chris blogs: February 2014

02feb2014 · The road to OpenSSH bliss: ED25519 and the IdentityPersist patch

The recent release of OpenSSH 6.5 had many convincing new features to make me update to it early, quoting from the release notes:

  • support for key exchange using elliptic-curve Diffie–Hellman in Daniel Bernstein’s Curve25519
  • support for Ed25519 as a public key type
  • a new private key format that uses a bcrypt KDF to better protect keys at rest
  • a new transport cipher that combines Daniel Bernstein’s ChaCha20 stream cipher and Poly1305 MAC to build an authenticated encryption mode

Since OpenSSH 5.7, there is support for ECDSA keys according to RFC5656. The problem with this schema is that it uses NIST curves generally. Due to recent events, everyone is (rightfully) more paranoid now, and there are reasons to consider these curves to be problematic. Thus, I decided to disable support for ECDSA host keys and use the superior ED25519 scheme for new keys.

However, I also need to access many machines which don’t run the latest version of OpenSSH, but for these we can at least make use of the new, safer public key format.

First, I will show how to update your OpenSSH installation to make use of the new features, and then I’ll explain what else I had to do to make everything work correctly.

Upgrading OpenSSH

Arch is, at the time of writing, providing binaries in “testing”, but plucking a single package is easy:

# pacman -U

After merging configuration files (if required), we edit /etc/ssh/sshd_config and spell out the HostKeys to disable the built-in defaults, which include ECDSA.

HostKey /etc/ssh/ssh_host_rsa_key
HostKey /etc/ssh/ssh_host_dsa_key
HostKey /etc/ssh/ssh_host_ed25519_key

Upon restarting SSH, a new ED25519 hostkey will be generated. Using ignite:

# sv restart sshd

Essentially, that’s it. Let’s check that the ECDSA hostkey is disabled:

% ssh-keyscan -t ecdsa,ed25519 localhost
# localhost SSH-2.0-OpenSSH_6.5
no hostkey alg
# localhost SSH-2.0-OpenSSH_6.5
localhost ssh-ed25519 AAAA...

(AFAIU, ECDSA user keys in authorized_keys will still work. It’s your task to replace them with ED22519 ones, I have found no way to blacklist them.)

We continue by creating a new ED25519 key for the user:

% ssh-keygen -t ed25519
Generating public/private ed25519 key pair.
Enter file in which to save the key (/home/chris/.ssh/id_ed25519): 
Enter passphrase (empty for no passphrase): 
Enter same passphrase again:

For testing, we can add it to the locally accepted keys:

% cat .ssh/ >>.ssh/authorized_keys

Finally, we can test it:

% ssh chris@localhost
Enter passphrase for key '/home/chris/.ssh/id_ed25519':

It seems to work! (Perhaps you’ll see an update of the fingerprint if you had the ECDSA one saved in .ssh/known_hosts.)

I was happy about that until I realized that ssh asked for the passphrase, and not my gnome-keyring-daemon… which brings us to part two of this post.

Migrating to ssh-agent and the IdentityPersist patch

Since new ED25519 keys are always stored in the new bcrypt format, they won’t work (as of right now) with SSH agents that don’t support it (I know of gnome-keyring and mate-keyring; gpg-agent stores the key itself, anyway). Essentially, only plain ssh-agent supports it, but I used to hate ssh-agent since it doesn’t support adding keys upon use: gnome-keyring will ask for the password and keep the key unlocked. Since I try to keep my keys locked when I’m not using them, I don’t want to keep them unlocked in every session, and neither unlock them manually, because that is inconvenient.

Luckily, I found this patch which adds the key automatically. I hope it gets accepted, because it is very useful. I couldn’t wait and patched OpenSSH myself. With this tiny patch, I could finally drop my use of gnome-keyring, which is one more step towards a GTK3-free desktop.

I run ssh-agent from my .xinitrc:

eval $(ssh-agent -s)
xscreensaver-ssh-helper &

Now, we can convert our old keys to the new storage format:

% ssh-keygen -p -o -f ~/.ssh/id_dsa
Enter old passphrase: 
Key has comment 'id_dsa'
Enter new passphrase (empty for no passphrase): 
Enter same passphrase again: 
Your identification has been saved with the new passphrase.

The key file should now start with -----BEGIN OPENSSH PRIVATE KEY-----. The public key format is unchanged.

For the IdentityPersist patch, we need to add a line to .ssh/config, stating the lifetime of the key (or true for infinity):

IdentityPersist 300

Now, we can try everything together:

% ssh myhost
Enter passphrase for key '/home/chris/.ssh/id_dsa': 
Identity added: /home/chris/.ssh/id_dsa (id_dsa)

Another SSH connect within the next 5 minutes will connect without asking for a password.

I noticed one more difference between gnome-keyring and ssh-agent: gnome-keyring would unlock any key that has a *.pub in ~/.ssh, while ssh-agent requires an explicit list in .ssh/config:

IdentityFile ~/.ssh/id_ed25519
IdentityFile ~/.ssh/id_dsa
IdentityFile ~/.ssh/id_work

Fixing Gnus

I had one final issue, which is rather niche: My newsreader gnus connects to my NNTP feed via a SSH hop. The configuration looked like this:

(setq gnus-select-method
      '(nntp "localhost"
             (nntp-address "myshellhost")
             (nntp-rlogin-program "ssh")
             (nntp-open-connection-function nntp-open-rlogin)
             (nntp-end-of-line "\n")
             (nntp-rlogin-parameters ("nc" "mynntpserver" "nntp"))))

A configuration like this will fail, because Emacs runs this ssh process with a pty, and there ssh will stupidly ask for the password to unlock (if required). But I learned this configuration is outdated anyway, and the recommended version using nntp-open-via-rlogin-and-netcat works correctly, and asks for the password (if required) using x11-ask-sshpass. This is mentioned in a comment in nntp.el, so I’m not the first on to stumble on this.

A fixed version of above would be:

(setq gnus-select-method
      '(nntp "localhost"
             (nntp-address "mynntpserver")
             (nntp-via-address "myshellserver")
             (nntp-via-rlogin-command "ssh")
             (nntp-via-rlogin-command-switches ("-C"))
             (nntp-open-connection-function nntp-open-via-rlogin-and-netcat)))

Enjoy your safer OpenSSH setup!

NP: Slime—A.C.A.B.

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