The information on the chip-including the photograph-is "signed" using a type of electronic seal, called a digital signature. It is virtually impossible to counterfeit an epassport, because no one can duplicate the authentic U.S. digital seal on the electronic data. Furthermore, any change to the chip information breaks the seal, so tampering is evident to a border agent.
At passport control, border agents can compare the person, the paper passport and the signed information on the security chip. These all have to match to confirm the identity of the person presenting the passport.
Want to learn more about how the epassport protects your privacy and safety? Read JustAskGemalto.com's Tutorial on the U.S. Electronic Passport.