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Comment by slasaus | Hacker News Reader

My hardened ~/.ssh/config on OS X 10.11:

  Host *
    Ciphers chacha20-poly1305@openssh.com,aes256-gcm@openssh.com,aes128-gcm@openssh.com,aes256-ctr
    MACs hmac-sha2-512-etm@openssh.com,hmac-sha2-256-etm@openssh.com,hmac-sha2-512,hmac-sha2-256
    KexAlgorithms curve25519-sha256@libssh.org,diffie-hellman-group-exchange-sha256
    HostKeyAlgorithms ssh-ed25519,ssh-rsa
    ChallengeResponseAuthentication no
    UseRoaming no

If you only connect to newer servers you can further restrict ciphers to only use AEADs (only list the chacha20-poly1305 and aes-gcm ciphers). I assume using AEADs-only makes the MACs keyword obsolete, is this correct?

Config is based on tips from https://stribika.github.io/2015/01/04/secure-secure-shell.ht...

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> I assume using AEADs-only makes the MACs keyword obsolete, is this correct?

Correct. You can test this by running SSH with -v/--verbose and observing these log lines:

    debug1: kex: server->client aes256-gcm@openssh.com <implicit> none
    debug1: kex: client->server aes256-gcm@openssh.com <implicit> none

When using a cipher that does not support AEAD, the 2nd field will include the MAC instead of <implicit>

    debug1: kex: server->client aes256-ctr hmac-sha2-256-etm@openssh.com none
    debug1: kex: client->server aes256-ctr hmac-sha2-256-etm@openssh.com none

I wasn't sure what the 3rd "none" field is, but after some digging it appears to be the compression algorithm: https://github.com/openssh/openssh-portable/blob/master/kex....

And I confirmed this with ssh -C:

    debug1: kex: server->client aes256-ctr hmac-sha2-256-etm@openssh.com zlib@openssh.com
    debug1: kex: client->server aes256-ctr hmac-sha2-256-etm@openssh.com zlib@openssh.com

Mozilla also maintains a SSH guide for both clients and servers: https://wiki.mozilla.org/Security/Guidelines/OpenSSH

Funny story, I erroneously threw `UseRoaming no` on my server config and freaked the fuck out when I couldn't get into via SSH.

Six rescue sessions later, I figured out it shouldn't have been there ...

In the future, I'm definitely going to read documentation & test config files (sshd -t -f /path/to/sshd_config, in this case)

EDIT: Moral of the story, I am not a smart person sometimes.

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