ADCS with Linux/BSD based Offline Root CA

Overview

General best practices suggest using an offline Root CA for security. This makes sense, but the challenge I always have it why waste a Windows Server license for a host that will be offline 99% of the time. This article describes how to use a free open source alternative for your Root CA. Keep in mind that only the Root CA will be Linux based, the Issuing/Subordinate Certificate Authorities will be running ADCS.

Linux/BSD

Linux is a free open source operating system (ya I know Linux is actually just the kernel). It provides a very robust and solid platform for this task. Case in point most Internet facing Certificate Authorities are likely running on Linux or a BSD derivative.

Distribution selection is entirely up to you, and most of what I put in this article will apply to BSD based OSes also. In this example I will use Debian, but FreeBSD is an equally capable.

XCA

I find Certificate Management a real pain, so I will be using XCA. XCA is a GUI frontend for certificate management. You can quickly create keys, self signed certs, and CRLs with just a few clicks of a button.

Installation

apt-get install xca

or

pkg install xca

Configuration

• Create a Key
  • Private Keys Tab -> 'New Key'
  • Name: YourDomain-RootCA (Something that makes sense)
  • Keytype: RSA (or your preferred encryption algorithm)
  • Keysize: 2048 (minimum)
• Create a self signed certificate
  • Certificates Tab -> 'New Certificate'
  • Select your desired 'Signature algorithm' = SHA256 (minimum)
  • Select the '[default] CA' template and click 'Apply all'
  • Subject Tab -> fill in all the fields and make sure the Private Key you created is selected at the bottom.
  • Extensions Tab
    • Type: Certificate Authority
    • Path length: 2 (or greater depending on how many levels of Subordinate CA's you plan to implement).
    • Set the Validity period to something reasonable: 10 to 20 years is typical.
    • CRL distribution point: Leave this blank
  • Key Usage Tab: 'Certificate Sign' and 'CRL Sign' should be highlighted (at minimum).
  • Click 'OK' to create the Root Certificate.

Export the Root Cert to a USB Drive, and add it to the Subordinate CA's Trusted Root Authorities Certificate Store.

Sign Subordinate CSRs

Setup ADCS on your designated Issuing CA hosts. Export CSR files to a USB Drive. Plug the USB drive into the Offline Root, then sign the CSR in XCA

• Certificate signing requests Tab
• Click Import, open the CSR on the USB Drive
• Highlight the imported CSR, Right click and select 'Sign'
  • Extensions Tab
  • Path length: 0 (or greater depending on how many down stream Issuing CAs you plan to have).
  • Validity: Set the Issuing CA time range to something reasonable like half the life of the Root CA (5yrs).
  • CRL distribution point: URI:http://pki.yourdomain.com/your.crl (comma separated for multiple URIs)
• Click 'OK'

Create CRLs

• Create the CRL
  • Certificates Tab
    • Right Click the Root Cert, click 'CA', click 'Generate CRL'
    • Specify the last and next updates, and set the validity period. 6mo ~ 1yr is probably reasonable. Keep in mind that this will determine how often you have to turn on your Offline CA and publish a new CRL.
    • Hashing algorithm: SHA256 (minimum)
    • Click 'OK'
  • Export the CRL
    • Revocation lists Tab
    • Select the CRL you just created and click 'Export'
    • Filename should match the CRL URI you used when signing the SubCA Cert. In this example it was your.crl
    • Export Format: PEM
    • Click 'OK'

Copy the CRL to the USB Drive, then copy the CRL to the web server you specified in the CDP URI (http://pki.yourdomain.com/your.crl). This needs to be reachable by the ADCS SubCA prior to installing the Signed SubCA Certificate. Shutdown the Offline CA and store it in a secure location. Optionally create an encrypted USB Drive and copy the XCA database to it and store it in a secure off-site location (gotta have backups).

Install the Subordinate CA Cert

Export the Subordinate Certs to the USB Drive and Import them into the ADCS SubCAs

• Certificate Authority MMC Snap-in
  • Select the CA Node, Right Click and select 'Install Certificate'

Security

Take precautions to secure your Root CA's operating system.

1. Perform a minimal GUI install. (GUI only because of XCA, I suggest installing just X11 and a lightweight Window Manager like OpenBox)
2. Use strong passwords
3. Don't install SSH. Remove it if OpenSSH-Server is installed by default.
4. Disable Networking after OS and XCA installation (optionally removing network kernel modules/drivers so it can't be easily re-enabled).
5. Create an encrypted volume to store your Keys and the XCA database.
6. Transfer CSR and CRL files using a USB drive.

Reference

http://www.mbse.eu/linux/homeserver/essential/certificatesxca/

http://xca.sourceforge.net/

https://sourceforge.net/projects/xca/