IPsec Protection Mechanisms

IPsec provides two security protocols for protecting data:

Authentication Header (AH)
Encapsulating Security Payload (ESP)

An AH protects data with an authentication algorithm. An ESP protects data with an encryption algorithm. Optionally, an ESP protects data with an authentication algorithm. Each implementation of an algorithm is called a mechanism.

Authentication Header

The authentication header provides data authentication, strong integrity, and replay protection to IP datagrams. AH protects the greater part of the IP datagram. As the following illustration shows, AH is inserted between the IP header and the transport header.

Diagram shows the AH header between the IP header and the TCP header.

The transport header can be TCP, UDP, SCTP, or ICMP. If a tunnel is being used, the transport header can be another IP header.

Encapsulating Security Payload

The encapsulating security payload (ESP) module provides confidentiality over what the ESP encapsulates. ESP also provides the services that AH provides. However, ESP only provides its protections over the part of the datagram that ESP encapsulates. ESP provides optional authentication services to ensure the integrity of the protected packet. Because ESP uses encryption-enabling technology, a system that provides ESP can be subject to import and export control laws.

ESP encapsulates its data, so ESP only protects the data that follows its beginning in the datagram, as shown in the following illustration.

Diagram shows the ESP header between the IP header and the TCP header. The TCP header is encrypted by the ESP header.

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.

If self-encapsulation is set, a copy of the IP header is made to construct an IP-in-IP datagram. For example, when self-encapsulation is not set on a TCP socket, the datagram is sent in the following format:

[ IP(a -> b) options + TCP + data ]

When self-encapsulation is set on that TCP socket, the datagram is sent in the following format:

[ IP(a -> b) + ESP [ IP(a -> b) options + TCP + data ] ]

For further discussion, see Transport and Tunnel Modes in IPsec.

Security Considerations When Using AH and ESP

The following table compares the protections that are provided by AH and ESP.

Table 18-2 Protections Provided by AH and ESP in IPsec

Protocol	Packet Coverage	Protection	Against Attacks
AH	Protects packet from the IP header to the transport header	Provides strong integrity, data authentication: Ensures that the receiver receives exactly what the sender sent Is susceptible to replay attacks when an AH does not enable replay protection	Replay, cut-and-paste
ESP	Protects packet following the beginning of ESP in the datagram.	With encryption option, encrypts the IP datagram. Ensures confidentiality	Eavesdropping
		With authentication option, provides the same protection as AH	Replay, cut-and-paste
		With both options, provides strong integrity, data authentication, and confidentiality	Replay, cut-and-paste, eavesdropping

Authentication and Encryption Algorithms in IPsec

IPsec security protocols use two types of algorithms, authentication and encryption. The AH module uses authentication algorithms. The ESP module can use encryption as well as authentication algorithms. You can obtain a list of the algorithms on your system and their properties by using the ipsecalgs command. For more information, see the ipsecalgs(1M) man page. You can also use the functions that are described in the getipsecalgbyname(3NSL) man page to retrieve the properties of algorithms.

IPsec on a Solaris system uses the Solaris cryptographic framework to access the algorithms. The framework provides a central repository for algorithms, in addition to other services. The framework enables IPsec to take advantage of high performance cryptographic hardware accelerators. The framework also provides resource control features. For example, the framework enables you to limit the amount of CPU time spent in cryptographic operations in the kernel.

For more information, see the following:

Authentication Algorithms in IPsec

Authentication algorithms produce an integrity checksum value or digest that is based on the data and a key. The AH module uses authentication algorithms. The ESP module can use authentication algorithms as well.

Encryption Algorithms in IPsec

Encryption algorithms encrypt data with a key. The ESP module in IPsec uses encryption algorithms. The algorithms operate on data in units of a block size.

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