# Limitations of certificates in VPNs

Certificates in VPNs have these limitations.

- All gateways in the same VPN must support the same CA algorithm. Otherwise, VPN communication fails. For example, if you use an Internal ECDSA CA for Gateways as the default CA, all other gateways used in the same VPN must support ECDSA.
- Certificates created for VPN gateways for establishing the VPN are stored on the VPN gateway devices (Engines). These certificates are not included in the Management Server backup, and are not changed in any way when a Management Server backup is restored.
- Certificates can become unusable if the private key for that certificate is lost. The key can be lost, for example, if the Security Engine hardware fails and must be replaced. Engine Clusters share each VPN certificate and can synchronize the private key from node-to-node as needed. If the private key is erased from a Single Engine or from all the nodes of a Engine Cluster, a new certificate must be created.
- Externally issued VPN certificates can be revoked by the certificate authority that issued them. This safety measure is used when the certificate is suspected to be compromised.
- A single VPN Gateway can use only one type of certificate (for example, RSA or ECDSA). However, if different VPNs require different certificate types, then different VPN Gateway elements must be created for those VPNs.