OpenSSH with X509 certificates HOW TO

In this post I will explain how to test a connection with OpenSSH using PKIXSSH fork from Roumen Petrov.

The full process followed to test a SSH connection between a client and a server machine using X509 certificates will be detailed. To make the test we will use a third machine, that we will call control machine, machine that will act as a "Certification Authority", which is the entity that will validate the authenticity of the certificates presented by the user who wants to make a connection and by the destination server.

We will use a custom compiled version of PKIXSSH, as our client demands. So the directories mentioned here will not be the standard.

To understand how it works I have read the following documents:

In a quick summary, and if I have correctly understood, this is how it works

  • X509 is a standard to sign public keys. Signed public keys are considered valid if the Certification Authority is known.
  • We can sign public keys for hosts and users
  • With X509 certificates we can sign in a OpenSSH server without using passwords and without using the traditional OpenSSH private-public key authentication. This means that no public keys must be distributed.

We are going to make two tests

  • Test the connection for an user from the client machine to the server using a X509 certificate
  • In a second step add authentication for the server host

The next steps must be followed

  • In the control machine:

    • Creation of CA Certificate
    • Deploy of CA Certificate in certificate signers directory of OpenSSH server and client machines
  • In the client machine:

    • Creation of keys and certificate for the user in the client machine
  • In the server machine:

    • Configuration of the server to accept X509 certificates for the user
    • Creation of a X09 certificate for the host
  • Again in the client machine:

    • Configuration of the client to accept X509 certificates from the server

So a bidirectional authentication will be made: the user is going to be verified by the server, and the server host is going to be verified by the client.

Creation of CA Certificate

In the control server we run the following commands:

  • The first one creates a key for the CA
  • Then we create Certificate Signature Request for this key
  • And then we create a self-signed certificate, valid for 10 years, for this key
openssl genrsa -des3 -out ca.key 2048
openssl req -new -key ca.key -out ca.csr
openssl x509 -req -days 3650 -in ca.csr -signkey ca.key -out ca.crt

Some info is requested. The important is the "Common Name". Set as the server's hostname.

The results files are:

  • ca.key: private key for this "fake" certification authority
  • ca.crt: certificate we will use to sign user and host certificates

Deploy of CA Certificate in client and server machines

Then we send the CA certificate to the OpenSSH on server and client machines, under the path signaled in CACertificatePath directive of OpenSSH configuration file sshd_config.

In the control machine:

scp ca.crt server:/opt/openssh/data/ca/crt/
scp ca.crt client:/opt/openssh/data/ca/crt/

We should also create a link with the form [HASH].[NUMBER]. The hash can be obtained with the command:

openssl x509 -in ca.crt -noout -hash

Then, in the server and client machines, we add the link with:

cd /opt/openssh/data/ca/crt/
ln -s ca.crt `openssl x509 -in ca.crt -noout -hash`.0

So, this CA will be recognized as a valid authority and the certificates signed by it seen as valid.

It is needed in both sides, server and client, as the user certificate will be verified by the server, an the server host will be verified by the client before opening a SSH session.

If the CA certificate is not saved in ca/crt directory the following warning will appear (in verbose mode)

cannot build certificate chain, code=20, msg='unable to get local issuer certificate'

Creation of keys and certificate for the user in the client machine

First, in the client machine:

  • create the private key for the user
ssh-keygen -t rsa -b 2048 -f .ssh/id_rsa -N ""
  • generate a signing request and send it to the control server to be signed
openssl req -new -key .ssh/id_rsa -out id_rsa.csr
scp id_rsa.csr control:/tmp

Now, in the control server, where the CA files are stored:

  • create a matching signed certificate for the user's private key
cd /tmp
openssl x509 -req -in id_rsa.csr -out id_rsa.crt -CA ca.crt -CAkey ca.key -CAcreateserial

The result file, id_rsa.crt is what we want

Again, in the client,

  • add the generated certificate to the client SSH private key and create also the public key
scp control:/tmp/id_rsa.crt .
cat id_rsa.crt >> .ssh/id_rsa
ssh-keygen -y -f .ssh/id_rsa > .ssh/

Configuring the server to accept X509 certificates for the user

Now came the really interesting part.

