Abstract--- Quantum cryptography 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 result from a fundamental part of quantum mechanics: the process of measuring a quantum system in general disturbs the system.
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. The algorithm most commonly associated with QKD is the one-time pad, as it is provably secure when used with a secret, random key.
INTRODUTION
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.
Cryptography operates by a sender scrambling or encrypting the original message or plaintext in a systematic way that obscures its meaning. The encrypted message or cryptotext is transmitted, and the receiver recovers the message by unscrambling or decrypting the transmission.
Originally, the security of a cryptogram depended on the secrecy of the entire encrypting and decrypting procedures. Today, however, we use ciphers in which the algorithm for encrypting and decrypting could be revealed to anybody without compromising the security of a particular message. In such ciphers a set of specific parameters, called a key, is used together with the plaintext as an input to the encrypting algorithm, and together with the cryptotext as an input to the decrypting algorithm. The encrypting and decrypting algorithms are publicly announced; the security of the cryptogram depends entirely on the secrecy of the key. To prevent this being discovered by accident or systematic search, the key is chosen as a very large number.
Once the key is established, subsequent secure communication can take place by sending cryptotext, 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). The idea of PKC is for each user to randomly choose a pair of mutually inverse transformations -- a scrambling transformation and an unscrambling transformation -- and to publish the directions for performing the former but not the latter. The transformation is designed so that the unscrambling operation cannot be deduced easily from the scrambling operation, enabling only the user to read scrambled messages. In these systems users do not need to agree on a secret key before they send a message. They work similarly to a drop mailbox with two locks. The owner of the mailbox provides everybody with a key for dropping mail into his box, but only he has the key to open it and read the messages inside. PKC was introduced in 1976 .
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 cryptosystem, RSA (Rivest-Shamin-Adleman), 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 cryptosystems 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. This Hot Topic will discuss some of the basics of how this can be achieved.
CONCLUSION
For the first time in history, the security of cryptography does not depend any more on the computing resources of the adversary, nor does it depend on mathematical progress. Quantum cryptography allows exchanging encryption keys, whose secrecy is future-proof and guaranteed by the laws of quantum physics. Its combination with conventional secret-key cryptographic algorithms allows raising the confidentiality of data transmissions to an unprecedented level. Recognizing this fact, the MIT Technology Review and Newsweek magazine identified in 2003 quantum cryptography as one of the “ten technologies that will change the world”.
Quantum cryptography promises to revolutionize secure communication by providing security based on the fundamental laws of physics, instead of the current state of mathematical algorithms or computing technology. The devices for implementing such methods exist and the performance of demonstration systems is being continuously improved. Within the next few years, if not months, such systems could start encrypting some of the most valuable secrets of government and industry
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