Comment 11 for bug 2031942

Verified for lunar and jammy:

# lxc launch ubuntu:jammy test-openssh-lunar
# lxc exec test-openssh-lunar bash

# cat <<EOF >/etc/apt/sources.list.d/ubuntu-$(lsb_release -cs)-proposed.list
# Enable Ubuntu proposed archive
deb http://archive.ubuntu.com/ubuntu/ $(lsb_release -cs)-proposed restricted main multiverse universe
EOF

# apt update && apt dist-upgrade -y

# mkdir reproducer
# cd reproducer
# mkdir certuser keyonlyuser
# cd keyonlyuser

# ssh-keygen -t ed25519 -f key
Generating public/private ed25519 key pair.
Enter passphrase (empty for no passphrase):
Enter same passphrase again:
Your identification has been saved in key
Your public key has been saved in key.pub
...

# cd ../certuser/

# ssh-keygen -t rsa -f ca
Generating public/private rsa key pair.
Enter passphrase (empty for no passphrase):
Enter same passphrase again:
Your identification has been saved in ca
Your public key has been saved in ca.pub

# ssh-keygen -t ed25519 -f key
Generating public/private ed25519 key pair.
Enter passphrase (empty for no passphrase):
Enter same passphrase again:
Your identification has been saved in key
Your public key has been saved in key.pub

# ssh-keygen -s ca -I key_id -n certuser key.pub
Signed user key key-cert.pub: id "key_id" serial 0 for certuser valid forever

# cd ..
# cat <<EOF >authorized_principals
#!/bin/sh
if [ "$1" = "otheruser" ]; then
echo certuser
fi
EOF

# chmod 755 authorized_principals

# adduser --disabled-password otheruser
Adding user `otheruser' ...
Adding new group `otheruser' (1001) ...
Adding new user `otheruser' (1001) with group `otheruser' ...
Creating home directory `/home/otheruser' ...
Copying files from `/etc/skel' ...
Changing the user information for otheruser
Enter the new value, or press ENTER for the default
 Full Name []:
 Room Number []:
 Work Phone []:
 Home Phone []:
 Other []:
Is the information correct? [Y/n] y

# adduser --disabled-password keyonlyuser
Adding user `keyonlyuser' ...
Adding new group `keyonlyuser' (1002) ...
Adding new user `keyonlyuser' (1002) with group `keyonlyuser' ...
Creating home directory `/home/keyonlyuser' ...
Copying files from `/etc/skel' ...
Changing the user information for keyonlyuser
Enter the new value, or press ENTER for the default
 Full Name []:
 Room Number []:
 Work Phone []:
 Home Phone []:
 Other []:
Is the information correct? [Y/n] y

# cat <<EOF >authorized_keys
#!/bin/sh
if [ "$1" = "keyonlyuser" ]; then
echo "ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIBbBpE1TDzG48CLKI6bWQbIE87ke4ZJmgFQ0LFnYyikT root@test-openssh-lunar"
fi
EOF

# chmod 755 authorized_keys

# vi /etc/ssh/sshd_config
i
AuthorizedKeysCommand /root/reproducer/authorized_keys %u
AuthorizedKeysCommandUser root

AuthorizedPrincipalsCommand /root/reproducer/authorized_principals %u
AuthorizedPrincipalsCommandUser root

TrustedUserCAKeys /root/reproducer/certuser/ca.pub
[esc]
:wq

# systemctl restart ssh

# ssh keyonlyuser@localhost -i /root/reproducer/keyonlyuser/key
The authenticity of host 'localhost (127.0.0.1)' can't be established.
ED25519 key fingerprint is SHA256:hCVcIK0Q/jBlQqqyEAcGnxgXnd46/kaT7j+78BobN2E.
This key is not known by any other names
Are you sure you want to continue connecting (yes/no/[fingerprint])? yes
Warning: Permanently added 'localhost' (ED25519) to the list of known hosts.
Welcome to Ubuntu 23.04 (GNU/Linux 5.15.0-83-generic x86_64)
...

