Quantum Cryptography

Quantum Cryptography uses the Quantum Computation Effects to perform Cryptographic tasks and to break into Cryptographic systems. The word quantum itself refers to the most fundamental behavior of the smallest particles of matter and energy: quantum theory explains everything that exists and nothing can be in violation of it.

Essentially, quantum cryptography is based on the usage of individual particles/waves of light (photon). It is also based on their intrinsic quantum properties to develop an unbreakable cryptosystem. It is so because it is impossible to measure the quantum state of any system without disturbing that system. It is theoretically possible that other particles could be used, but photons offer all the necessary qualities needed, their behavior is comparatively well-understood, and they are the information carriers in optical fiber cables, the most promising medium for extremely high-bandwidth communications.

The idea behind quantum cryptography is that two people communicating using a quantum channel can be absolutely sure no one is eavesdropping. Heisenberg’s uncertainty principle requires anyone measuring a quantum system to disturb it, and that disturbance alerts legitimate users as to the eavesdropper’s presence. No disturbance, no eavesdropper. In simple word quantum cryptography is completely secure.

There are companies out there selling very high end, and “provably secure” cryptography gear, all based on fundamental principles of quantum mechanics. Yet, despite being fundamentally unbreakable, there have been quite a few publications on more-or-less practical ways for Eve to eavesdrop on people whispering quantum sweet-nothings in darkened rooms. The word *quantum* itself refers to the most fundamental behavior of the smallest particles of matter and energy: quantum theory explains everything that exists and nothing can be in violation of it.

The most widely and well known use of quantum cryptography is to use quantum communication to securely exchange a quantum key distribution and using the quantum computers that would allow us to break public-key Encryption such as RSA and ElGamal.

The first basic science behind quantum crypto was developed in the 1980s by two computer scientists Charles Bennette and Giles Brassard. Since then there have been steady advancement in the field of quantum cryptography.

How Does Quantum Cryptography Works?

Quantum cryptography uses photons to transmit a key. Once the key is transmitted, coding and encoding using the normal secret-key method can take place. But how does a photon become a key? How do you attach information to a photon’s spin?

This is where binary code comes into play. Each type of a photon’s spin represents one piece of information — usually a 1 or a 0, for binary code. This code uses strings of 1s and 0s to create a coherent message. For example, 11100100110 could correspond with h-e-l-l-o. So a binary code can be assigned to each photon — for example, a photon that has a vertical spin ( | ) can be assigned a 1. Alice can send her photons through randomly chosen filters and record the polarization of each photon. She will then know what photon polarizations Bob should receive.

When Alice sends Bob her photons using an LED, she’ll randomly polarize them through either the X or the + filters, so that each polarized photon has one of four possible states: (|), (–), (/) or ( ) [source: Vittorio]. As Bob receives these photons, he decides whether to measure each with either his + or X filter — he can’t use both filters together. Keep in mind, Bob has no idea what filter to use for each photon, he’s guessing for each one. After the entire transmission, Bob and Alice have a non-encrypted discussion about the transmission.

Reference: HowStuffWorks

Advantages Of Quantum Cryptography:

The advantage of quantum cryptography lies in the fact that it allows the completion of various cryptographic tasks that are proven or conjectured to be impossible using only classical (i.e. non-quantum) communication. For example, quantum mechanics guarantees that measuring quantum data disturbs that data; this can be used to detect eavesdropping in quantum key distribution.

We should look at it this way. Suppose we are able to build a quantum computer (not to mention Science Fiction right now). It will be able to factor numbers do mathematical calculations very quickly. Simply, it could break all our used Public-Keys. It will halve the key length. For example if a key is of 256-bit, then it would like breaking into a 128-bit encryption key with a quantum computer.

Possibility of such a computer is very far from the near future.

Application In Real Life:

In practice, quantum cryptography has been demonstrated in the laboratory by IBM and others, but over relatively short distances. Recently, over longer distances, fiber optic cables with incredibly pure optic properties have successfully transmitted photon bits up to 60 kilometers. Beyond that, BERs (bit error rates) caused by a combination of the Heisenberg Uncertainty Principle and microscopic impurities in the fiber make the system unworkable. Some research has seen successful transmission through the air, but this has been over short distances in ideal weather conditions. It remains to be seen how much further technology can push forward the distances at which quantum cryptography is practical.

What Is Quantum Key Distribution?

Quantum key distribution (QKD) uses quantum mechanics to guarantee secure communication. It enables two parties to produce a shared random secret key known only to them, which can then be used to encrypt and decrypt messages.

The most well-known and developed application of quantum cryptography is quantum key distribution. QKD describes the process of using quantum communication to establish a shared key between two parties (usually called Alice and Bob) without a third party (Eve) learning anything about that key. even if Eve can eavesdrop on all communication between Alice and Bob. This is achieved by Alice encoding the bits of the key as quantum data and sending them to Bob. If Eve tries to learn these bits, the messages will be disturbed and Alice and Bob will notice. The key is then typically used for encrypted communication.

Quantum Key Distribution (QKD) is a technology, based on the quantum laws of physics, rather than the assumed computational complexity of mathematical problems, to generate and distribute provably secure cipher keys over unsecured channels. It does this using single photon technology and can detect potential eavesdropping via the quantum bit error rates of the quantum channel. Sending randomly encoded information on single photons produces a shared secret that is a random string and the probabilistic nature of measuring the photon state provides the basis of its security.

Quantum key distribution is only used to produce and distribute a key, not to transmit any message data. This key can then be used with any chosen encryption algorithm to encrypt (and decrypt) a message, which can then be transmitted over a standard communication channel. The algorithm most commonly associated with QKD is the one-time pad, as it is provably secure when used with a secret, random key.

Reference: Wikipedia

Limitations of Quantum Cryptography:

Quantum Cryptography has its limitations also. Bruce Schneier, an American cryptographer, says “ I don’t see any commercial value in it. I don’t believe it solves any security problem that needs solving. I don’t believe that it’s worth paying for. I can’t imagine anyone but a few technophiles buying and deploying it. Systems that use it don’t magically become unbreakable, because the quantum part doesn’t address the weak points of the system.

Security is a chain; it’s as strong as the weakest link. Mathematical cryptography, as bad as it sometimes is, is the strongest link in most security chains. Our symmetric and public-key algorithms are pretty good, even though they’re not based on much rigorous mathematical theory. The real problems are elsewhere: computer security, network security, user interface and so on. “

Even quantum cryptography doesn’t “solve” all of cryptography: The keys are exchanged with photons, but a conventional mathematical algorithm takes over for the actual encryption.

Author: Naveen Singh

Originally Published in Hacker5 Magazine

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