The lab creates users alice and bob (password Lab-Pass-1), and the files /home/alice/secret.txt and /home/alice/notes.txt. Do each task as the user it names: log in as them directly, on the console or with ssh alice@localhost (after su, gpg often can't ask for a passphrase).

The peer, at 10.99.0.2 (name peer), is the second VM. It shares this VM's users and home directories, but runs its own sshd (le-lab-peer-sshd.service, which allows passwords for now) with its own configuration and host keys. Get a root shell there with sudo ip netns exec lepeer bash: in that shell, /etc/ssh is the peer's (/etc/netns/lepeer/ssh seen from this VM). Restart its sshd with sudo systemctl restart le-lab-peer-sshd.

Tasks

As alice, make an ed25519 key with a passphrase, copy it to the peer with ssh-copy-id [email protected], and log in. Load it into an agent with ssh-add and log in again without typing the passphrase.
Turn off password logins on the peer, keeping one session open, and check in a new terminal that keys still work and passwords don't.
Compare the peer's host key fingerprint (ssh-keygen -lf) with the one your client showed. Then regenerate the peer's host keys (in the peer's shell: rm /etc/ssh/ssh_host_* and ssh-keygen -A, then restart its sshd), connect as alice, and read the warning. Then remove the old key from alice's known_hosts and connect again.
Start a web server that listens only on the peer's localhost (python3 -m http.server --bind 127.0.0.1 8000, in the peer's shell), and reach it from this VM with ssh -L, so that curl http://localhost:8000/ here shows its page while the tunnel is up.
Make GnuPG keys for alice (Alice <[email protected]>) and bob (Bob <[email protected]>), each with a passphrase, and exchange public keys. As alice, encrypt secret.txt for bob as /home/alice/secret.txt.gpg; as bob, decrypt it to /home/bob/secret.txt.
As alice, make a detached signature /home/alice/notes.txt.sig of notes.txt and check it. Then change one byte of notes.txt, and check again.
As alice, generate a revocation certificate for her key with --generate-revocation, import it, and look at gpg -k.

Hints

ssh-keygen -t ed25519; eval "$(ssh-agent)", ssh-add, ssh-add -l.
PasswordAuthentication no in the peer's sshd_config; test with ssh -o PubkeyAuthentication=no [email protected].
ssh-keygen -lf /etc/ssh/ssh_host_ed25519_key.pub; ssh-keygen -R 10.99.0.2.
Run the web server in the background (&) in the peer's shell; ssh -N -f -L 8000:127.0.0.1:8000 [email protected].
gpg --full-generate-key or --quick-generate-key, --export -a, --import, -e -r, -d -o; copy files through /tmp.
gpg --detach-sign, gpg --verify notes.txt.sig notes.txt.
gpg --output revoke.asc --generate-revocation [email protected], gpg --import revoke.asc.