Add OpenSSH 8 recommendations

Hi Joe,

The tool currently suggests recommended settings for OpenSSH 7.6 but I currently have OpenSSH_8.0p1, OpenSSL 1.1.1c 28 May 2019, so it would be great to have up-to-date kex/mac/etc recommendations what to enable, disable, and so on. There might be such in the official docs but the tool should make it easy to find them via its output.

# algorithm recommendations (for OpenSSH 7.6)
...

[jtesta]

I followed this guide since published: https://stribika.github.io/2015/01/04/secure-secure-shell.html

That’s funny, because that exact same guide is what got me into this SSH hardening topic years ago! It hasn’t been updated in a while, though, so I made my own based on it here: https://www.sshaudit.com/hardening_guides.html.

The Encrypt-and-MAC design introduced much more attack surface (see DJM’s blog post about it here; he’s the OpenSSH maintainer: http://blog.djm.net.au/2013/11/chacha20-and-poly1305-in-openssh.html). Encrypt-then-MAC (ETM) doesn’t have these problems, but they introduce another issue: the plaintext packet lengths are exposed. So the question is: is it better to use the EAM algorithms with their problems, or ETM and get a higher risk of traffic analysis? Generally, it seems people are leaning towards ETM being a little bit better.

(rec) -hmac-sha2-256                        -- mac algorithm to remove         
(rec) -hmac-sha2-512                        -- mac algorithm to remove         
(rec) [e-mail]                 -- mac algorithm to remove

The above algorithms are flagged as warnings (not failures), because they use the EAM method instead of ETM. The tool recommends that all algorithms that have at least warnings be removed. I didn’t change this behavior; that’s how it worked before.

[immanuelfodor]

Wow, bookmarked it!