Highlights
-Quantum cryptography is new secure method to conduct Internet communications
-Used recently during elections in Switzerland
-Will likely be secure for short to mid-term, until eavesdropping technology catches up
According to media reports, the Swiss government planned to use quantum cryptography to secure ballot box data for the national elections conducted on October 21, 2007 (source). Quantum cryptography has been widely publicized as the ultimate secure communication, but has yet to be tested in practical mass applications, such as the Swiss election forum.
The Promise of Quantum Cryptography
In theory, quantum cryptography provides completely secure communications between two parties. Proponents of quantum cryptography claim it enables 100 percent secure communications. Quantum cryptography greatly simplifies the key distribution process by enabling a sender to quickly and efficiently distribute a key to a receiver, ensuring that the key to unlock the scrambled communications has not been tampered with.
Quantum Key Distribution
The quantum key distribution process relies on manipulating photons with a polarized filter. The sender can polarize a photon via two separate filters. One filter can polarize a photon horizontally or vertically, known as rectilinear, and a second filter can polarize a photon diagonally. In turn, the receiver can accept the polarized photons with the same set of filters.
To create a key, the sender can send a random stream of photons polarized by the rectilinear and diagonal filters without telling the receiver how the photons were polarized. The receiver will likewise select a random order of filters to receive the stream of photons and then measure the results.
Alice and Bob
For example, Alice can send a vertically polarized photon to the receiver, Bob. Should Bob choose a horizontal filter to receive the photon, he will detect a negative result. However, should Bob choose a vertical filter to receive Alice’s photon, he will receive a positive result. Interestingly, if Bob chooses a diagonal filter to receive Alice’s photons he will still receive a positive result since the vertical polarized photon sent by Alice will continue to pass through the diagonal filter used by Bob.
As a result, a second step of the key distribution process is required to ensure that Alice and Bob are using the proper key. Alice must communicate via a separate channel – such as a telephone. Alice will tell Bob which filter she used to polarized her photon, but not what direction the photon was polarized. Returning to the previous example, Alice tells Bob that she polarized her photon with a rectilinear filter. Therefore, Bob would know that a rectilinear receiver on his end would generate the only reliable result and all misleading results from the diagonal filter should be ignored. To create a secure key, Alice would send Bob a lengthy stream of photons. Their shared key would be created from the string of photons that generated a positive result – i.e. the instances in which Bob correctly guesses the right filter used by Alice.
Blocking Eve
This process stymies Eve, the eavesdropper, because she does not have access to all the information required to build the key. Eve could also randomly select the rectilinear and diagonal polarized filters to intercept Alice’s traffic but would likely not have access to the out of band communication between Alice and Bob and therefore would not know which of her results were reliable.
Moreover, Alice and Bob would be able to detect Eve’s eavesdropping because Eve’s interference would alter the polarization of Alice’s photons. For example, if Alice sent a diagonal photon to Bob and Eve tried to intercept it with a vertical filter; Eve’s actions would change the photons polarized to vertical.
Therefore, when Alice told Bob that she sent him a diagonal photon Bob would immediately know their traffic was intercepted because he would have detected a vertical photon instead of a diagonal photon.
Outlook
Although quantum cryptography has apparently developed a solution to the thorny problem of key distribution that doomed previously unbreakable systems like Germany’s World War II era “Enigma” machine, history has shown that no communication system is completely secure.
Not surprisingly, researchers from the Massachusetts Institute of Technology (MIT) have developed a technique to gain information about the key used to secure communications without altering the transmitted photons and thereby not informing the sender and the receiver that their communications are being intercepted. As a result, it is theoretically possible under the right conditions to eavesdrop on a communications secured via quantum cryptography.
It therefore remains to be seen whether quantum cryptography can live up to its promised hype of complete privacy and security. History has demonstrated that technology development operates in a spiral-like fashion.
As the defenders introduce a technology like quantum cryptography, attackers will begin a concerted effort to crack the security and will eventually succeed.
It is therefore more than likely that the introduction of quantum cryptography will be followed by the introduction of a technology to eavesdrop on quantum cryptography.
