Earlier in the book, we laid out some of the dangers of sniffers. They really are easy to obtain and you would be amazed at what you can see and do as information crosses the wire. Data integrity is what protects information from any type of unauthorized modification while it is on the wire. This makes sure that the information that has been received is exactly the same information that was sent. This is done by making use of mathematical hash functions to mark, or "sign," each and every packet. The receiving computer is responsible for checking the signature before it opens the packet. If the signature has been changed, it means the packet has been changed. That means the packet will be discarded to guard against a possible network attack. The Authentication Headers provide this authentication, integrity, and anti-replay protection for the IP header and for the entire data payload that is carried in the packet. The process means that the data is readable while it travels the wire, but it is protected from modification during the time it is being transported.
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