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
_____________________
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.
