ABSTRACT1. INTRODUCTION2. WORKING OF QUANTUM CRYPTOGRAPHY3. CONCLUSION4. REFERENCES:5. WEBSITES:Quantum Cryptography – Security through Uncertainty Narayana KashyapQUANTUM CRYPTOGRAPHY – SECURITYTHROUGH UNCERTAINTYNARAYANA D KASHYAPSpring 2003ABSTRACTQuantum cryptography is governed by the laws of physics. It basically utilizesHeisenberg’s Uncertainty Principle. The way it works is by a system which includes atransmitter and receiver. A sender transmits photons which have been polarized (usingeither rectilinear polarization or diagonal polarization). The receiver at the other endmeasures this polarization for each photon by using any of the two types. The receiverrecords the measurement (rectilinear or diagonal). He announces the measurement but notthe result and the sender announces back to the receiver which of the measurements wereof the correct type. Both the parties keep the cases in which the receiver measurementswere of the correct type. These cases are then translated into bits (1's and 0's) calledqubits and thereby become the key. An eavesdropper is bound to introduce errors to thistransmission because he/she does not know in advance the type of polarization of eachphoton and quantum mechanics does not allow him/her to acquire sharp values of twonon-commuting observables (here rectilinear and diagonal polarizations). The twolegitimate users of the quantum channel test for eavesdropping by revealing a randomsubset of the key bits and checking (in public) the error rate. Although they cannotprevent eavesdropping, they will never be fooled by an eavesdropper because any effortto tap the channel, however subtle and sophisticated, will be detected. Whenever they arenot happy with the security of the channel they can try to set up the key distributionagain. 1. INTRODUCTIONCryptography Gentlemen do not read each other's mail - Henry Stimson, U.S. Secretary of StateDuring early days when people wanted information to be protected or to be destined foronly a few designated people, they used to hide information in such a way that theonlooker would be puzzled and does not understand what the writing is. The earliestknown technique is Caesar Cipher, a substitution technique where the letters of Englishwere replaced with the letters further down the alphabet set. Such techniques andmethods where information is not easy to perceive and understand leads to what is knownas Cryptography.With the usage of the internet, where information is exposed to everyone andcould be tampered by anyone, the need for information hiding has become even moreimportant and prioritized in the modern day society. Hackers and cryptanalysts find thisinformation easy to watch, modify and sometimes even block the information fromgetting across to the destined person. Hence, a person who would like to haveinformation sent across to only a few selected trusted parties takes help fromCS 265 Term Paper Page 1 of 5 April 14, 2003Quantum Cryptography – Security through Uncertainty Narayana Kashyaporganizations which help in hiding the information and securely transferring theinformation to the designated parties. These organizations use what is called as Public /Private Key Cryptography. The algorithms and the techniques used are quite robust and itwould take a cryptanalyst considerable amount of time and effort to decipher theencrypted message. The time, in fact, is quite huge; a 128-bit key used for encryption issaid to have a time factor of the order of millions of years to decipher using brute forcemethods. These algorithms are based on mathematical concepts, which can be solved orbroken through better decryption techniques and faster and parallel computers in thefuture. One method goes beyond mathematics and its concepts and instead, the laws ofphysics govern the security of encrypted message. This form is called QuantumCryptography. Quantum Cryptography I can't speak without an interception.This is private; please get off my line.Please tell me when I can have my privacy. - Ray and Dave Davies This cryptographic technique allows a secure channel between two people who want tocommunicate in privacy. Here initial parameters are agreed in forefront and during thecourse of communication can change to the desired configuration or sometimes theseparameters are not even needed. It is as powerful as classical cryptography and can takeconventional cryptography to the next level, as Quantum Cryptography can sense even apassive attack or eavesdropping on the quantum channel.“Quantum theory is believed to govern all objects, large and small, but its consequencesare most conspicuous in microscopic systems such as individual atoms or subatomicparticles. The act of measurement is an integral part of quantum mechanics, not just apassive, external process as in classical physics. So it is possible to design a quantumchannel - one that carries signals based on quantum phenomena - in such a way that anyeffort to monitor the channel necessarily disturbs the signal in some detectable way.”(http://www.cyberbeach.net/~jdwyer/quantum_crypto/quantum1.htm)This is due to Heisenberg’s Uncertainty Principle which asserts that certain pairs ofphysical properties are completely complimentary to each other, in the sense thatmeasuring one of them disturbs the other property. This principle can be applied to the quantum channel, which is used by QuantumCryptography. The Quantum channel carries small particles called “Quantums” or“Photons” of light within them. Hence the name, Quantum Cryptography. When lighttravels, it generates oscillating electric field, and the direction of oscillation is known asthe polarization of the photon. The Quantum channel can be constructed with the help of two polarizing filters which areplaced at either ends; one is the sender and the other is the receiver. These crystals aremade of birefringent crystal (such as calcite). The sender can place his crystal in aCS 265 Term Paper Page 2 of 5 April 14, 2003Quantum Cryptography – Security through Uncertainty Narayana Kashyapdirection that he wants the photons to be polarized and transmitted. The
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