The predecessor of PS/2-interface was the PC / XT keyboard interface, which is cognate to the operation only allows the transfer of data from the keyboard to the computer. The AT keyboard interface is bi-directional, in order to represent certain states such as Num Lock, Caps Lock and Scroll Lock LEDs.
USB interface
The keyboard is handled by the host controller as a Class Human Interface Device. The necessary device drivers are already integrated into any USB compatible operating system. The data transfer is done using interrupt transfer. Many wireless keyboards have a USB SPI i2c receiver, the log on system is equal to wired keypads.
The USB interface has certain advantages over the PS2 format: Devices can be connected or removed while in operation (hotplug mechanism). The device port can be used universally, not only for keyboards. Any number of keyboards are connected simultaneously, for example, additional keypad for left-handers.
The integration of other devices with a USB port in the keyboard housing for space and cost savings are possible, for example, trackball, smart card reader, USB SPI i2c hub. More distinct key codes are possible and thus additional specialized function keys.
However, there are also disadvantages of the USB interface: The HID protocol supports a maximum of 6 keys pressed simultaneously, while there is no restriction on PS / 2.
Behind the one used in standard USB label (interrupt transfer), it actually hides a polling mode. The CPU polls the keyboard in a fixed time frame – typically every 8 ms – according to their condition, while PS / 2 offers real-time interrupt mode.
This not only leads to higher latency, but also energy saving functions because the CPU simply hibernates until the next keystroke, but must actively check for input. Smart cards, USB keys and other intelligent devices may increase reliability. Smart cards can be used for secure storage of private keys of the user and safer to perform cryptographic transformations. Of course, intelligent authentication device does not provide absolute protection, but their security is far superior.
Store and use of the private key can be different, and manufacturers use varying approaches. The simplest of them is the use of smart device as a floppy disk. If necessary, map exports the private key and cryptographic operations are performed on the workstation.
This approach is not the most advanced in terms of safety, but relatively easy to implement and have few requirements for intelligent devices. Two other approaches are more secure because they assume implementation of an intelligent cryptographic operations.
The first user generates the keys on the workstation and stores them in memory. The second user generates the keys by the device. In both cases, after the private key is stored, it can not be removed from the device and used any other way.