HP UX IPSec Software Chapter, shared key hash, secret key hash, Symmetric key hash, Authentication Header (AH), Authentication Header (AH)

HP-UX IPSec Overview

Authentication Header (AH)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

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