Kish cypher - AbsoluteAstronomy.com

The Kish cypher is a technique purported to maintain secure communications utilizing classical statistical physics due to Laszlo B. Kish

Laszlo B. Kish

László Bela Kish is a physicist and professor of Electrical and Computer Engineering at Texas A&M University. His activities include a wide range of issues surrounding the physics and technical applications of stochastic fluctuations in physical, biological and technological systems, including...

. The Kish cypher is a physical secure layer (hardware-based technique) where the security is provided by the laws of physics (the second law of thermodynamics

Second law of thermodynamics

The second law of thermodynamics is an expression of the tendency that over time, differences in temperature, pressure, and chemical potential equilibrate in an isolated physical system. From the state of thermodynamic equilibrium, the law deduced the principle of the increase of entropy and...

and Kirchhoff's laws

Kirchhoff's laws

There are several Kirchhoff's laws, all named after Gustav Robert Kirchhoff:* Kirchhoff's circuit laws* Kirchhoff's law of thermal radiation* Kirchhoff's equations* Kirchhoff's three laws of spectroscopy* Kirchhoff's law of thermochemistry-See also:...

) and it should not be confused with a software-based approach called the Kish–Sethuraman (KS) cypher.

The Kish cypher scheme

The communication channel is a standard wire, and conceptually the sender can transmit a message by simply switching between two different resistor values at one end of the wire. At the other end, the receiver can also reciprocate by switching in and out resistors. No signals are sent along the line. The receiver simply uses a spectrum analyser to passively measure the Johnson noise of the line. From the noise, the total resistance of the line can be calculated. The receiver knows his/her own resistor value, so can then deduce the sender's resistor. In this way messages can be simply encoded in terms of binary states dependent on two resistor values. The system is thought to be secure because although an eavesdropper can measure the total resistance, the eavesdropper has no knowledge of the individual values of the receiver and sender.

The use of resistors is an idealization for visualization of the scheme, however, in practice, one would use artificially generated noise with higher amplitude possessing Johnson-like properties. This removes the restriction of operation within thermal equilibrium. It also has the added advantage that noise can be ramped down to zero before switching and can be ramped up back to the nominal value after switching, in order to prevent practical problems involving unwanted transients.

To protect the Kish cypher against invasive attacks, including man-in-the-middle attack

Man-in-the-middle attack

In cryptography, the man-in-the-middle attack , bucket-brigade attack, or sometimes Janus attack, is a form of active eavesdropping in which the attacker makes independent connections with the victims and relays messages between them, making them believe that they are talking directly to each other...

s, the sender and receiver continuously monitor the current and voltage amplitudes and broadcast them via independent public channels. In this way they have full knowledge of the eavesdropper's information.

An eavesdropper may potentially crack the Kish cypher by evaluating a resistor value at one end of the wire, in the time window where the resistor at the other end is being switched out. The response to this claim is that this attack is completely avoidable by the simple trick of doing the switching when both the voltage and the current are zero in the line. In the hardware demonstration of the cypher, the voltage (and current) was ramped down to zero before the switching took place in order to create this situation in an easy way . A simpler method to eliminate this problem utilizes the fact that accurate noise measurement is slow, as it requires an averaging process. The resistors are switched faster than the noise measurement time.

The use of the Johnson noise formula to evaluate the resistor values requires thermal equilibrium. In the Kish cypher method this is far from the case. For example, it cannot be guaranteed that the receiver and sender are at the same temperature. This is addressed by using artificial noise sources with Johnson-like characteristics rather than actual resistor values.

Attacking physical realizations of the Kish scheme

While the mathematical concept is unconditionally secure, hacking attacks against the actual physical realization of the Kish scheme, utilizing non-ideal features, such as inaccuracies and stray resistive elements, can be exploited to extract a fraction of the transmitted key bits. In 2005, Bergou proposed a method of finding such a weakness in the Kish scheme by utilizing the wire resistance. Then in 2006, Scheuer and Yariv analyzed Bergou's attack in detail. In 2010, Kish and Scheuer critically revisited the old Scheuer and Yariv results and showed that the original calculations of the Bergou-Scheuer-Yariv-attack were incorrect; moreover the new calculations indicate that the actual effect is about 1000 times weaker.
Back in 2006, a defense against the Bergou-Yariv-Scheuer attack was mounted and then experimentally confirmed in 2007, where Mingesz et al. showed that it was possible to build a hardware realization communicating over two thousand kilometers with 99.98% fidelity and a maximum of a 0.19% leak to an eavesdropper. It also turns out that the sender can exactly calculate which of the bits have been detected by the eavesdropper—this was mathematically analyzed by Kish and Horvath in 2009.

Privacy amplification for the Kish cypher

Recently, T. Horvath, et al have studied the practical effectiveness of privacy amplification for the Kish cypher and for two subsequent classical key-distribution schemes inspired by it. They find that the high fidelity of the raw key generated in these key-exchange protocols allow Alice and Bob to always extract a secure key provided they have an upper bound on Eve's chances to correctly guess the bits. They conclude that this property can make the Kish cypher highly useful for practical applications.

Securing computers and hardware by integrating the Kish cypher on chips

A specific advantage of the Kish system is that it can be integrated on digital chips to provide unconditionally secure key exchange (both for the first and the refreshed keys) for secure data communication between hardware units, such as processors, memories, hard drives, etc, within a computer or an instrument. Another advantage of such system is that, due to the short distances and the relevant range of frequency, the main non-idealities (wire resistance, inductance and capacitance) are negligible thus the Kish system can run under idealistic conditions to provide unconditional security without further precautions or processing, such as privacy amplification.

External links

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