Quantum cryptography

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Quantum cryptography is an emerging technology in which two parties can secure network communications by applying the phenomena of quantum physics. The security of these transmissions is based on the inviolability of the laws of quantum mechanics. The quantum cryptography relies on two important elements of quantum mechanics the Heisenberg Uncertainty principle and the principle of photon polarization. This report summarizes the current state of quantum cryptography, and the real world application environment and the future direction in which the technology is headed forwards. Go through the attached report for design and implementation. 

Cryptography is the art of devising codes and ciphers. In the literature of cryptography, a sender scrambles or encrypts the original message or plain-text in a systematic way that obscures its meaning. The encrypted message or crypto-text or cipher-text is transmitted, and the receiver recovers the message by unscrambling or decrypting the  transmission. The parameters of the encryption algorithm that transforms the plain-text are collectively called a key. The keys used to encrypt most messages, are themselves encrypted before being sent. The schemes used to disguise keys are thought to be secure, because discovering them would take too long, even for the fastest computers. 

The purpose of cryptography is to transmit information in such a way that access to it is restricted entirely to the intended recipient, even if the transmission itself is received by others. This science is of increasing importance with the advent of broadcast and network communication, such as electronic transactions, the Internet, e-mail, and cell phones, where sensitive monetary, business, political, and personal communications are transmitted over public channels. Once the key is established, subsequent secure communication can take place by sending crypto-text, even over a public channel that is vulnerable to total passive eavesdropping, such as public announcements in mass media. 

However, to establish the key, two users, who may not be in contact or share any secret information initially, will have to discuss it, using some other reliable and secure channel. But since interception is a set of measurements performed by an eavesdropper on a channel, however difficult this might be from a technological point of view, any classical key distribution can in principle be passively monitored, without the legitimate users realizing that any eavesdropping has taken place. Cryptographers have tried hard to solve this key distribution problem. The 1970s brought a clever mathematical discovery in the form of public key cryptography (PKC).

PKC systems exploit the fact that certain mathematical operations are easier to do in one direction than the other. The systems avoid the key distribution problem, but unfortunately their security depends on unproven mathematical assumptions about the intrinsic difficulty of certain operations. The most popular public key crypto system, RSA, gets its security from the difficulty of factoring large numbers. This means that if ever mathematicians or computer scientists come up with fast and clever procedures for factoring large numbers, then the whole privacy and discretion of widespread crypto systems could vanish overnight. Indeed, recent work in quantum computation suggests that in principle quantum computers might factorize huge integers in practical times, which could jeopardize the secrecy of many modern cryptography techniques. 

But quantum technology promises to revolutionize secure communication at an even more fundamental level. While classical cryptography relies on the limitations of various mathematical techniques or computing technology to restrict eavesdroppers from learning the contents of encrypted messages, in quantum cryptography the information is protected by the laws of physics. Quantum cryptography, or quantum key distribution (QKD), uses quantum mechanics to guarantee secure communication. It enables two parties to produce a shared random bit string known only to them, which can be used as a key to encrypt and decrypt messages. 

An important and unique property of quantum cryptography is the ability of the two communicating users to detect the presence of any third party trying to gain knowledge of the key. This results from a fundamental aspect of quantum mechanics: the process of measuring a quantum system in general disturbs the system. A third party trying to eavesdrop on the key must in some way measure it, thus introducing detectable anomalies. By using quantum super positions or quantum entanglement and transmitting information in quantum states, a communication system can be implemented which detects eavesdropping.

If the level of eavesdropping is below a certain threshold, a key can be produced that is guaranteed to be secure (i.e. the eavesdropper has no information about), otherwise no secure key is possible and communication is aborted. Quantum cryptography 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.

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