of the NSA
> documents provided or the CNSA document reference.
> > Otherwise, what seems to be the current obstacle with making progress
> > on this?
> I think that work on the document is desirable. Does anyone wish to be a
> co-author with me?
> I would like to see more opinions on the list about which algorithms are
> to be 'SHOULD NOT' and which are to be 'MUST' ... in general, I would
> like to see this document as a KEX refernce that may be updated every
> few years as we learn more about which KEX algorithms are best to use.
> My opinion for Section 5 as I write this email today is:
>       Key Exchange Method Name             Reference  Implement
>       ------------------------------------ ---------- ----------
>       curve25519-sha256                    RFC8731    SHOULD
>       curve448-sha512                      RFC8731    MAY
>       diffie-hellman-group-exchange-sha1   RFC4419    SHOULD NOT
>       diffie-hellman-group-exchange-sha256 RFC4419    MAY
>       diffie-hellman-group1-sha1           RFC4253    SHOULD NOT
>       diffie-hellman-group14-sha1          RFC4253    SHOULD NOT
>       diffie-hellman-group14-sha256        RFC8268    SHOULD
>       diffie-hellman-group15-sha256        RFC8268    MAY
>       diffie-hellman-group16-sha512        RFC8268    MUST
>       diffie-hellman-group17-sha512        RFC8268    MAY
>       diffie-hellman-group18-sha512        RFC8268    MAY
>       ecdh-sha2-*                          RFC5656    MAY
>       ecdh-sha2-nistp256                   RFC5656    SHOULD
>       ecdh-sha2-nistp384                   RFC5656    SHOULD
>       ecmqv-sha2                           RFC5656    MAY
>       ext-info-c                           RFC8308    SHOULD
>       ext-info-s                           RFC8308    SHOULD
>       gss-*                                RFC4462    MAY
>       gss-curve25519-sha256-*              RFC8732    SHOULD
>       gss-curve448-sha512-*                RFC8732    MAY
>       gss-gex-sha1-*                       RFC4462    SHOULD NOT
>       gss-group1-sha1-*                    RFC4462    SHOULD NOT
>       gss-group14-sha256-*                 RFC8732    SHOULD
>       gss-group15-sha512-*                 RFC8732    MAY
>       gss-group16-sha512-*                 RFC8732    SHOULD
>       gss-group17-sha512-*                 RFC8732    MAY
>       gss-group18-sha512-*                 RFC8732    MAY
>       gss-nistp256-sha256-*                RFC8732    SHOULD
>       gss-nistp384-sha384-*                RFC8732    MAY
>       gss-nistp521-sha512-*                RFC8732    MAY
>       rsa1024-sha1                         RFC4432    MUST NOT
>       rsa2048-sha256                       RFC4432    MAY
> The above list of KEX algorithms comes from the IANA ssh-parameters list
> URL:
> Please let me know if I have missed any of the KEX algorithms in the
> list.
> Of these, I am not sure if rsa2048-sha256 has support for a 'MAY' or if
> its lack of use would drive it to a 'SHOULD NOT' in the table.
> To be honest, I am really not sure which KEX algorithms should be listed
> as Mandatory To Implement (MTI) for key exchanges going forward.
> Which diffie-hellman FFC group should be listed as MTI? group14-sha256
> or group16-sha512? (I tentatively selected this one). Is that wise?
> Should any FFC Diffie-Hellman group size be MTI?
> I would like to hear if others on this list believe that
> curve25519-sha256 should be a MUST or a SHOULD.
> I also do not know if the expired draft-ietf-curdle-ssh-kex-sha2
> document should bother to give opinions on any of the KEX options other
> than those being deprecated or thrust into MTI. Opinions please?
> It seems clear to me that removing the *-sha1* KEX algorithms is a good
> idea. I would love to move diffie-hellman-group14-sha1, but I honestly
> suspect that some hardware is deployed for which it is the only KEX
> algorithm that may still need to be supported... which is the only
> reason it is a 'SHOULD' on my list instead of a 'SHOULD NOT' ...
>         Be safe, stay healthy,
>         -- Mark