Authentication Header (Ah); Authentication Algorithms - Alcatel-Lucent OmniSwitch 6850-48 Network Configuration Manual

IPsec Overview

Authentication Header (AH)

An Authentication Header (AH) provides connectionless integrity and data origin authentication. This
protocol permits communicating parties to verify that data was not modified in transit and that it was genu-
inely transmitted from the apparent source. AH helps verify the authenticity/integrity of the content and
origin of a packet. It can optionally protect against replay attacks by using the sliding window technique
and discarding old packets. It authenticates the packet by calculating the checksum via hash-based
message authentication code (HMAC) using a secret key and either HMAC-MD-5 or HMAC-SHA1 hash
functions.

Authentication Algorithms

•
HMAC-MD5 - An algorithm that produces a 128-bit hash (also called a digital signature or message
digest) from a message of arbitrary length and a 16-byte key. The resulting hash is used, like a finger-
print of the input, to verify content and source authenticity and integrity.
•
HMAC-SHA1 - An algorithm that produces a 160-bit hash from a message of arbitrary length and a
20-byte key. It is generally regarded as more secure than MD5 because of the larger hashes it produces.
•
AES-XCBC-MAC-96 - An algorithm that uses AES [AES] in CBC mode [MODES] with a set of
extensions [XCBC-MAC-1] to overcome the limitations of the classic CBC-MAC algorithm. It uses
the AES block cipher with an increased block size and key length (128 bits) which enables it to with-
stand continuing advances in crypto-analytic techniques and computational capability. Its goal is to
ensure that the datagram is authentic and cannot be modified in transit.
Unlike ESP, AH does not encrypt the data. Therefore, it has a much simpler header than ESP. The figure
below shows an AH-protected IP packet.
Next Header(8 bits)
AH is identified by a value of 51 in the IP header. The Next header field indicates the value of the upper
layer protocol being protected (for example, UDP or TCP) in the transport mode. The payload length field
in the AH header indicates the length of the header. The SPI, in combination with the source and destina-
tion addresses, helps distinguish multiple SAs configured for the same source and destination combina-
tion. The AH header provides a means to verify data integrity. It is similar to the integrity check provided
by the ESP header with one key difference. The ESP integrity check only verifies the contents of the ESP
payload. AH's integrity check also includes portions of the packet header as well.
page 27-8
Payload Length(8 bits)
Security association identifier (SPI) (32 bits)
Sequence Number (32 bits)
Authentication Data (Variable)
(Integrity Check Value)
IP Packet protected by AH
OmniSwitch AOS Release 6 Network Configuration Guide
Configuring IPsec
Reserved (16 bits)
September 2009