Keyboard Protected from Information Leakage via Electromagnetic Channel.

Nowadays Keyboard is used everywhere and it is often used for inputting valuable data, loss or leakage of which can cause a big harm to its owner.

First of all such a data is: passwords, security and access codes (f.e. used when operating with private data, commercial classified information, bank accounts, etc.).

Every key on the Keyboard has it's own Scan Code. The common PS/2 or USB Keyboard's Scan Codes are transferred via double-wire asymmetrical cable as an 8-bit sequence of unidirectional pulses. So all of the Keystrokes has their own unique scan-codes and the unique radiation is emitted for each scan-code transferred via keyboard cable.

The maximum probability of Key recognition is measured with a "Ideal Observer" Method as a rule. First, 'the cross correlation functions of all Scan Codes are calculated. Then we this functions of Scan Code are compared with each other and with themselves. According to this "Ideal Observer" Method the success of 'Receiver' is guaranteed if the meanings of cross correlation function on the main diagonal are discernible on the background even in the presence of noises. The visual presentation of this method is shown in the Figure 1. There the signal/noise (S/N) ratio is about 5/1. The main diagonal (resembling a kind of 'mountain ridge') is well discernible. That illustrates that the recognition of Keystrokes is perfectly correct even if the signal quality is bad.

Figure 1

In practice this Method proved to be effective for receiving and decoding Scan Code signalson the distances up to several tens of meters. And the shielding of the Keyboard turned out to be too expensive and not effective.

So concluding the stated above we can see that Keyboard is one of the most vulnerable parts of computer and is a source of radio-signals containing important put data.

But how to protect the inputted data effectively and in a not very expensive way?
Here's a decision.

The result of our company's many years research became the developing (and patenting) of an effective and high-technology method that prevents information typed from Keyboard from leakage via electro-magnetic channel.
This method is based on replacing an 8-bit sequence by special signals system so that the electro-magnetic field emitted is equal for all Keystrokes.
Unlike the encryption this method's effectiveness doesn't depends on whether malefactor is informed about Keyboard's construction and principles of operation or not.
The cross-correlation diagram is presented in the Figure 2, and it should be noted that S/N ration is about 1000/1. The main diagonal is not discernible even if the quality of signal is high, so the correct decoding of Keystrokes scan codes rather hard.

Figure 2

Actually the S/n ratio is less than 1000/1 on the distance of 0,1 meters from keyboard, that makes the recognition of Scan Code signals is practically impossible. This method of protection of information inputted from Keyboard from leakage is implemented in the following device produced by our company - the protected keyboard AN 140.