Which of the following services are provided by encapsulating security payload?

The encapsulating security payload [ESP] protocol provides confidentiality over what the ESP encapsulates. ESP also provides the services that AH provides. However, ESP does not protect the outer IP header. ESP provides 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.

The ESP header and trailer encapsulate the IP payload. When encryption is used with ESP, it is applied only over the IP payload data, as shown in the following illustration.

In a TCP packet, the ESP header is authenticated and it encapsulates the TCP header and its data. If the packet is an IP-in-IP packet, ESP protects the inner IP packet. Per-socket policy allows self-encapsulation, so ESP can encapsulate IP options when necessary.

Self-encapsulation can be used by writing a program that uses the setsockopt[]. If self-encapsulation is set, a copy of the IP header is made to construct an IP-in-IP packet. For example, when self-encapsulation is not set on a TCP socket, the packet is sent in the following format:

An Encapsulating Security Payload is primarily designed to provide encryption, authentication and protection services for the data or payload that is being transferred in an IP network. ESP doesn’t protect the packet header; however, in a tunnel mode if the entire packet is encapsulated within another packet as a payload/data packet, it can encrypt the entire packet residing inside another packet. Typically, in an IP network packet, the ESP header is placed after the IP header. The components of an ESP header include sequence number, payload data, padding, next header, an integrity check and sequenced numbers.

Encapsulating Security Payload [ESP] provides all encryption services in IPSec based on integrity for the payload and not for the IP header, confidentiality and authentication that using encryption, without authentication is strongly discouraged because it is insecure.

Any translations in readable message format into an unreadable format are encrypted and used to hide the message content against data tampering.

IPSec provides an open framework, such as SHA and MD5 for implementing industry standard algorithms.

Encryption/decryption allows only the sender and the authorised receiver to make the data to be received in readable form and only after the integrity verification process is complete, the data payload in the packet is decrypted.

IPSec uses a unique identifier for each packet, which is a data equivalent of a fingerprint and checks for packets that are authorised or not. It doesn't sign the entire packet unless it is being tunnelled—ordinarily, for this IP data payload is protected, not the IP header. In Tunnel Mode, where the entire original IP packet is encapsulated with a new packet header added.

ESP in transport mode does not provide integrity and authentication for the entire IP packet.

ESP Format

The ESP format is diagrammatically represented as follows −

Explanation

Security Parameters Index [32 bits] − Identifies a security association. This field is mandatory. The value of zero is reserved for local, implementation- specific use and MUST NOT be sent on the wire.

Sequence Number [32 bits] − A monotonically increasing counter value; this provides an anti-replay function, as discussed for AH. The first packet sent using a given SA will have a Sequence number of 1.

Payload Data [variable] − This is a transport-level segment [transport mode] or IP packet [tunnel mode] that is protected by encryption. The type of content that was protected is indicated by the Next Header field.

Padding [0-255 bytes] − Padding for encryption, to extend the payload data to a size that fits the encryption's cipher block size, and to align the next field.

Pad Length [8 bits] − Indicates the number of pad bytes immediately preceding this field.

Next Header [8 bits] − Identifies the type of data contained in the payload data field by identifying the first header in that payload.

Authentication Data [variable] − A variable-length field [must be an integral number of 32-bit words] that contains the Integrity. Check Value computed over the ESP packet minus the Authentication Data field. This field is optional and is included only if the authentication service has been selected for the SA in question.

Encapsulating Security Payload [ESP]

Security Encyclopedia

Encapsulating Security Payload [ESP]

Encapsulating Security Payload [ESP] is a member of the Internet Protocol Security [IPsec] set of protocols that encrypt and authenticate the packets of data between computers using a Virtual Private Network [VPN]. The focus and layer on which ESP operates makes it possible for VPNs to function securely. 

The enhanced version of IPsec in use is an Internet-layer security protocol. It is pre-programmed for IP-layer application security whereas other protocols such as  Transport Layer Security [TLS] and Secure Shell [SSH] function on the application layer. 

Security Authentication Header [AH] is another IPsec member protocol. ESP and AH can operate between hosts and between networks. The can also operate in two modes: the less-secure Transport Mode that encrypts the data packet, for use between two workstations that are running a VPN client; and Tunnel Mode, which is more secure. Tunnel Mode encrypts the whole packet including header info and source, and is used between networks.

Example:

“Security for a VPN involves IPsec, and with IPsec’s protocols of AH and ESP, the connection between a user and a network is secure. Going further ESP, on the application layer, can run in its more secure Tunnel Mode offering the most privacy.”

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