Embodiments of the invention provide a portable consumer device configured to store dynamic authentication data in memory. The portable consumer device also includes an interface for transmitting data to and receiving power from an external device. The dynamic authentication data is read from the memory by a read-write device located on the portable consumer device. The authentication data is updated and the updated data may be written into memory using the read-write device. In some embodiments, an authentication value read from the memory may be used to generate another authentication value based on an algorithm. The portable consumer device is further configured to transmit authentication data to an external device. The process of reading, updating, generating, transmitting, and rewriting the authentication data may occur each time external power is provided to the portable consumer device via the interface. The power for operating the components of the portable consumer device may be drawn exclusively from the power received from the external device (i.e., the external power). In some embodiments, a portion of the external power may be temporarily stored on an energy storage means of the portable consumer device.