This post takes a look at using Hashicorp's Vault to manage secrets for SSH authentication.

For this post, I started working with Vault pretty quickly via this docker-compose setup I found via GitHub. It's a very quick way to get a Vault instance with a Consul backend. You'd never do this for production, since they are single instances, but for functional testing, it's enough.

Requirements

What are we trying to accomplish?

Let's verify the ability to store, retrieve, generate and revoke SSH keys.

Setup, Unseal, and Use Vault

Refer to this github repo for bootstrapping the containers, and unsealing the vault as well as environment set up, and creating/accessing a credential, and creating a one-time token, and using it for access to a key (secret) stored in Vault.

SSH Authentication

Configuring Vault and a Target Host

We'll use the guide at https://www.vaultproject.io/docs/secrets/ssh/signed-ssh-certificates.html for reference.

We start by mounting the backend:

vault mount -path=ssh-client-signer ssh
Successfully mounted 'ssh' at 'ssh-client-signer'!

Then we create a CA. You can also use an internal CA, but we'll use an auto_generated one (generate_signing_key=true) for demo purposes:

vault write ssh-client-signer/config/ca generate_signing_key=true

Next we create the trusted keys pem file on the target host (I am using my localhost) so that certificates issued by our CA can be verified:

vault read -field=public_key ssh-client-signer/config/ca > /etc/ssh/trusted-user-ca-keys.pem

Now we update the sshd_config on our target host to accept certificates trusted by this CA (again, I am using localhost for this test):

# /etc/ssh/sshd_config
# ...
TrustedUserCAKeys /etc/ssh/trusted-user-ca-keys.pem

Now we create a role in Vault for signing SSH keys, including an extension to allow pseudo ttys and with a short TTL for demo purposes:

vault write ssh-client-signer/roles/my-role -<<"EOH"
{
  "allow_user_certificates": true,
  "allowed_users": "*",
  "default_extensions": [
    {
      "permit-pty": ""
    }
  ],
  "key_type": "ca",
  "default_user": "csuttles",
  "ttl": "2m0s"
}
EOH

Configuring the Client

First we add our SSH key to Vault:

vault write ssh-client-signer/sign/my-role \
    public_key=@$HOME/.ssh/id_rsa.pub

Now we save the signed public key to disk:

vault write -field=signed_key ssh-client-signer/sign/my-role \
    public_key=@$HOME/.ssh/id_rsa.pub > signed-cert.pub

If we inspect the extensions and metadata of the signed key, we can verify the parameters we specified, like the short TTL:

csuttles@csuttles-Mac:[~]: ssh-keygen -Lf ~/.ssh/signed-cert.pub
/Users/csuttles/.ssh/signed-cert.pub:
        Type: ssh-rsa-cert-v01@openssh.com user certificate
        Public key: RSA-CERT SHA256:1d2oFnwq0s6F+QRHr6DudpsVwfxeI9rX5UOaC6y+OLc
        Signing CA: RSA SHA256:mrYCyfzQD1sv7Wn4Afj7sbwuOT4edV0to7NJKdxRUlw
        Key ID: "vault-root-d5dda8167c2ad2ce85f90447afa0ee769b15c1fc5e23dad7e5439a0bacbe38b7"
        Serial: 15839214298381429970
        Valid: from 2018-01-02T14:15:49 to 2018-01-02T14:18:19
        Principals:
                csuttles
        Critical Options: (none)
        Extensions:
                permit-pty


We can authenticate to test. Here I am using the signed key, within the expiry we see from the previous command to authenticate via ssh to localhost and run the date command:

csuttles@csuttles-Mac:[~]: ssh -i ~/.ssh/signed-cert.pub -i ~/.ssh/id_rsa localhost date
Tue Jan  2 14:18:18 PST 2018
[Exit: 0] 14:18

Once the TTL expires, it is no longer a sufficient authentication method, and I am prompted for my password:

csuttles@csuttles-Mac:[~]: ssh -i ~/.ssh/signed-cert.pub -i ~/.ssh/id_rsa localhost date
Password:

[Exit: 130] 14:20
csuttles@csuttles-Mac:[~]:

After our TTL has expired, we can simply refresh the signed public key and validate that things work again (until our new TTL expires):

csuttles@csuttles-Mac:[~]: vault write -field=signed_key ssh-client-signer/sign/my-role     public_key="$(cat $HOME/.ssh/id_rsa.pub)" > ~/.ssh/signed-cert.pub
[Exit: 0] 14:21
csuttles@csuttles-Mac:[~]: ssh -i ~/.ssh/signed-cert.pub -i ~/.ssh/id_rsa localhost date
Tue Jan  2 14:21:52 PST 2018
[Exit: 0] 14:21

Revoking Credentials

I looked around for information on revoking the certificates used with SSH and Vault, and I don't think it is supported at this time, since I was unable to find docs for how to do so, and this feature request asking for a CRL to be published when using the SSH backend.

The current best practice (at the time of writing) seems to be not revoking the certificates at all:

We generally recommend making certificate lifetimes useful for only a single connection. As a result CRLs are unnecessary. This workflow can be made easier using vault ssh.

Since we are not using a CRL, the small TTL of two minutes might not be a bad choice, if the intention is to make the certificate good only for a single connection. This means any certificate we issue using the TTL of 2 minutes would have a max "revocation time" of 2 minutes, and quite likely less. The automatic expiration within 2 minutes seems like it would be faster or equal in speed to manual recovation in most circumstances.