Quantum Cryptography

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Arka Deb, Nagaraja.H, Noor Afshan Fathima

III sem Computer Science and Engineering
P E S College of Engineering Mandya, India.

Keywords— Cryptography, Quantum Cryptography, Photons,
Polarization, Key.

I. Introduction
Privacy is paramount when communicating sensitive information, and humans have invented some unusual ways to encode their conversations. Quantum cryptography describes the use of quantum mechanical effects (in particular quantum communication and quantum computation) to perform cryptographic tasks or to break cryptographic systems. The goal of quantum cryptology is to thwart attempts by a third party to eavesdrop on the encrypted message.

Cryptology is the process of encoding (cryptography) and decoding (crypto analysis) information or messages (called plaintext). All of these processes combined are cryptology. Earlier cryptology was based on algorithms -- a mathematical process or procedure which were created by a sender and transmitted to a receiver. These algorithms are used in conjunction with a key, a collection of bits (usually numbers). Without the proper key, it's virtually impossible to decipher an encoded message, even if you know what algorithm to use. In modern cryptology, the third party (adversaries) can passively intercept sender and receiver’s encrypted message -- he can get his hands on the encrypted message and work to decode it without the sender and receiver knowing he has their message. The adversary can accomplish this in different ways, such as wiretapping sender or receiver’s phone or reading secure e-mails. Quantum cryptology is the first cryptology that safeguards against passive interception as photons come into play. Since we can't measure a photon without affecting its behaviour, Heisenberg's Uncertainty Principle emerges when the third party makes his own eavesdrop measurements.

A. Photon Properties:
Photons are some pretty amazing particles. They have no mass, they're the smallest measure of light, and they can exist in all of their possible states at once, called the wave function. This means that whatever direction a photon can spin in -- say, diagonally, vertically and horizontally -- it does all at once. Light in this state is called unpolarized. This is exactly the same as if we constantly moved east, west, north, south, and up-and-down at the same time. The foundation of quantum physics is the unpredictability factor. This unpredictability is pretty much defined by Heisenberg's Uncertainty Principle. This principle says, essentially, that it's impossible to know both an object's position and velocity -- at the same time. But when dealing with photons for encryption, Heisenberg's principle can be used to our advantage. To create a photon, quantum cryptographers use LEDs -- light emitting diodes, a source of unpolarized light. LEDs are capable of creating just one photon at a time, which is how a string of photons can be created, rather than a wild burst. Through the use of polarization filters, we can force the photon to take one state or another -- or polarize it. If we use a vertical polarizing filter situated beyond a LED, we can polarize the photons that emerge: The photons that aren't absorbed will emerge on the other side with a vertical spin ( | ). The thing about photons is that once they're polarized, they can't be accurately measured again, except by a filter like the one that initially produced their current spin. So if a photon with a vertical spin is measured through a diagonal filter, either the photon won't pass through the filter or the filter will affect the photon's behaviour, causing it to take a diagonal spin. In this sense, the information on the photon's original polarization is lost, and so, too, is any information attached to the photon's spin....
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