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  QUANTAM KEY DISTRIBUTION

1.  INTRODUCTION

Classical cryptography can be divided into two major branches; secret or symmetric key cryptography

and public key cryptography, which is also known as asymmetric cryptography. Secret key

cryptography represents the most traditional form of cryptography in which two parties both encrypt

and decrypt their messages using the same shared secret key.

 While some secret key schemes, such as one-time pads, are perfectly secure against an attacker with

arbitrary Computational power, they have the major practical disadvantage that before two parties can

communicate securely they must somehow establish a secret key.

n order to establish a secret key over an insecure channel, key distribution schemes based on publickey cryptography, such as !iffie-"ellman, are typically employed.

#he advent of a feasible $uantum computer would make current public key cryptosystems obsolete

and threaten key distribution protocols such as !iffie-"ellman, some of the same principles that

empower $uantum computers also offer an unconditionally secure solution to the key distribution

 problem. %oreover, $uantum mechanics also provides the ability to detect the presence of an

eavesdropper who is attempting to learn the key, which is a new feature in the field of cryptography.

& '(! system consists of a $uantum channel and a classical channel.

• #he $uantum channel consists of a transparent optical path t is a lossy and probabilistic

channel.• #he classical channel can be a conventional ) channel

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2.  PROTOCOLS

&ll the )rotocols are based on two major principles described below.

2.1 Heisenberg Uncer!in" Princi#$e

"eisenberg *ncertainty )rinciple +"*) which states that in a $uantum system only one property of a pair of conjugate properties can be known with certainty. "eisenberg, who was initially referring to

the position and momentum of a particle, described how any conceivable measurement of a particles

 position would disturbs its conjugate property, the momentum. t is therefore impossible to

simultaneously know both properties with certainty. 'uantum cryptography can leverage this

 principle but generally uses the polariation of photons on different bases as the conjugate properties

in $uestion. #his is because photons can be e/changed over fibre optic links and are perhaps the most

 practical $uantum systems for transmission between two parties wishing to perform key e/change.

0ne principle of $uantum mechanics, the no cloning theorem, intuitively follows from "eisenbergs

*ncertainty )rinciple.

   BB84 protocol,

   BB92 protocol,  SARG04 protocol.

2.2 Q%!n%& En!ng$e&en

#he other important principle on which '(! can be based is the principle of $uantum entanglement.

t is possible for two particles to become entangled such that when a particular property is measured

in one particle, the opposite state will be observed on the entangled particle instantaneously. #his is

true regardless of the distance between the entangled particles. t is impossible, however, to predict

 prior to measurement what state will be observed thus it is not possible to communicate via entangled

 particles without discussing the observations over a classical channel.

 E91 protocol,

COW protocol,

 DPS protocol,

 KMB09 Protocol,

S09 protocol,

 S13 protocol  

BB'( Pr))c)$

1elow are the steps of the 1123 protocol for e/change the secret key in the 1123 protocol 4567, client& and client 1 must do as follow8

S#&9: ) or diagonal polariation +?

therefore Client & transmit photons in the four polariation states +@, 3A, 6@,BA degree.

• Client & records the polariation of each photon and sends it to Client 1.

• Client 1 receives a photon and randomly records its polariation according to the rectilinear

or diagonal basis. #he Client 1 records the measurement type +basis used and the resulting

 polariation measured. Client 1 doesnt know which of the measurement are deterministic,

i.e. measured in the same basis as the one used by client &. "alf the time Client 1 will be

lucky and chose the same $uantum alphabet as the third person. n this case, the bit resulting

from his measurement will agree with the bit sent by Client &. "owever the other half timehe will be unlucky and choose the alphabet not used by client &. n this case, the bit resulting

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from his measurement will agree with the bit sent by client & only A@D of the time. &fter all

these measurement, client 1 now has in hand a binary se$uence

Client & and Client 1 now proceed to communicate over the public two-way channel using the

following stage 5 protocol.

S#&9: 5 )

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,.  Pr))c)$s - A##$ic!i)ns

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(.  *inings

/.  Re0erences

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• C. ". 1ennett and 9. 1rassard, EQu!tu" cr#pto$rp%#& pu'l(c )*# +(tr('ut(o! !+ co(!

to(!$ ,F )roc. of the ::: nternational Conference on Computers, Systems and Signal

)rocessing, 1angalore, ndia, pp. GA-G6, 623.

• :. &rtur E'uantum cryptography based on 1ells theorem.F, )hysical review =etters, Hol. IG,

 Jo, I,A august 66, pp II-IIB.• :duin ".Serna, E'uantum (ey !istribution from a random seedF arKiv8 B.A25v5 $uant-

 ph 5th Jov 5@B

• :duin :steban, "ernande Serna, E'uantum (ey !istribution protocol with private- public

keyF arKiv8 @6@2.53Iv3 $uant-ph 5th may 5@5

• 41ennet657 1ennett, C., L'uantum cryptography using any two nonorthoganol states.L, )hys.