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Enable SSH

For network engineers, this guide will help you to authenticate with your PIV/CAC and SSH to a Linux server from your Windows or macOS computer.  For server administrators, this guide will help you to configure a Linux server for remote access.

This guide uses open-source, smart-card software for Windows (PuTTY-CAC) and macOS (OpenSC). Commercial solutions are also available.

Your PIV/CAC contains an authentication certificate key pair (public and private) for smart card logon. Using a PIV/CAC key pair is very similar to using a self-signed key pair for SSH. The setup below is meant for PIV/CAC based authentication.

Your Chief Information Security Officer must determine that security controls are in place and approve SSH scenarios. You should also review your agency's policies and use your physical or virtual jump servers to restrict users from using SSH directly from workstations.

SSH from Windows

These steps use PuTTY-CAC v0.70u2, which supports Cryptographic API (CAPI) integration. Pageant software is not required.

  1. You’ll need to download PuTTY-CAC to C:\ssh\putty.exe, or a similar folder. Select the 32-bit or 64-bit executable, based on your Windows OS. You don’t need the MSI Installers.
  2. Double-click on putty.exe to launch PuTTY-CAC and insert your PIV/CAC into your smart card reader.
  3. At the PuTTY Configuration window side-bar, go to Connection > SSH > Certificate. Click Set CAPI Cert….
    PuTTY Configuration Window
  4. At the Windows Security window, select your certificate.
  5. If you don’t know which certificate to choose, view its properties. At the Certificate Details tab, click Enhanced Key Usage. Select the one whose value is Client Authentication or Smart Card Logon and click OK.
    PuTTY Certificate Display Details
  6. Back at the PuTTY Configuration window, you’ll see the certificate thumbprint. Click the Copy to Clipboard button and paste your certificate’s SSH key into a text file. The key will look like this:

    ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQCyPn2dShOF... CAPI:05bf4653b3098a87b67816d81049f489d5b5ffb4

  7. Provide the text file to the administrator to set up your account.
  8. At the PuTTY Configuration window, you’ll now see that the Attempt Certificate Authentication box is checked.
  9. You can create and save session profiles for each target server. Click Session and enter a remote server’s hostname or IP address. For Connection type, click SSH and 22 will appear under Port. Enter a session name in Saved Sessions and click Save.
  10. Once you have an account, select a Saved Session and click Load to load the server configuration. Click Open to connect to the Linux server. A dialog box will open and display the server’s key fingerprint as a hash value. Verify and accept the server key and enter your username.
  11. When the server detects your smart card authentication, it will prompt for your PIV/CAC PIN. Enter your PIN. Once it’s validated, you’ll be logged into the server via SSH.

The card reader may flash. Do not remove the PIV/CAC until the login process has been completed.

SSH from macOS

To enable PIV/CAC authentication for your macOS, you’ll need to install third-party software, such as OpenSC:

  1. Install OpenSC.
  2. Insert your PIV/CAC into your card reader.
  3. To view all of the certificates on your Mac, enter:

         pkcs15-tool --list-public-keys  
  4. Note the ID for the PIV AUTH pubkey RSA key:

     Using reader with a card: SCR35xx Smart Card Reader
     Public RSA Key [PIV AUTH pubkey]
         Object Flags   : [0x0]
         Usage          : [0xD1], encrypt, wrap, verify, verifyRecover
         Access Flags   : [0x2], extract
         ModLength      : 2048
         Key ref        : 154 (0x9A)
         Native         : yes
         ID             : 01
         DirectValue    : <absent>
     Public RSA Key [SIGN pubkey]
         Object Flags   : [0x0]
         Usage          : [0x2C1], encrypt, verify, verifyRecover, nonRepudiation
         Access Flags   : [0x2], extract
         ModLength      : 2048
         Key ref        : 156 (0x9C)
         Native         : yes
         ID             : 02
         DirectValue    : <absent>
  5. To view your public SSH key, enter:

     pkcs15-tool --read-ssh-key 01

    Note:  The 01 value is the ID from above. When prompted, enter your PIV/CAC PIN. The SSH key will look like this:

    ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQCyPn2dShOFLBnMraiP2MnLU .... PIV AUTH pubkey  
  6. Send the SSH key to the server administrator to set up your account.
  7. Once you have your account, you can log into the Linux server. Enter:

    ssh -I /usr/lib64/opensc-pkcs11.so <username>@<remote-host>
    Note:  If you don’t want to specify the opensc-pkcs11.so using -I, you can update the setting in the /etc/ssh_config file to:

         PKCS11Provider /usr/lib64/opensc-pkcs11.so&nbsp;
  8. The server will prompt for your PIV/CAC PIN. Enter your PIN. Once it’s validated, you’ll be logged into the server via SSH.

The card reader may flash. Do not remove the PIV until the login process has been completed.

Configure a Linux Server

Server administrators need to have root privileges for these steps.

These SSH configurations are examples only. Other configuration options are available, including Pluggable Authentication Modules (PAM) that look up user accounts and authorizations through directories. You can automate account set-ups by using centralized configuration management tools that can push or remove authorized_keys.

  1. By default, SSH keys are read from the .ssh/authorized_keys file in your home directory. You’ll need to create a /home/<username>/.ssh directory and change it to the requestor’s ownership. Then, create an authorized_keys file in the .ssh directory and copy the requestor’s SSH key to the /home/>user>/.ssh/authorized_keys file starting with ssh-rsa <public key> <key_name>:

         mkdir /home/<user>/.ssh
         chown <user> .ssh
         chgrp <user> .ssh
         chmod 700 .ssh
         cat > authorized_keys 
         ssh-rsa AAAAB3NzaC1yc2EAAAADAQA... CAPI:05bf4653b3098a87b67816d81049f489d5b5ffb4    
  2. Set the permissions for authorized_keys to 600 and change the authorized_keys ownership to the user:

          chmod 600 authorized_keys
          chown <user> authorized_keys
          chgrp <user> authorized_keys
  3. This step is only needed if you want to change the default SSH settings. You can change the location for the authorized_keys file in the /etc/ssh/sshd_config file and restart the sshd. You can also disable any alternative means of access (i.e., passwords), as needed:

        AuthorizedKeysFile /etc/ssh/authorized_keys/%u  
        PasswordAuthentication no

    Note:  If you change the default settings, you’ll need to create a corresponding directory for authorized_keys under /etc/ssh and place the authorized_keys there instead of in the user’s home folder.