Kish cypher

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The Kish cypher[1], is a classical technique for maintaining secure communications without resort to the complexity of quantum cryptography. There have been a number of attempts to crack the Kish cypher method, but so far it has stood up to scrutiny, and it remains an open question as to whether it could ultimately replace secure quantum cryptography. The Kish cypher is a hardware-based technique and should not be confused with a software approach called the Kish-Sethuraman (KS) cypher. [2][3]

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[edit] 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 voltages or 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 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, he/she 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 sources 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.

[edit] Controversy

There have been a number of objections to this scheme, but as far as is known it has not yet been cracked. A number of objections are as follows:

  • The Kish cypher is very slow. The reply to this objection is that quantum cryptography is also very slow. Quantum cryptographers therefore only claim their technique is to be used for secure key distribution to make classical encryption more secure. Similarly the Kish cypher can be used for secure key distribution and not the whole message.
  • An eavesdropper can 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 reply to this objection is that accurate noise measurement is slow, as it requires an averaging process. The resistors are switched faster than the noise measurement. Thus security is maintained, as the Kish cypher is elegantly based on classical time-amplitude measurement uncertainty, in the same way that quantum uncertainty is at the heart of secure quantum cryptography.
  • 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 objection is addressed by using artificial noise sources with Johnson-like characteristics rather than actual resistor values.

[edit] See also

[edit] References

  • 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).
  • L. B. Kish, "Totally secure classical communication utilizing Johnson (-like) noise and Kirchoff's law," Physics Letters A, 352(3):178-182, (2006).
  • 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).
  • A. Cho, "Cryptography - Simple noise may stymie spies without quantum weirdness," Science 309(5744):2148-2148, (2005).

[edit] Notes

  1. ^ Noise keeps spooks out of the loop - tech - 23 May 2007 - New Scientist Tech
  2. ^ L. B. Kish and S. Sethuraman, "Non-breakable data encryption with classical information," Fluctuation and Noise Letters, 4:C1–C5, (2004).
  3. ^ A. Klappenecker, "Remark on a 'non-breakable data encryption' scheme by Kish and Sethuraman," Fluctuation and Noise Letters, 4:C25, (2004).

[edit] External links