IPsec (Internet Protocol Security)

IPsec (Internet Protocol Security) is a framework of open standards for ensuring private, secure communications over Internet Protocol (IP) networks through the use of cryptographic security services. It is designed to protect data flows between a pair of hosts (host-to-host), between a pair of security gateways (network-to-network), or between a security gateway and a host (network-to-host). IPsec supports network-level peer authentication, data origin authentication, data integrity, data confidentiality (encryption), and replay protection.

The IPsec suite is an essential tool for implementing virtual private networks (VPNs) and for securing internet communication. It operates at the network layer, allowing it to secure applications at the IP level, which means that it can secure nearly any application without modifications to the application itself.

IPsec includes protocols for establishing mutual authentication between agents at the beginning of a session and negotiation of cryptographic keys to be used during the session. The two main protocols involved in IPsec are:

1. IKE (Internet Key Exchange): IKE is used to set up a security association (SA) in the IPsec protocol suite. IKE builds upon the Oakley protocol and ISAKMP. IKE uses X.509 certificates for authentication ‒ either pre-shared or distributed using DNS (preferably with DNSSEC) and a Diffie–Hellman key exchange to set up a shared session secret from which cryptographic keys are derived.
2. ESP (Encapsulating Security Payload) and AH (Authentication Header): ESP provides confidentiality, data origin authentication, connectionless integrity, an anti-replay service (a form of partial sequence integrity), and limited traffic flow confidentiality. AH is designed to provide integrity and data origin authentication for IP datagrams and provides protection against replay attacks.

ESP can be used alone, in combination with AH, or in a nested mode, thereby providing various levels of security. The choice between AH and ESP, and the choice of which security services to use, is determined by the security policy in the IPsec implementation.

Because of its robust security mechanisms, IPsec is widely used in creating secure connections between networks (site-to-site VPNs), between remote users and an entire network (remote access VPNs), or securing data in transit between servers across insecure networks. IPsec's flexibility and strong security features make it a popular choice for protecting internet traffic in an array of industries and applications.