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The Kish cypher^[1]^[2] is an insecure[3] [4] [5] technique for communication that has been reported to maintain secure communications utilizing classical statistical physics, due to Laszlo B. Kish. The Kish cypher^[2] is a physical secure layer (hardware-based technique) where the security is claimed to be provided by the laws of physics (the second law of thermodynamics and Kirchhoff's laws)^[3] and it should not be confused with a software-based approach called the Kish–Sethuraman (KS) cypher.^[4]^[5]

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 conventional signal is sent along the line, although the varying Johnson noise effectively propagates down the wire like any other electrical signal. 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 claimed 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 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.^[2] 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 attacks, the sender and receiver continuously monitor the current and voltage amplitudes^[6] and broadcast them via independent public channels. In this way they are thought to have full knowledge of the eavesdropper's information, although existing security analyses have covered only specific attacks.^[3]

One possible attack against the Kish cypher is to evaluate 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.^[7] 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. Thus the heart of the Kish cypher scheme is based on exploiting classical time-amplitude measurement uncertainty, in contrast to quantum uncertainty that is central to quantum cryptography.

Attacking physical realizations of the Kish scheme

While the idealized mathematical concept is secure against certain idealized attacks, 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 transmitted key bits. In 2005, Bergou proposed a method of finding such a weakness in the Kish scheme by utilizing the wire resistance.^[8] Then in 2006, Scheuer and Yariv analyzed Bergou's attack in detail.^[9] 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. ^[10] Back in 2006, a defense against the Bergou-Yariv-Scheuer attack was mounted^[11] and then experimentally confirmed in 2007,^[7] 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.^[3]

Privacy amplification for the Kish cypher

Recently, Horvath, et al.^[12] 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 ^[13] of the Kish system is that it can be integrated on digital chips to provide 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^[3] (wire resistance, inductance and capacitance) are negligible thus the Kish system can run under idealistic conditions ^[2] to provide security without further precautions or processing, such as privacy amplification.^[12]

Teleportation of classical bits

The telecloning of classical bits in quantum communicator networks without telecloning the quantum states was proposed by Kish and Mingesz.^[14]^[15] The telecloning scheme was originally proposed to realize a high-speed secure network of Kish cypher loops for network-based key distribution instead of point-to-point key exchange. The key exchange steps take place through the chain of these loops and a parallel authenticated network. The combination of bits arriving in these separate channels results in the secure telecloning of classical bits. It was recognized in the paper that the same idea could be used in quantum key distribution networks.

References

^ Noise keeps spooks out of the loop - tech - 23 May 2007 - New Scientist Tech
^ ^a ^b ^c ^d L. B. Kish, "Totally secure classical communication utilizing Johnson (-like) noise and Kirchhoff's law," Physics Letters A, 352(3):178-182, (2006).
^ ^a ^b ^c ^d L. B. Kish and T. Horvath, "Notes on recent approaches concerning the Kirchhoff-Law-Johnson-Noise-based secure key exchange," Physics Letters A. 373 (32) 2858-2868 (2009) http://arxiv.org/pdf/0903.2071
^ L. B. Kish and S. Sethuraman, "Non-breakable data encryption with classical information," Fluctuation and Noise Letters, 4:C1–C5, (2004).[1]
^ A. Klappenecker, "Remark on a 'non-breakable data encryption' scheme by Kish and Sethuraman," Fluctuation and Noise Letters, 4:C25, (2004).[2]
^ L. B. Kish, "Protection against the man-in-the-middle-attack for the Kirchhoff-loop-Johnson(-like)-noise cipher and expansion by voltage-based security," Fluctuation and Noise Letters, 6:L57-L63, (2006), http://arxiv.org/abs/physics/0512177
^ ^a ^b R. Mingesz, Z. Gingl, L. B. Kish, "Johnson(-like)-noise-Kirchhoff-loop based secure classical communicator characteristics, for ranges of two to two thousand kilometers, via model-line," Physics Letters A, 372(7):978-984, (2008). Cite error: The named reference "Mingesz" was defined multiple times with different content (see the help page).
^ A. Cho, "Cryptography - Simple noise may stymie spies without quantum weirdness," Science 309(5744):2148-2148, (2005)
^ J. Scheuer and A. Yariv, "A classical key-distribution system based on Johnson (like) noise - How secure?" Physics Letters A, 359(6):737-740, (2006).
^ L.B. Kish and J. Scheuer, "Noise in the wire: the correct results for the Johnson (-like) noise based secure communicator," Physics Letters A, 374 : 2140-2142, (2010).
^ L. B. Kish, "Response to Scheuer–Yariv: 'A classical key-distribution system based on Johnson (like) noise—how secure?' " Physics Letters A, 359:741–744, (2006).
^ ^a ^b T.Horvath, L.B. Kish, J.Scheuer, "Effective Privacy Amplification for Secure Classical Communications" EPL 94 (2011) 28002-p1 - 28002-p6. http://arxiv.org/abs/1101.4264
^ L.B. Kish, O. Saidi, "Unconditionally secure computers, algorithms and hardware, such as memories,...", Fluctuation and Noise Letters 8 (2008) L95-L98. http://arxiv.org/abs/0803.4479
^ http://arxiv.org/ftp/physics/papers/0603/0603041.pdf - Totally secure classical networks with multipoint telecloning (teleportation) of classical bits through loops with Johnson-like noise
^ Information theoretic security by the laws of classical physics - http://arxiv.org/ftp/arxiv/papers/1206/1206.2534.pdf

