Recently, Kish proposed a “totally secure communication system” that uses only resistors, wires and Johnson noise. His paper—“Totally Secure Classical Communication Utilizing Johnson (-like) Noise and Kirchoff’s Law”—was published on Physics Letters (March 2006).

The above paper had been featured in Science magazine (Vol. 309), reported in News articles (Wired news, Physorg.com) and discussed in several weblogs (Schneier on security, Slashdot). The initial sensation created was that Quantum communication could now be replaced by a much cheaper means. But not quite so …

This paper—to appear in IEE Information Security—shows that the design of Kish’s system is fundamentally flawed. The theoretical model, which underpins Kish’s system, implicitly assumes thermal equilibrium throughout the communication channel. This assumption, however, is invalid in real communication systems.

Kish used a single symbol ‘T’ to denote the channel temperature throughout his analysis. This, however, disregards the fact that any real communication system has to span a distance and endure different conditions. A slight temperature difference between the two communicating ends will lead to security failure—allowing an eavesdropper to uncover the secret bits easily (more details are in the paper).

As a countermeasure, it might be possible to adjust the temperature difference at two ends to be as small as possible—for example, by using external thermal noise generators. However, this gives no security guarantee. Instead of requiring a fast computer, an eavesdropper now merely needs a voltage meter that is more accurate than the equipments used by Alice and Bob.

In addition, the transmission line must maintain the same temperature (and noise bandwidth) as the two ends to ensure “thermal equilibrium”, which is clearly impossible. Kish avoids this problem by assuming zero resistance on the transmission line in his paper. Since the problem with the finite resistance on the transmission line had been reported before, I will not discuss it further here.

To sum up, the mistake in Kish’s paper is that the author wrongly grafted assumptions from one subject into another. In circuit analysis, it is common practice to assume the same room temperate and ignore wire resistance in order to simplify the calculation; the resultant discrepancy is usually well within the tolerable range. However, the design of a secure communication is very different, as a tiny discrepancy could severely compromise the system security. Basing security upon invalid assumptions is a fundamental flaw in the design of Kish’s system.