We don't need to copy the public key on server's SSH configuration for the user. Just add the "subject" information of x509 certificate to authorized_keys in destination server.

To get the subject run in the client

openssl x509 -noout -subject -in .ssh/id_rsa

On the server, add this line with the prefix x509v3-sign-rsa subject= to the server's .ssh/authorized_keys.
This line will have a content similar to this one:

x509v3-sign-rsa subject= /C=ES/ST=Pontevedra/L=Vigo/CN=testssh/

As we can see, the authentication is really made trusting the CA for any valid x509 certificate from the user. We could generate a new certificate and it will be accepted with no intervention on server side.

Test the connection

Now we should be able to connect from client to server without a password. For example, to list the /home directory on server we could use

ssh server ls -l /home

If we run in with option -v for verbose mode we could see an info line like this one, telling that x509 certificates are being used:

Offering public key: 'x509v3-ssh-rsa' /users/login/testssh/.ssh/id_rsa RSA+cert SHA256:+uPwjPPsVBZrdsKTSvpPthpafa8re4qLaEj8YWqxpIE

And it should work 🙂

Creation of certificate for the host in the server machine

The first time we try to connect to an OpenSSH server, the public key of the destination host is added to the client's known_hosts file. The user must accept it interactively of use the option "StrictHostKeyChecking no" to don't check remote host identity.

It will be more interesting if the server's identity could be verified by a external certification authority. With OpenSSH we can configure it the same way we have done with the user.

So, first in the server machine

  • generate a signing request for the host rsa key and send it to the control server to be signed.

The host RSA key is already present, we don't have to create it, as the OpenSSH daemon generates one when it's installed.

As "Common Name" we will use the host name with the domain

openssl req -new -key /opt/openssh/data/ssh_host_rsa_key -out ssh_host_rsa_key.csr
scp ssh_host_rsa_key.csr control:/tmp

Now, in the control server, where the CA files are stored, we create a signed certificate for this key

  • create a matching signed certificate for the host's private key
cd /tmp
openssl x509 -req -in ssh_host_rsa_key.csr -out ssh_host_rsa_key.crt -CA ca.crt -CAkey ca.key

The result file, ssh_host_rsa_key.crt is what we want

Again, in the server,

  • add the generated certificate to the server SSH private key and create also the public key
scp control:/tmp/ssh_host_rsa_key.crt .
cat ssh_host_rsa_key.crt >> /opt/openssh/data/ssh_host_rsa_key
ssh-keygen -y -f /opt/openssh/data/ssh_host_rsa_key > /opt/openssh/data/

Test the connection to the server

Now, in the client machine, we can delete the known_hosts file and try to make a connection to the server.

We will have a message similar to this one:

$ ssh server ls -l /home
The authenticity of host 'server (' can't be established.
RSA+cert key fingerprint is SHA256:u/tkQ1WQUqptnj8Fbyn1nHOPU+HgchxJ9a13MQKq7FQ.
Distinguished name is 'C=ES,ST=Pontevedra,L=Vigo,O=Internet Widgits Pty Ltd,,'.
Are you sure you want to continue connecting (yes/no/[fingerprint])?

After telling "yes", we will have the following line in known_hosts

server, x509v3-sign-rsa Subject:C=ES,ST=Pontevedra,L=Vigo,O=Internet Widgits Pty Ltd,,

Once again, no public key is added to the file. With the host name, ip and certificate description OpenSSH has enough. Is the X509 certificate presented by the server which is used to validate the host as as legitimate one.

We could verify that the remote host X509 certificate is being used connecting with very verbose level information set

$ ssh server -vvv ls -l /home

and look for the lines similar to this:

debug1: Server host key: x509v3-sign-rsa SHA256:u/tkQ1WQUqptnj8Fbyn1nHOPU+HgchxJ9a13MQKq7FQ
debug1: Host 'server' is known and matches the RSA+cert host key.
debug1: Found key in /users/login/testssh/.ssh/known_hosts:1

Done 🙂