Message Authentication Code
Uses an algorithm known as HMAC
HMAC takes as input a portion of the message and a secret key and produces a MAC as output
This MAC value is stored in the Authentication Data field of the AH header
The calculation takes place over the entire enclosed TCP segment plus the authentication header
When this IP packet is received at the destination, the same calculation is performed using the same key
- If the calculated MAC equals the value of the received MAC, then the packet is assumed to be authentic