Jean F. Martinelle <JFMart@overthere.com> wrote:

I believe that, in essence, what the authors meant is that "ssh-
rsa" will be the same as before - i.e. RSA with SHA-1 - and that the RSA
key used for "ssh-rsa" can be used, without any changes to the key itself, with "rsa-sha2-256" and "rsa-sha2-512".

Is this the correct interpretation?

Yes, I agree with all that. If the client and server agree on one of
the new host key algorithm names, say "rsa-sha2-256", then the string "rsa-sha2-256" will appear in the wire encoding of the _signature_,
but the wire encoding of the _key_ will still begin with the string
"ssh-rsa", because it will be the same key that would be used for
original SHA-1-based signatures.
--
import hashlib; print((lambda p,q,g,y,r,s,m: (lambda w:(pow(g,int(hashlib.sha1( m.encode('ascii')).hexdigest(),16)*w%q,p)*pow(y,r*w%q,p)%p)%q)(pow(s,q-2,q))==r and m)(0xb80b5dacabab6145,0xf70027d345023,0x7643bc4018957897,0x11c2e5d9951130c9 ,0xa54d9cbe4e8ab,0x746c50eaa1910, "Simon Tatham <anakin@pobox.com>" ))

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I sent this to the wrong forum by mistake.

I am looking into RFC 8832, where the RSA-based host algorithms
that use SHA-2 are defined. I find the following paragraph in that
document confusing:

"All aspects of the "ssh-rsa" format are kept, including the
encoded string "ssh-rsa". This allows existing RSA keys to be used with
the new public key algorithms, without requiring re-encoding or affecting already trusted key fingerprints."

There are nwe identifiers for the RSA-based algorithms that use
SHA-2: "rsa-sha2-256" and "rsa-sha2-512". From this I gather that when a
client specifies preference for (say) "rsa-sha2-256", the server will
offer an RSA host key with a SHA-256 algorithm for digests. If the client prefers "sha-rsa", I would have thought that the server would use an RSA
key and the SHA-1 algorithm.

My first assumption seems to be borne out by the second sentence
in the paragraph above. What is throwing me a bit off balance is the
"including the encoded string "ssh-rsa"" part of the first sentence.

I believe that, in essence, what the authors meant is that "ssh-
rsa" will be the same as before - i.e. RSA with SHA-1 - and that the RSA
key used for "ssh-rsa" can be used, without any changes to the key itself,
with "rsa-sha2-256" and "rsa-sha2-512".

Is this the correct interpretation?

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