Monthly Archives: October 2016

Use a smart card or HSM to securely store your SSH CA keys

Depending on the use case, SSH CA keys can be extremely sensitive. Possession of the private key gives you the ability to sign new certificate for arbitrary usernames that will grant access to all machines where this CA is trusted. This is why you should keep your CA keys very safe, preferably offline and of course encrypted (password protected).

One big problem with digital encryption keys is that it is extremely difficult to find out if you lost control over they keys. If they where copied you have no way of finding out. This is where hardware tokens comes in. If you can be sure that the key will never leave the hardware token, you have a better chance of knowing when they keys are lost or stolen.

OpenSSH have support for storing keys onto hardware tokens that talk PKCS#11 such as smart cards and hardware security modules.

If you want to store your personal SSH key on a smart card you can read more about that here: Using Smart Card enabled yubikey for ssh authentication in FreeBSD

Since a SSH CA is just a regular SSH key they can also be stored on smart cards. But there is a few subtle differences in how you use it. First of all you need a token with a RSA key present. There is no option in OpenSSH to ask for a specific key stored on the token, instead we specifiy the public key that matches the private key we want to use. Lets download the public key from the token:

$ ssh-keygen -D /usr/local/lib/opensc-pkcs11.so
ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQCOvhEqFq9Ualp1iYiNs0JFs3MgvGU+By/VvyW4qkymW+w/MmHAaHl+/UFnE+kgXChdYHaGEVxxGi6dQlSq+1ZKAWPJsOEbkysK6cgjgvP21gVNjL62TlQz+QfGF82mv0hfSGXQrZQR7VDs+6xJOe3S/0i1HvnnRTdR2v9QSJzd2EWNLmUcPy7+4x4rEB11G0oPt+Xyx60WaleJctwJHHhJS/jqHdvuf7HO6MS/EQn2NTnwIjChlmm2kUT7obnev/r6uEwz87NubnYJUrYImRDMafjS9taq8l7y33ydT00QHEI76kmrSSi7hTfmxUgStQWuQ2mq10YEVd8kZ2sqmC3N

We put this into a file, let say ssh_ca.pub. Now we are ready to start using the token for certificate signing.

$ ssh-keygen -s ssh_ca.pub -D /usr/local/lib/opensc-pkcs11.so -I key_id wack.pub 
Enter PIN for 'Framkant HSM Test (UserPIN)': 
Signed user key wack-cert.pub: id "key_id" serial 0 valid forever

To look at the certificate we can use ssh-keygen.

% ssh-keygen -L -f wack-cert.pub 
wack-cert.pub:
        Type: ssh-rsa-cert-v01@openssh.com user certificate
        Public key: RSA-CERT SHA256:AtPyAu1DL5cFruTo9XnsVz7tdec7xF9SbpX8DzsQrbs
        Signing CA: RSA SHA256:Pbdx6TAvXvwZTKQVMRYWsWYPomw6AxBVoqbtXgy9pfs
        Key ID: "key_id"
        Serial: 0
        Valid: forever
        Principals: (none)
        Critical Options: (none)
        Extensions: 
                permit-X11-forwarding
                permit-agent-forwarding
                permit-port-forwarding
                permit-pty
                permit-user-rc

In a real life scenario I would recommend against this kind of certificate. Give it some validity and specify principals.

Two nice affordable options for hardware tokens are the Yubikey and the Smart Card HSM

Setting up a SSH Certificate Authority (CA)

Are you managing a couple of machines over ssh and have begun to feel frustrated about the key management? Find it tedious to distribute your public key to every machine you want to administer? Well, there is a simple solution, that does not include LDAP or some other central authentication server. A little known fact is that OpenSSH have support for both server and client certificates (not x509) since version 5.4. These can be used to set up a trusted Certificate authority on every server once and for all.

This is a very useful tool in environments where server access are harmonized (Where the same set of users should have access to all servers).

The other part is the signing of host keys. This solves the problem of having to manually check and verify the host key fingerprint every time you connect to a new server. If the server key is signed by a CA that you choose to trust you will not be asked to verify the fingerprint. If you have some configuration orchestration like Puppet you can have the puppetmaster create the host certificates on the fly.

