Encapsulating Security Payload

The ESP provides confidentiality over what the ESP encapsulates, as well as the services that AH provides. However, ESP only provides its protections over the part of the datagram that ESP encapsulates. ESP's authentication services are optional. These services enable you to use ESP and AH together on the same datagram without redundancy. Because ESP uses encryption-enabling technology, ESP must conform to U.S. export control laws.

ESP encapsulates its data, so ESP only protects the data that follows its beginning in the datagram. In a TCP packet, ESP encapsulates only the TCP header and its data. If the packet is an IP-in-IP datagram, ESP protects the inner IP datagram. Per-socket policy allows self-encapsulation, so ESP can encapsulate IP options when ESP needs to. Unlike the authentication header (AH), ESP allows multiple kinds of datagram protection. Using only a single form of datagram protection can make the datagram vulnerable. For example, if you use ESP to provide confidentiality only, the datagram is still vulnerable to replay attacks and cut-and-paste attacks. Similarly, if ESP protects only integrity, ESP could provide weaker protection than AH. The datagram would be vulnerable to eavesdropping.

Algorithms and the ESP Module

IPsec ESP implements ESP as a module that is automatically pushed on top of IP. The /dev/ipsecesp entry tunes ESP with ndd(1M). ESP allows encryption algorithms to be pushed on top of ESP, in addition to the authentication algorithms that are used in AH. Encryption algorithms include Data Encryption Standard (DES), Triple-DES (3DES), Blowfish, and AES. Each encryption algorithm has its own key size and key format properties. Because of export laws in the United States and import laws in other countries, not all encryption algorithms are available outside of the United States.

Security Considerations for ESP

An ESP without authentication is vulnerable to cut-and-paste cryptographic attacks and to replay attacks. When you use ESP without confidentiality, ESP is as vulnerable to eavesdropping as AH is.