Authentication Header (AH)

The IPSec Authentication Header (AH) provides integrity and authentication but no privacy--the IP data is not encrypted. The AH contains an authentication value based on a symmetric-key hash function.

Symmetric key hash functions are a type of cryptographic hash function that take the data and a key as input to generate an authentication value. Cryptographic hash functions are usually one-way functions, so that starting with a hash output value, it is difficult to create an input value that would generate the same output value. This makes it difficult for a third party to intercept a message and replace it with a new message that would generate the same authentication value.

Symmetric key hash functions are also known as shared key hash functions because the sender and receiver must use the same (symmetric) key for the hash functions. In addition, the key must only be known by the sender and receiver, so this class of hash functions is sometimes referred to as secret key hash functions.

In the example below, the sender uses the plaintext and shared secret key to calculate an authentication value and sends the authentication value with the plaintext. The recipient computes its own authentication value using the same shared secret key and the plaintext. The recipient then compares the result with the transmitted authentication value. If the values match, the recipient is assured that the sender knows the same secret key, confirming the identity of the sender. The recipient is also assured that the data was not altered during transit.

Figure 1-1 Symmetric Key Authentication

HP-UX IPSec supports the following algorithms for AH:

Transport and Tunnel Modes

The IPSec headers (AH and ESP) can be used in transport mode or tunnel mode.

Transport Mode

In transport mode, IPSec inserts the AH header after the IP header. The IP data and header are used to calculate the AH authentication value. Mutable fields in the IP header (fields that need to change in transit), such as “hop count,” and “time to live,” are assigned a zero value before IPSec calculates the authentication value, so the actual value of the mutable fields are not authenticated.

IPv6

In IPv6 AH transport mode, IPSec inserts the AH after the following headers and extensions:

• the basic IPv6 header

• hop-by-hop options

• any destination options needed to interpret the AH header

• routing extensions

• fragment extensions

The items listed below follow the AH:

• any destination options needed only for the “final” destination and not needed to interpret the AH header

• the IP data or payload (e.g., TCP or UDP packet)

The entire packet is used to calculate the authentication value. Mutable and unpredictable fields and options, such as timestamp and traceroute options, are assigned a zero value before calculating the authentication value.

Figure 1-2 AH in Transport Mode

Tunnel Mode

In tunnel mode, IPSec encloses, or encapsulates, the original IP datagram, including the original IP header, within a second IP datagram. All of the original IP datagram, including all fields of the original header, is authenticated.

IPv6

In IPv6 AH tunnel mode, the packet layout is the same as IPv4 AH tunnel mode, except that the original and new (outer) IP headers may include header extensions.

Figure 1-3 AH in Tunnel Mode