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# Quantum Cryptography The Quantum Cryptography Presented by Vinod V 1BT00CS052 Introduction

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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.

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```									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
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.
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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