The iButton® monetary device (DS1963S) with SHA-1 Function is a rugged 4kbit read/write data carrier that can be easily accessed with minimal hardware. Its NV memory acts as a localized database for public as well as protected data belonging to the owner of the device and the environment in which it is used. An integrated 512-bit SHA-1 engine can be activated to compute 160-bit message authentication codes (MAC) based on information stored in the device. Data is transferred serially via the 1-Wire® protocol, which requires only a single data lead and a ground return. Using the TMEX™ file format (see Application Note 114) a single DS1963S can serve up to four independent applications, such as secure change purses for electronic payment at local transit systems, pay phones, parking systems or vending machines. The DS1963S is also intended to function as a coprocessor that assists the host in computing signatures, using a secure signing secret, when writing back the new balance to a roaming device after a purchase.
The DS1963S, like other SRAM-based iButton devices, has an additional memory area called the scratchpad that acts as a buffer when writing to the main memory. The DS1963's scratchpad is also used for feeding data segments to the SHA-1 engine or receiving/comparing message authentication codes.
Data is first written to the scratchpad from where it can be read back. After the data has been verified, a copy scratchpad command will transfer the data to main memory. This process ensures data integrity in an environment that does not provide a reliable electric contact.
Each DS1963S has its own 64-bit ROM registration number that is factory lasered into the chip inside to provide a guaranteed unique identity for absolute traceability. The durable MicroCan package is highly resistant to environmental hazards such as dirt, moisture, and shock. Its compact coin-shaped profile is self-aligning with mating receptacles, allowing the DS1963S to be easily used by human operators. Accessories permit the DS1963S to be mounted on almost any surface including plastic key fobs, photo-ID badges and printed circuit boards.