External links

[NewScientist-1] Noise keeps spooks out of the loop - tech - 23 May 2007 - New Scientist Tech

[kish-2] L. B. Kish, "Totally secure classical communication utilizing Johnson (-like) noise and Kirchhoff's law," Physics Letters A, 352(3):178-182, (2006).

[horvath-3] L. B. Kish and T. Horvath, "Notes on recent approaches concerning the Kirchhoff-Law-Johnson-Noise-based secure key exchange," Physics Letters A. 373 (32) 2858-2868 (2009) http://arxiv.org/pdf/0903.2071

[4] L. B. Kish and S. Sethuraman, "Non-breakable data encryption with classical information," Fluctuation and Noise Letters, 4:C1–C5, (2004).[1]

[5] A. Klappenecker, "Remark on a 'non-breakable data encryption' scheme by Kish and Sethuraman," Fluctuation and Noise Letters, 4:C25, (2004).[2]

[6] L. B. Kish, "Protection against the man-in-the-middle-attack for the Kirchhoff-loop-Johnson(-like)-noise cipher and expansion by voltage-based security," Fluctuation and Noise Letters, 6:L57-L63, (2006), http://arxiv.org/abs/physics/0512177

[Mingesz-7] R. Mingesz, Z. Gingl, L. B. Kish, "Johnson(-like)-noise-Kirchhoff-loop based secure classical communicator characteristics, for ranges of two to two thousand kilometers, via model-line," Physics Letters A, 372(7):978-984, (2008). Cite error: The named reference "Mingesz" was defined multiple times with different content (see the help page).

[8] A. Cho, "Cryptography - Simple noise may stymie spies without quantum weirdness," Science 309(5744):2148-2148, (2005)

[9] J. Scheuer and A. Yariv, "A classical key-distribution system based on Johnson (like) noise - How secure?" Physics Letters A, 359(6):737-740, (2006).

[10] L.B. Kish and J. Scheuer, "Noise in the wire: the correct results for the Johnson (-like) noise based secure communicator," Physics Letters A, 374 : 2140-2142, (2010).

[11] L. B. Kish, "Response to Scheuer–Yariv: 'A classical key-distribution system based on Johnson (like) noise—how secure?' " Physics Letters A, 359:741–744, (2006).

[horvath_etal-12] T.Horvath, L.B. Kish, J.Scheuer, "Effective Privacy Amplification for Secure Classical Communications" EPL 94 (2011) 28002-p1 - 28002-p6. http://arxiv.org/abs/1101.4264

[saidi-13] L.B. Kish, O. Saidi, "Unconditionally secure computers, algorithms and hardware, such as memories,...", Fluctuation and Noise Letters 8 (2008) L95-L98. http://arxiv.org/abs/0803.4479

[14] ttp://arxiv.org/ftp/physics/papers/0603/0603041.pdf - Totally secure classical networks with multipoint telecloning (teleportation) of classical bits through loops with Johnson-like noise

[15] Information theoretic security by the laws of classical physics - http://arxiv.org/ftp/arxiv/papers/1206/1206.2534.pdf

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