Figure out sshd algorithms for noble

[legoktm]

Description

Our sshd_config ships with:

# Cipher selection

Ciphers [email protected],[email protected],aes256-ctr,aes128-ctr
# Don't use SHA1 for kex
KexAlgorithms [email protected],diffie-hellman-group-exchange-sha256
# Don't use SHA1 for hashing, don't use encrypt-and-MAC mode
MACs [email protected],[email protected],[email protected]

We should update this for noble.

I was reading through https://blog.stribik.technology/2015/01/04/secure-secure-shell.html because I was trying to understand the whole /etc/ssh/moduli and it seems like we could drop diffie-hellman-group-exchange-sha256 for diffie-hellman-group14-sha256/diffie-hellman-group16-sha512/diffie-hellman-group18-sha512 and not have to deal with the moduli file. But I guess the group exchange one is preferred?

Also instead of setting a fixed list, we could just subtract the ones we don't want, so we get the advantage of newer algos without needing to manually update our lists each time. It also supports wildcards so theoretically do something like KexAlgorithms -*sha1.

Per https://www.man7.org/linux/man-pages/man5/sshd_config.5.html the upstream default currently is:

Ciphers

[email protected],
aes128-ctr,aes192-ctr,aes256-ctr,
[email protected],[email protected]

KexAlgorithms

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

MACs

[email protected],[email protected],
[email protected],[email protected],
[email protected],
[email protected],[email protected],
hmac-sha2-256,hmac-sha2-512,hmac-sha1