How to blacklist certain ciphers and key exchange algorithms in ssh and sshd

Question

To get a list of all supported algorithms, I can run ssh -Q kex, which on my machine outputs

diffie-hellman-group1-sha1
diffie-hellman-group14-sha1
diffie-hellman-group14-sha256
diffie-hellman-group16-sha512
diffie-hellman-group18-sha512
diffie-hellman-group-exchange-sha1
diffie-hellman-group-exchange-sha256
ecdh-sha2-nistp256
ecdh-sha2-nistp384
ecdh-sha2-nistp521
curve25519-sha256
[email protected]
[email protected]

I also know I can add a line into my sshd_config and ssh_config files like KexAlgorithms [email protected],diffie-hellman-group-exchange-sha256 if I want to whitelist certain algorithms.

However, I don't think I will stay on top of keeping this list up to date. If my version of ssh is updated to include better ciphers, I want to be able to take advantage of those automatically (which is what would happen if I don't modify my configuration file). By trying to eliminate algorithms I do not want to use (for instance, diffie-hellman-group1-sha1), I have also locked myself in to not using anything better than the best currently supported algorithm.

Is there a way to configure ssh and sshd to say "Do not consider these algorithms" without also saying "And also don't consider anything except these other algorithms"?

I would accept answers specific to openssh and Linux, and you can assume I am using very new versions of relevant software.

Answer 1

From the man page for ssh_config:

KexAlgorithms
Specifies the available KEX (Key Exchange) algorithms. Multiple algorithms must be comma-separated. If the specified list begins with a '+' character, then the specified methods will be appended to the default set instead of replacing them. If the specified list begins with a '-' character, then the specified methods (including wildcards) will be removed from the default set instead of replacing them.

Use the '-' option with all the ciphers you want to blacklist from the default list on the client.

Unfortunately, the man page for sshd_config doesn't seem to document the same option at the server end, but it does work.