Basic Introduction to X.509 Public Key Infrastructure¶
One authentication option for VPNs is to use X.509 keys. An in depth discussion of X.509 and Public Key Infrastructure (PKI) is outside the scope of this book, and is the topic of a number of entire books for those interested in details. This chapter provides the very basic understanding necessary for creating and managing certificates in pfSense® software.
With PKI, first a Certificate Authority (CA) is created. This CA then signs all of the individual certificates in the PKI. The certificate of the CA is used on VPN servers and clients to verify the authenticity of server and client certificates used. The certificate for the CA can be used to verify signing on certificates, but not to sign certificates. Signing certificates requires the private key for the CA. The secrecy of the CA private key is what ensures the security of a PKI. Anyone with access to the CA private key can generate certificates to be used on a PKI, hence it must be kept secure. This key is never distributed to clients or servers.
Never copy more files to clients than are needed, as this may compromise the security of the PKI.
A certificate is considered valid if it has been trusted by a given CA. In this case of VPNs, this means that a certificate made from a specific CA would be considered valid for any VPN using that CA. For that reason the best practice is to create a unique CA for each VPN that has a different level of security. For instance, if there are two mobile access VPNs with the same security access, using the same CA for those VPNs is OK. However if one VPN is for users and another VPN is for remote management, each with different restrictions, then a unique CA for each VPN should be used.
Certificate Revocation Lists (CRLs) are lists of certificates that have been compromised or otherwise need to be invalidated. Revoking a certificate will cause it to be considered untrusted so long as the application using the CA also uses a CRL. CRLs are generated and signed against a CA using its private key, so in order to create or add certificates to a CRL in the GUI the private key for a CA must be present.