View source for 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 [https://sourceforge.net/projects/xca/ 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:  URI:http://pki.yourdomain.com/your.crl (comma separated for multiple URIs)
** 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).
* 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 creating the Root 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.
# Perform a minimal GUI install.
# Use strong passwords
# Don't install SSH. Remove it if OpenSSH-Server is installed by default.
# Disable Networking after OS and XCA installation (optionally removing network kernel modules/drivers so it can't be easily re-enabled).
# Create an encrypted volume to store your Keys and the XCA database.
# 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/





[[Category:Active_Directory_Certificate_Services]]
[[Category:Linux]]