The Common Access Card, also commonly referred to as the CAC or CAC card, is a smart card about the size of a credit card. It is the standard identification for Active Duty United States Defense personnel, to include the Selected Reserve and National GuardUnited States Department of Defense (DoD) civilian employees, United States Coast Guard (USCG) civilian employees and eligible DoD and USCG contractor personnel. It is also the principal card used to enable physical access to buildings and controlled spaces, and it provides access to defense computer networks and systems. This card is also used by most law enforcement agencies as an official photo identification card for law enforcement officers to carry. These photo identification cards normally authorize law enforcement officers to carry their handgun off duty.  It also serves as an identification card under the Geneva Conventions (esp. the Third Geneva Convention). In combination with a personal identification number, a CAC satisfies the requirement for two-factor authentication: something the user knows combined with something the user has. The CAC also satisfies the requirements for digital signature and data encryption technologies: authentication, integrity and non-repudiation.