


I am looking into RFC 8832, where the RSAbased host algorithms
that use SHA2 are defined. I find the following paragraph in that document confusing: "All aspects of the "sshrsa" format are kept, including the encoded string "sshrsa". This allows existing RSA keys to be used with the new public key algorithms, without requiring reencoding or affecting already trusted key fingerprints." There are nwe identifiers for the RSAbased algorithms that use SHA2: "rsasha2256" and "rsasha2512". From this I gather that when a client specifies preference for (say) "rsasha2256", the server will offer an RSA host key with a SHA256 algorithm for digests. If the client prefers "sharsa", I would have thought that the server would use an RSA key and the SHA1 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 "sshrsa"" 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 SHA1  and that the RSA key used for "sshrsa" can be used, without any changes to the key itself, with "rsasha2256" and "rsasha2512". Is this the correct interpretation? 


On 20200224, Jean F. Martinelle <JFMart> wrote:
[RFC 8332] > "All aspects of the "sshrsa" format are kept, including the > encoded string "sshrsa". This allows existing RSA keys to be used with > the new public key algorithms, without requiring reencoding or affecting > already trusted key fingerprints." > I believe that, in essence, what the authors meant is that "ssh > rsa" will be the same as before  i.e. RSA with SHA1  and that the RSA > key used for "sshrsa" can be used, without any changes to the key > itself, with "rsasha2256" and "rsasha2512". Yes. It also means that references to a key proper, e.g. this entry from my ~/.ssh/known_hosts, will remain unchanged. sushi.unixag.unikl.de sshrsa AAAAB3NzaC1yc2EAAAABIwAAAIEA126tYr4R3gmXSBeY8pt2gd AZxmNhnIUqRK9lxlaTxRfZxB9i5Q9Kc2m0IVOtcKcbGppX3cPC 9mWmDA/desbB1guEDOLuc5emNn1K0bXMtFyUhD9W7EsHTF5mDyjDj9GuV 7thWG91dQX+PXNPcer5KThMaI3qqYh70XEJveCUr/c= This is still an "sshrsa" host key and can be used with any of the sshrsa, rsasha2256, or rsasha2512 as host key algoritm. 