Re: Fixing exchange of host keys in the SSH key exchange

Yes – that is my understanding as well.


From: Peter Gutmann 
Sent: Friday, April 7, 2017 21:48
To: Mouse ; [email protected] 
Subject: Re: Fixing exchange of host keys in the SSH key exchange

Mouse <[email protected]> writes:

>They say, when describing [email protected] and hostkeys-
>[email protected], that "[i]t also supports graceful key rotation: a
>server may offer multiple keys of the same type for a period (to give clients
>an opportunity to learn them using this extension) before removing the
>deprecated key from those offered".
>
>I cannot see how this is even possible, at least not without a custom kex
>algorithm. With, for example, Diffie-Hellman as defined in 4253 section 8,
>the server presents only one host key to the client, and must choose which
>one to present before kex (and thus authentication) completes.  This then
>gives no room to "offer multiple keys of the same type".

I assumed the offered keys are via "[email protected]", not in the
keyex.  As the text says, it offers those for awhile, then when it seems all
clients have got a copy it switches the keyex from the old to the new key.

Peter.