Kish cypher

The Kish cypher, is a technique for maintaining secure communications using classical physics in CONTRAST to quantum cryptography, due to L. B. Kish. The Kish cypher is a hardware-based technique and should not be confused with a software 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, 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 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 and broadcast them via independent public channels. In this way they have full knowledge of the eavesdropper's information.

Debate

There have been a number of objections to this scheme as follows:

  • 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 time. 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. Whilst the time it takes to perform a noise measurement gives the Kish scheme its security, the downside is the resulting slow operation.
  • 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.
  • 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.

Attempts at cracking the scheme

In order to fully crack the security of a communications scheme, the goal of an eavesdropper is to extract 100% of the key bits without being detected. So far, there have been no attempts to crack the idealized Kish scheme, which is claimed to be totally secure. However, in an actual circuit realization of the Kish scheme, 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. Later, in 2006, a defense against the Bergou-Yariv-Scheuer attack was mounted and then experimentally confirmed in 2008, where Mingesz et al. showed that it was possible to build a hardware realization communicating over two thousand kilometers with a maximum of a 0.18% leak to an eavesdropper. It also turns out that the sender can exactly calculate which of the bits have been detected by the eavesdropper and this is possible as the Kish scheme is classical rather than quantum—this was mathematically analyzed by Kish and Horvath in 2009.

See also

  • Cryptography
  • Quantum cryptography
  • Secure communication
  • Topics in cryptography

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