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Quantum Cryptography provides means for two parties to exchange an enciphering key over a private channel with complete security of communication.
The Quantum Cryptography Presented by Vinod.V 1BT00CS052 Introduction What is Cryptography? Cryptography is the art of devising codes and ciphers. Crypto analysis is the art of breaking them. Cryptology is the combination of the two i. e Cryptography and Crypto analysis What is Quantum Cryptography? Quantum Cryptography is an effort to allow two users of a common communication channel to create a body of shared and secret information. This information, which generally takes the form of a random string of bits, can then be used as a conventional secret key for secure communication. The Heisenberg Uncertainty principle and quantum entanglement can be exploited in as system of secure communication often referred to as “quantum Cryptography”. Differences Cryptography Quantum Cryptography 1.Classical Cryptography employs In Quantum mechanics the laws of various mathematical techniques to physics protect the information. restrict eavesdropping from learning the contents of encrypted messages. 2.In Classical Cryptography, an Quantum Cryptography provides means absolute security of information for two parties to exchange an cannot be guaranteed. enciphering key over a private channel with complete security of communication History of Quantum Cryptography The roots of Quantum Cryptography are in a proposal by Stephen Weisner called “Conjugate Coding” from the early 1970’s. It was eventually published in 1983 in Sigact News, and by that time Bennett and Brassard, who were familiar with Weisner’s ideas were ready to publish ideas of their own. Bennett and Brassard produced “BB84”, the first quantum cryptography protocol in 1984, but it was not until 1991 that the first experimental prototype based on this protocol was made operable (over a distance of 32 centimeters) . Most recent systems have been tested successfully on fibre optic cable over distances in the kilometers. A short introduction to Quantum Computation To explain what makes quantum computers so different from their classical counterparts we begin by having a closer look at a basic chunk of information namely one bit. From a physical point of view a bit is a physical system which can be prepared in one of the two different states representing two logical values --- no or yes, false or true, or simply 0 or 1 For example, in digital computers, the voltage between the plates in a capacitor represents a bit of information: a charged capacitor denotes bit value 1 and an uncharged capacitor bit value 0. One bit of information can be also encoded using two different polarisations of light or two different electronic states of an atom. However, if we choose an atom as a physical bit then quantum mechanics tells us that apart from the two distinct electronic states the atom can be also prepared in a coherent superposition of the two states. This means that the atom is both in state 0 and state 1. Idea of Quantum Object in ‘Two states at once’. Steps for establishing secret key Alice sends photons with one of four polarisations, which she has chosen at random. For each photon, Bob chooses at random the type of measurement: either the rectilinear type(+) or the diagonal type (x). Bob records the result of his measurement but keeps it a secret. Bob publicly announces the type of measurements he made, and Alice tells him which measurements were of the correct type. Alice and Bob keep all cases in which Bob measured the correct type. These cases are then translated into bits (1’s and 0’s ) and thereby become the key. Using this key, they check for eavesdropping at any time by sending a few number of bits out of the key and measuring the error rate as any eavesdropping would randomise the system. Quantum Cryptographic Communication Quantum communication uses the following properties of quantum mechanics for transfer of bits: 1. Polarisation of light. 2. Qubits. 3. Heisenberg’s Uncertainty principle. Qubit A photon can poses two states at a time. A photon being in such a state is called as qubit. Thus sending photon in such states can reduce the traffic by sending more information on less number of photons. A qubit can exist as a zero, a one, or simultaneously as both 0 and 1, like a spinning coin that has not yet landed. Hiesenberg’s Uncertainty Principle This states that any two conjugate property of photon cannot be measured at a time. Depending on how the observation is carried out, different aspects of the system can be measured – for example, polarisation of photons can be expressed in any of three different bases: rectilinear circular, and diagonal – but observing in one basis randomizes the conjugates. Advantage over Traditional Cryptography The advantage of quantum cryptography over traditional key exchange methods is that the exchange of information can be shown to be secure in a very strong sense, without making assumptions about the intractability of certain mathematical problems. Even when assuming hypothetical eavesdroppers with unlimited computing power, the laws of physics guarantee (probabilistically) that the secret key exchange will be secure, given a few other assumptions. Scientists have harnessed the properties of light to encrypt information into code that can be cracked only one-way: by breaking the physical laws of nature. Quantum physics guarantees that the properties of the photon will change if anyone intercepts it and tries to read the information off it. Limitations All quantum cryptography techniques only work over dedicated fibre – optic lines. Distances not to be greater than 90kms from one point to another. For instance there is a limit to the distance that photons can travel before they lose coherence which makes it impossible to read key information. The current record for long-distance quantum key distribution is 120km. Another practical problem arises from the fact that available detectors sometimes produce a response even when no photon has arrived. Such dark counts and other imperfections in the apparatus lead to errors even when there has been no eavesdropping and make it impractical for Alice and Bob simply to reject their data whenever they find and error in them. Future Aspects Recent developments in quantum optics have resulted in laser beams and other new photon sources, new photo-detectors, and better optical fibres that enable the transmission of relevant quantum phenomena over much larger distances. Using 1.55 – micron wavelength could increase the distance to about 100km. Beyond this distance, the Heisenberg’s uncertainty principle becomes unreliable. For industry sensitivity information, this technique has potential. This is why BT and other telecommunication companies are busily doing the R&D required before commercializing it into a product. Conclusion With the increasingly computing power of computers via qubit technology, it has become the real matter of concern to save information under transmission. A threat from quantum mechanics for classical cryptography has opened other way for encryption itself, which is unbreakable as long as physical laws of nature exist in tact. Quantum cryptography would be able to move beyond the constraints of a dedicated fibre – optic line between two points and extend out to wider network like the internet. Using properties of quantum physics, the technique encrypts data with keys. Governments and armed forces are thought to be among the first users of the technology Information about the key is encoded on to a single photon of light. Quantum physics guarantees that the properties of the photon will change if anyone intercepts it and tries to read the information off it.
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