Re: [Curdle] Looking for comments on draft-ietf-curdle-ssh-kex-sha2

Ron Frederick <> Sun, 16 August 2020 21:00 UTC

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From: Ron Frederick <>
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Date: Sun, 16 Aug 2020 14:00:27 -0700
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To: "Mark D. Baushke" <>
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Subject: Re: [Curdle] Looking for comments on draft-ietf-curdle-ssh-kex-sha2
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Hi Mark,

On Aug 5, 2020, at 9:57 AM, Mark D. Baushke <> wrote:
> Before IETF-108, I uploaded a new edition of
>  Key Exchange (KEX) Method Updates and Recommendations for Secure Shell (SSH)
> with the hope of
>  a) Providing a survey of existing IANA Secure Shell (SSH) Protocol
>     Parameters for Key Exchange Method Names
>  b) identifying a KEX to replace the current MUST implement exchanges:
>       diffie-hellman-group1-sha1
>       diffie-hellman-group14-sha1
>  c) the desire to be deprecate any KEX which use sha1 as the secure hash:
>       diffie-hellman-group-exchange-sha1
>       diffie-hellman-group1-sha1
>       diffie-hellman-group14-sha1
>       gss-group1-sha1-*
>       gss-group14-sha1-*
>       gss-gex-sha1-*
>       rsa1024-sha1
>  d) A desire that the IANA KEX parameters contain another column in the
>     table to indicate MUST, SHOULD, SHOULD NOT, MUST NOT, and MAY.
> I have tried to indicate general guidance for the rest of the Method
> Names, but many of the guidelines may be my personal opinion even though
> I have tried to be objective throughout.

This generally looks good to me. Here are a few more detailed comments:

- Sections 3.14 and 3.15 list the ext-info values as SHOULD (which I agree with). However, your table in section 5 has them marked as MAY.

- I noticed you dropped diffie-hellman-group14-sha256 back from MUST to SHOULD, leaving no algorithms listed as MUST. I’d still like to see at least one algorithm be listed as MUST, and think this is probably the safest candidate for that.

- I’m also thinking diffie-hellman-group15-sha512 might be a good candidate for a SHOULD rather than a MAY, but I’m not sure we have consensus on that.

- I agree with the downgrade of diffie-hellman-group16-sha512 from SHOULD to MAY.

- Regarding possible ECDSA algorithms, I implemented the secp256k1 curve as ecdh-sha21. in AsyncSSH after seeing it was implemented by Bitvise. I don’t know if it is worth mentioning here explicitly, but it’s one real-world example of an ecdh-sha2-* algorithm not explicitly given a name in RFC 5656. The ‘endsa-sha2’ algorithm with this curve is also supported.
Ron Frederick