QUANTUM CRYPTOGRAPHY

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Download free Quantum Cryptography.pdf Classical digital cryptography is concerned with the problem of providing secure and secret communication over a medium which guarantees neither of these functions. This provision is achieved through the encryption of data at the sender, and the decryption of that cyphertext at the receiver (an encrypted message is known in cryptography as the cyphertext of the message). There are further considerations concerning cryptography such as fair exchange and non-repudiation, but these notions are separate to the security issue which we will be discussing here.

The majority of cryptographic techniques in common use today rely on the notion of computational complexity to ensure the difficulty (but not impossibility) of decrypting cyphertexts. Such methods make use of cryptographic keys to encrypt and decrypt data, and depending on the encryption chosen can be extremely secure.

The problem with this system is that the receiver must know which encryption was used by the sender in order to decrypt the cyphertext correctly. Obviously, this means that the decryption instructions (the decryption key) also need to be communicated from one party to another. Herein lies the quandary that quantum cryptography is looking to solve: the Key Distribution Problem.

Quantum Cryptography, which was first proposed by Stephen Weisner in the early 1970s, harnesses the Heisenberg Uncertainty Principle. Uncertainty is a major constituent in all quantum mechanics, and basically states the indeterminism of the universe around us. This indeterminism gives rise to certain mechanisms that can be put to great use in cryptography.

The first of those mechanisms specifies sets of conjugate pairs to which Heisenberg’s uncertainty is bound, such as position and velocity. In fact, the principle states that if you could know with absolute precision the position of a particle then you could never know with any degree of certainty the velocity of that particle (and vice versa). Depending on how these quantities are measured, different aspects of the system can be quantified - for example, polarisation of photons can be expressed in any of three different bases: rectilinear, circular and diagonal. Measuring polarisation in the rectilinear base destroys the certainty of the other two bases (i.e. randomises the conjugates). It therefore follows, that if the data is encoded using these conjugate values, then the sender and receiver must use the same measurement base otherwise this randomisation will destroy any meaningful information. This is the mechanism that was used in Stephen Weisner’s original proposal.

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