$ exit

# ssh otheruser@localhost -i /root/reproducer/certuser/key -o CertificateFile=/root/reproducer/certuser/key-cert.pub
Welcome to Ubuntu 23.04 (GNU/Linux 5.15.0-83-generic x86_64)

# lxc launch ubuntu:jammy test-openssh-jammy
# lxc exec test-openssh-jammy bash

# cat <<EOF >/etc/apt/sources.list.d/ubuntu-$(lsb_release -cs)-proposed.list
# Enable Ubuntu proposed archive
deb http://archive.ubuntu.com/ubuntu/ $(lsb_release -cs)-proposed restricted main multiverse universe
EOF

# apt update && apt dist-upgrade -y

# mkdir reproducer
# cd reproducer
# mkdir certuser keyonlyuser
# cd keyonlyuser

# ssh-keygen -t ed25519 -f key
Generating public/private ed25519 key pair.
Enter passphrase (empty for no passphrase):
Enter same passphrase again:
Your identification has been saved in key
Your public key has been saved in key.pub
...

# cd ../certuser/

# ssh-keygen -t rsa -f ca
Generating public/private rsa key pair.
Enter passphrase (empty for no passphrase):
Enter same passphrase again:
Your identification has been saved in ca
Your public key has been saved in ca.pub

# ssh-keygen -t ed25519 -f key
Generating public/private ed25519 key pair.
Enter passphrase (empty for no passphrase):
Enter same passphrase again:
Your identification has been saved in key
Your public key has been saved in key.pub

# ssh-keygen -s ca -I key_id -n certuser key.pub
Signed user key key-cert.pub: id "key_id" serial 0 for certuser valid forever

# cd ..
# cat <<EOF >authorized_principals
#!/bin/sh
if [ "$1" = "otheruser" ]; then
echo certuser
fi
EOF

# chmod 755 authorized_principals

# adduser --disabled-password otheruser
Adding user `otheruser' ...
Adding new group `otheruser' (1001) ...
Adding new user `otheruser' (1001) with group `otheruser' ...
Creating home directory `/home/otheruser' ...
Copying files from `/etc/skel' ...
Changing the user information for otheruser
Enter the new value, or press ENTER for the default
 Full Name []:
 Room Number []:
 Work Phone []:
 Home Phone []:
 Other []:
Is the information correct? [Y/n] y

# adduser --disabled-password keyonlyuser
Adding user `keyonlyuser' ...
Adding new group `keyonlyuser' (1002) ...
Adding new user `keyonlyuser' (1002) with group `keyonlyuser' ...
Creating home directory `/home/keyonlyuser' ...
Copying files from `/etc/skel' ...
Changing the user information for keyonlyuser
Enter the new value, or press ENTER for the default
 Full Name []:
 Room Number []:
 Work Phone []:
 Home Phone []:
 Other []:
Is the information correct? [Y/n] y

# cat <<EOF >authorized_keys
#!/bin/sh
if [ "$1" = "keyonlyuser" ]; then
echo "ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIIVm2xIPfP0Nm2blQVErar4K9iOFUVFSpVxyZhQIkTl4 root@test-openssh-jammy"
fi
EOF

# chmod 755 authorized_keys

# vi /etc/ssh/sshd_config
i
AuthorizedKeysCommand /root/reproducer/authorized_keys %u
AuthorizedKeysCommandUser root

AuthorizedPrincipalsCommand /root/reproducer/authorized_principals %u
AuthorizedPrincipalsCommandUser root

TrustedUserCAKeys /root/reproducer/certuser/ca.pub
[esc]
:wq

# systemctl restart ssh

# ssh keyonlyuser@localhost -i /root/reproducer/keyonlyuser/key
The authenticity of host 'localhost (127.0.0.1)' can't be established.
ED25519 key fingerprint is SHA256:4NFSX3Jx8X4+jOzo2xDnMcJy5lGJvTVnOupp9UAQ3J4.
This key is not known by any other names
Are you sure you want to continue connecting (yes/no/[fingerprint])? yes
Warning: Permanently added 'localhost' (ED25519) to the list of known hosts.
Welcome to Ubuntu 22.04.3 LTS (GNU/Linux 5.15.0-83-generic x86_64)
...

$ exit

# ssh otheruser@localhost -i /root/reproducer/certuser/key -o CertificateFile=/root/reproducer/certuser/key-cert.pub
Welcome to Ubuntu 22.04.3 LTS (GNU/Linux 5.15.0-83-generic x86_64)