strongSwan is a complete IPsec and IKEv1 implementation for Linux 2.4 and 2.6 kernels. It interoperates with most other IPsec-based VPN products. It is a descendant of the discontinued FreeS/WAN project. The focus of the strongSwan project is on strong authentication mechanisms using X.509 public key certificates and optional secure storage of private keys on smartcards through a standardized PKCS#11 interface. A unique feature is the use of X.509 attribute certificates to implement advanced access control schemes based on group memberships.