The Intrigued Ideologist Just another WordPress Blog

Quantum Cryptography: An Explorative Essay

Since the birth of communication, humans have strived to keep information from others - effectively lighting the privacy flame. Modern encryption began in the time of the early ___ century, when the Romans controlled most of the civilized world. Emperor Julius Caesar needed a way to communicate important battle orders to his war generals so they could move the information throughout the ranks. The problem with blatantly expressing his orders in plain text was that if a messenger got intercepted by the enemy, the Emperor's secret would be revealed. Caesar then proceeded to invent the "Caesar Shift" method of encryption (a rudimentary encryption method but effective for his time). Thus, the modern era of cryptography was born.

Much has changed since Caesar's famous shift. The birth of the Internet and other security heavy services now require strong encryption protocols to secure the sensitive information. However, no amount of encryption allows for full and "impossible-to-crack" protection. It is not until the world of mathematics and physics merged to create the idea of Quantum Cryptography that ciphers have literally become unbreakable. Quantum Cryptography differs from conventional methods of encryption in that it utilizes our modern understanding of physics and particle/wave duality instead of the usual mathematical algorithm scheme to transmit sensitive messages across long distances.

The Cryptographic Process
To explain the process of Quantum Cryptography, we will use the Alice, Bob and Eve notation standard for simplifying the process.
Suppose we have a stream of binary digits (also called the "key") moving in a rectilinear fashion from Alice to Bob. In conventional cryptography, these bits would be placed through a set of rules to change their values before leaving Alice. Once arrived at Bob, the rules would be reversed and the original string of information would be unveiled. The downside of this method is that anyone who knows the rule (in this case, Eve) can also read the information before it reaches Bob. Unlike bits of data, Quantum Cryptography deals with charged photons (units of light) moving in a linear path but that are also vibrating along an axis. When the data is sent from Alice, the photons are polarized through a directional filter and sent on their way to Bob. A directional filter restricts the photon's movement to only one axis.

Once they have left Alice, the photons are charged the way she wants and they make their way to Bob. Properties of quantum systems state that if a system is disturbed, it changes fundamentally -- which is where the strength of quantum cryptography lies. Let us say that, for example, Eve wants to eavesdrop on the chain of photons. The Heisenberg Uncertainty Principle states that:

"In the quantum world, the accuracy with which we can know the position and momentum of an object such as an electron or a photon is limited. This introduces a reciprocal relationship between position and momentum. Thus, if the position of an electron is known to a high degree of accuracy, then the momentum of the electron will be very uncertain, and vice versa."

This means that the second Eve sees the stream of photons, the sequence changes; meaning that the message is lost forever and must be retransmitted anew by Alice. This is the ultimate power behind quantum cryptography: physically unbreakable code.

Polarizing Filters
Quantum directional filters allow a non-polarized photon (spinning in all directions at once) to be "tamed" to a certain direction. For the simplicity of this paper, we will assume that there are only two types of filters, rectilinear and diagonal, as seen below:

Let us, however, assume that Eve is out of the picture and that the beam of photons successfully gets transmitted to Bob without interference. This means that Bob, sitting at his home computer, receives a string of polarized photons … but what can he do with that? He first uses a random sequence of filters to measure the incoming photons. Based on probability, he will get some right and others wrong. This is when Alice and Bob must initiate contact to speak about the filter basis used to decode on Bob's side. Alice will then apply the same filters used by Bob and, seeing as there was no intrusion by Eve, they will end up with the exact same key sequence. This key can then be used as a unique and 100% secret key.

Quantum cryptography is still in its beta stages, meaning that it is, for the most part, theoretical. However, Toshiba's Research Team in Cambridge successfully transmitted quantum information over a distance of 50 kilometers at a speed of over 1 megabit per second. This simply proves that technology is advancing at incredible speeds and that, over time, the domains of mathematics and physics will have forever greyer borders. Quantum cryptography is a step in the right direction (no pun intended).