RFID Readers Connected to a Linux System

In this configuration, one or more RFID readers are connected to a Linux-based PC. The readers are typically connected via serial lines or USB. Unfortunately, this is about where the similarity ends. Even for RFID systems using one given standard/protocol (e.g., ISO1443), there is no common communications protocol, USB device class, driver architecture, or software interface.

Most readers implement parts of the RFID protocol stack(s) inside the reader firmware and provide a relatively abstract communications interface on top. The communications interface is often based on the concept of a serial port, even if the reader hardware doesn't attach to a physical serial port. Some USB readers even actually contain a built-in USB-to-serial converter. Others go as far as emulating a USB serial adapter (CDC ACM or similar). Even the CompactFlash/PCMCIA readers often have a built-in legacy serial port or an emulated or real USB-serial converter. All of those readers in the end are accessed using a serial device node such as /dev/ttyS0, /dev/ttyACM0, or /dev/ ttyUSB0.

Some other reader manufacturers decided to make their readers emulate a contact-based chipcard reader compliant with the USB Chip Card Interface Device (CCID) specifications. Such readers are then driven by the pcsc-lite software package just like the contact-based chipcard readers.

Some more recent readers, particularly the inexpensive ones, implement the RFID protocol stack in the driver on the PC side. This mimics the concepts of the network world: An Ethernet card doesn't run the protocol stack; rather, the Linux OS runs the TCP/IP protocol stack. Examples of such readers are Omnikey CardMan 5121/5321 and the OpenPCD readers. This design simplifies the hardware requirements and eases development. Also, since all protocol logic is running on the PC, new protocols or workarounds for broken tags can be implemented by driver/software updates. For the security analyst and hacker, this type of reader provides the advantage of analyzing security aspects of the protocols itself.

Such readers usually provide a highly device-specific nonstandard USB interface to the underlying RFID reader ASIC. The RFID protocol stack (sometimes just called driver to hide the fact that it's a complete protocol stack and not just a device driver) then defines the upper-layer interfaces.

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