What makes an epassport hard to counterfeit?

In the redesigned U.S. passport books it is the contactless smart card chip that makes it virtually impossible to counterfeit.  First, when the passport is personalized for an individual, the information inside the chip is “signed” using a type of electronic seal.  That seal, called a digital signature, proves that the passport was issued by the U.S. government.  No one else can duplicate this authentic digital seal (making counterfeit passport impossible).  In addition, any attempt to change the passport information in the chip breaks the seal, so tampering with the digital security electronics is evident to a border agent.

A reader at passport control can verify this seal is authentic, reducing dependency on a border agent to determine if a passport is counterfeit.  Second, the information on the front page of the passport is stored electronically in the chip.