IPSec

Internet Protocol Security (IPSEC) is a standard architecture for setting up a secure channel between two entities.The entities can be anything like two system, two routers, two gateways or any other combination. IPsec uses Public Key Encryption to provide confidentiality, authentication, Integrity and Non-Repudiation. 

IPsec relies of security association and there are two main associations:
1) Authentication Header (AH) - Provides assurance of Message Integrity and Non Repudiation.
2) Encapsulation Security Payload (ESP) - Provides confidentiality and Integrity of the packet content. Provides encryption and prevent replay attacks (Captures the packet and replay it later). 
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Primary use for the IPsec is for the VPN. IPsec can operation in Two Modes
1) Tunnel Mode - Message + Header is encrypted
2) Transport - Only message is encrypted



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