Square Inc. has introduced a mobile payment device that lets users of Apple Inc.'s iPhone accept magnetic-stripe card payments.

The device works with a small, cube-shaped card reader that plugs in to the audio jack of an iPhone. People who enroll on Square's Web site can also upload a photo of themselves; when they use their enrolled cards with a Square reader, the picture is displayed on the phone, letting merchants verify that the customer is using his or her own card.

Square is also encouraging individuals to sign up to accept cards, which would let them pay each other with cards. Merchants typically must sign up with an acquirer to accept card payments, but Square says its enrollment process is much easier than of traditional acquirers.

The company is headed by Twitter Inc. founder Jack Dorsey, who used the microblogging Web site to announce Tuesday that the company had gone live.

Though the system was introduced on the iPhone platform, Square's Web site says it is hiring people experienced in Google Inc.'s Android and Research In Motion Ltd.'s Blackberry operating systems.