privsec.dev/content/posts/qubes/Using Split GPG and Split SSH on Qubes OS.md

title	date	tags	author
Using Split GPG and Split SSH on Qubes OS	2022-08-13	Operating Systems Qubes OS Security	Tommy

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

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.