Split GPG & SSH

This post will go over setting up Split GPG, then setting up Split SSH with the same PGP keys. Effectively, we are emulating what you can do with a PGP smartcard on Qubes OS.

Split GPG

Follow the official Qubes OS documentation to set this up.

Note that if you already have a PGP key with a passphrase, you can remove it by installing pinentry-gtk to vault’s TemplateVM, then run gpg2 --edit-key <key_id> and passwd to set an empty passphrase. The default non-graphical pinentry will just make an infinite loop and will not allow you to set an empty passphrase.

Split SSH

This part is based on the Qubes Community’s guide; however, I will deviate from it to use the PGP keys for SSH instead of generating a new key pair.

In dom0

  • Create /etc/qubes-rpc/policy/qubes.SshAgent with @anyvm @anyvm ask,default_target=vault as the content. Since the keys are not passphrase protected, you should not set the policy to allow.

In vault AppVM

  • Add enable-ssh-support to the end of ~/.gnupg/gpg-agent.conf
  • Get your keygrip with gpg --with-keygrip -k
  • Add your keygrip to the end of ~/.gnupg/sshcontrol

PGP Keygrip

In vault’s TemplateVM

  • Create /etc/qubes-rpc/qubes.SshAgent with the following content:
#!/bin/sh
# Qubes App Split SSH Script

# Activate GPG Agent and set the correct SSH socket
export SSH_AUTH_SOCK=$(gpgconf --list-dirs agent-ssh-socket)
gpgconf --launch gpg-agent

# safeguard - Qubes notification bubble for each ssh request
notify-send "[$(qubesdb-read /name)] SSH agent access from: $QREXEC_REMOTE_DOMAIN"

# SSH connection
socat - "UNIX-CONNECT:$SSH_AUTH_SOCK"
  • Make it executable with sudo chmod +x /etc/qubes-rpc/qubes.SshAgent
  • Turn off the templateVM. If the vault VM is running, turn it off, then start it to update the VM’s configuration.

In ssh-client AppVM

  • Add the following to the end of /rw/config/rc.local:
# SPLIT SSH CONFIGURATION >>>
# replace "vault" with your AppVM name which stores the ssh private key(s)
SSH_VAULT_VM="vault"

if [ "$SSH_VAULT_VM" != "" ]; then
  export SSH_SOCK="/home/user/.SSH_AGENT_$SSH_VAULT_VM"
  rm -f "$SSH_SOCK"
  sudo -u user /bin/sh -c "umask 177 && exec socat 'UNIX-LISTEN:$SSH_SOCK,fork' 'EXEC:qrexec-client-vm $SSH_VAULT_VM qubes.SshAgent'" &
fi
# <<< SPLIT SSH CONFIGURATION
  • Add the following to the end of ~/bash.rc:
# SPLIT SSH CONFIGURATION >>>
# replace "vault" with your AppVM name which stores the ssh private key(s)
SSH_VAULT_VM="vault"

if [ "$SSH_VAULT_VM" != "" ]; then
  export SSH_AUTH_SOCK="/home/user/.SSH_AGENT_$SSH_VAULT_VM"
fi
# <<< SPLIT SSH CONFIGURATION
  • Restart ssh-client and confirm if it’s working with ssh-add -L.

Limitations

A malicious ssh-client AppVM can hold onto the ssh-agent connection for more than one use until it is shut down. While your private key is protected, a malicious actor with access to the AppVM can still abuse the ssh-agent to log into your servers.