Re: [Curdle] RSA key transport for SSH (RFC 4432) and forward secrecy

Ron Frederick <> Thu, 11 February 2021 06:28 UTC

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From: Ron Frederick <>
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Date: Wed, 10 Feb 2021 22:27:55 -0800
Cc: Simon Tatham <>, Alexandre Becoulet <>, Keith Winstein <>, Hari Balakrishnan <>,, Peter Gutmann <>, Benjamin Kaduk <>, "" <>
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To: "Mark D. Baushke" <>
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Subject: Re: [Curdle] RSA key transport for SSH (RFC 4432) and forward secrecy
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On Feb 10, 2021, at 9:50 PM, Mark D. Baushke <> wrote:
> Summary:
>    Is anyone actively using rsa2048-sha256 for a Ssecure Shell Key
>    exchange per RFC 4432. 
>    The Security Area Director Benjamin Kaduk has concerns regarding
>    this Key Exchange Algorithm (see messagess below).
>    The IETF Draft
>    is presently in Last Call.
>    This draft is in the process of suggesting "MUST NOT" for
>    rsa1024-sha1.
>    The question on the table is if the same rating should be appled to
>    rsa2048-sha256 or if RFC 4432 should itself be moved to historical,
>    or if this is still a useful key exchange being actively used.
>    Ben desires data and it is my suggestion that the supporters for the
>    implementations that provide for rsa2048-sha256 may information on
>    this topic.
>    Comments welcome.

I added rsa1024-sha1 and rsa2048-sha256 to AsyncSSH back in 2018 to improve compatibility after noticing that there were other implementations out there. Since then, based on discussions on this mailing list, I disabled rsa1024-sha1 by default, but still allow it to be used if explicitly enabled. At this point, rsa2048-sha256 remains enabled in AsyncSSH, but it’s the very last entry in the list of advertised kex algorithms by default.

Depending on the outcome of this discussion, I would be happy to consider disabling both algorithms by default. I don’t see a reason in most cases to prefer them over other more widely implemented kex algorithms. However, I’d probably leave them available just in case someone wanted to use them.

One scenario which comes to mind where these algorithms could provide a benefit is precisely in the case which led to this thread suggesting we disable them, where someone might want the ability to passively monitor a stream and compute the session key when given the RSA private key. This is not possible (by design) with other kex algorithms without becoming an active man-in-the-middle, but it could be done passively with these RSA-based kex algorithms. I know such tools exist for TLS, but I don’t know if anyone has built something like that for SSH.
Ron Frederick