From the release notes of OpenSSH 5.4:

* Add support for certificate authentication of users and hosts using a
   new, minimal OpenSSH certificate format (not X.509). Certificates
   contain a public key, identity information and some validity
   constraints and are signed with a standard SSH public key using
   ssh-keygen(1). CA keys may be marked as trusted in authorized_keys
   or via a TrustedUserCAKeys option in sshd_config(5) (for user
   authentication), or in known_hosts (for host authentication).

Creating CA keys
First of all I strongly recommend that you create separate CA keys for hosts and users for security reasons.

The CA keys are just regular ssh keys, you can create them like this:

$ ssh-keygen -a 256 -o -t rsa -b 4096 -f user_ca
Generating public/private rsa key pair.
Enter passphrase (empty for no passphrase): 
Enter same passphrase again: 
Your identification has been saved in user_ca.
Your public key has been saved in user_ca.pub.

$ ssh-keygen -a 256 -o -t rsa -b 4096 -f host_ca
Generating public/private rsa key pair.
Enter passphrase (empty for no passphrase): 
Enter same passphrase again: 
Your identification has been saved in host_ca.
Your public key has been saved in host_ca.pub.

Securing the private key
Since we are in the process of essentially giving one ssh key access to all of our severs we want keep the private key used for certificate signing very secure. The absolute minimum is to have the key encrypted (password protected). But other than that you should also enable KDF which you can read more about here. I also recommend that you store your keys offline and only have them mounted to a system when you use them. One way could be a encrypted partition on a USB drive, but have two of them since USB drives have a tendency to fail.

Another way to secure your keys is to have them stored on a hardware token, you can read about that here.

Singning your ssh key
First of all you need a regular ssh key which you create with ssh-keygen. Then we can sign this key and create a certificate. This is also done with ssh-keygen:

$ ssh-keygen -s /path/to/ca_key -n peter -z 1234 -V +52w1d-I key_id /path/to/user_key.pub
Signed user key user_key-cert.pub: id "peter_cert" serial 1234 for peter valid from 2016-10-09T22:23:00 to 2017-10-09T22:24:57

The file user_key-cert.pub can now be used together with your private key to access machines that accept the user_ca.pub.

To look at the newly created certificate run:

% ssh-keygen -L -f user_key-cert.pub 
wack-cert.pub:
        Type: ssh-rsa-cert-v01@openssh.com user certificate
        Public key: RSA-CERT SHA256:AtPyAu1DL5cFruTo9XnsVz7tdec7xF9SbpX8DzsQrbs
        Signing CA: RSA SHA256:8PYQAJojSknTl3BqgBkFKigmaufDhL/7d8zYUNFm7Po
        Key ID: "peter_cert"
        Serial: 1234
        Valid: from 2016-10-09T22:23:00 to 2017-10-09T22:24:57
        Principals: 
                peter
        Critical Options: (none)
        Extensions: 
                permit-X11-forwarding
                permit-agent-forwarding
                permit-port-forwarding
                permit-pty
                permit-user-rc

Singning host keys
The signing of host keys is done exactly as above, the only difference is that you add -h and the value given to -n should be the hostname.

Configure sshd
To tell sshd to accept key signed with your newly created ca you just need to upload the user_ca.pub and add one line to sshd_config. To tell the server to provide a host certificate to the client you need to add one line per key type to the configuration.

TrustedUserCAKeys /etc/ssh/user_ca.pub
HostCertificate /etc/ssh/ssh_host_rsa_key-cert.pub
HostCertificate /etc/ssh/ssh_host_ecdsa_key-cert.pub
HostCertificate /etc/ssh/ssh_host_ed25519_key-cert.pub

and restart sshd.

Configure the client to accept host certificates
To accept host certificates signed by the host_ca you need to add a line to your known_hosts file.
Its just @cert-authority * < public key >

It should look something like this:

@cert-authority * ssh-rsa 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

Revoking Certificates
This will be added later.