On 18 December 2015 at 19:56, Lyor Goldstein <[email protected]> wrote: > I don't think what yo described is possible tor standard SSH. The KEX must > use a key type that is available for both the server and the flient. If you > look at the KEX INIT command you 2ill notice that there is only ONE FIELD tor > key signatures, unlike ciphers. MACs and compression wher each side can > choose what to use. Bottom line, you cannot force the server to present a > weaker key than the client. However, all is not lost - OpenSSH servers do > support a non-standard extension where they send ALL their keys. I only > recently added support for this - look for the OpenSSHHostKeys global request > handler
Thanks for the reference, I have saw this when looking at the recent changes, but did not make the connection between the two. This happens after user authentication, so in case of password authentication server already has the password, which is kinda strange, I will send a question to openssh devs. As far as I see at sshd-core, there is no support for the [email protected] at client, am I right? What is the sequence of usage at client side? I guess setGlobalRequestHandlers with my own instance inherited from OpenSshHostKeysHandler to accept keys, then I should ask for prove for selected keys, how can I do that? Thanks! > From: Alon Bar-Lev [[email protected]] > Sent: Thursday, December 17, 2015 6:19 PM > To: [email protected]; Lyor Goldstein > Subject: Selecting a specific KEX signature at client for server key > > Hello, > > I have a challenge... > > Let's assume I need to securely reconnect to a server, for that purpose I > store its public key. Every time I connect to this server I compare the > public key accepted by ServerKeyVerifier with the public key that is approved. > > This works perfectly and securely, well, until server adds a stronger > algorithm/public key, for example, a server with ssh-rsa key adds > ecdsa-sha2-nistp256 one. > > In this case KEX selects the stronger one, causing client to reject the > server as public key differs. > > I found a solution to this problem is to limit the signature algorithms based > on the public key type. We know the public key, so we can limit the KEX to > use only this type of signature, for example, assuming the approved key is > ssh-rsa we can: > > > client.setSignatureFactories(Collections.singletonList(BuiltinSignatures.rsa)) > > Now, even if the server has ecdsa-sha2-nistp256 key, the client will only > receive the ssh-rsa key. > > This works find as long as password authentication is used. > > However, as the signature factory is common to both client and server KEX, > this will not work in all cases for public key authentication, as the factory > must contain the union of both, for example: > > server has both ssh-rsa and ecdsa-sha2-nistp256 keys. > client caches the ssh-rsa. > client key is ecdsa-sha2-nistp256. > client must enable both rsa and nistp256 signatures for KEX to succeed. > server key will be ecdsa-sha2-nistp256 as it is the strongest among ssh-rsa > and ecdsa-sha2-nistp256. > > Keep in mind that my understanding of the code is limited, I reviewed the > session callbacks, but from what I do understand there is no way to separate > KEX algorithms of server from these of client, as both are in the same base > class KexFactoryManager. > > If I understand correctly, a solution could be implemented by having two > signature factories, one for client side and one for server side, or have a > filter for client and server, to enable forcing a specific set of algorithm > for each side. > > Maybe I mis-read the code and there is an option of forcing server to use a > specific key differently. > > Any clue? > > Thanks, > Alon Bar-Lev.
