Detailed explanation of Quantum Cryptgraphy

1. Name: K. Vidya Madhuri
Roll no: 14311A1201
3-2, IT-A

2.  Introduction
 Traditional vs Quantum Cryptography
 How Quantum Cryptography works
 Applications of Quantum Cryptography
 Limitations

3. 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

4. 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”.

6. StephenWiesner wrote “Conjugate Coding”
in the late sixties
Charles H. Bennett and Gilles Brassard
revived the field in 1982 by combining
quantum process with public key
cryptography

7.  Traditional cryptography is heavily based on mathematical
theory and computer science practice; cryptographic
algorithms are designed around computational hardness
assumptions, making such algorithms hard to break in
practice by any adversary.
 Quantum cryptography is the science of
exploiting quantum mechanical properties to
perform cryptographic tasks.

8.  The main practical problem with secret key encryption
(traditional cryptographic technique) is exchanging a secret
key. In principle any two users who wished to communicate
could first meet to agree on a key in advance, but in practice
this could be inconvenient.
 Public key cryptography (PKC) systems exploit the fact that
certain mathematical operations are easier to do in one
direction than the other.The systems avoid the key
distribution problem, but unfortunately their security
depends on unproven mathematical assumptions about the
intrinsic difficulty of certain operations.

9.  The most popular public key cryptosystem, RSA (Rivest-
shamir-Adleman), gets its security from the difficulty of
factoring large numbers.This means that if ever
mathematicians or computer scientists come up with fast
and clever procedures for factoring large numbers, then the
whole privacy and discretion of widespread cryptosystems
could vanish overnight.

10.  Quantum cryptography takes advantage of the unique and
unusual behavior of microscopic objects to enable users to
securely develop secret keys as well as to detect
eavesdropping.
 Quantum cryptography solves the problems of secret-key
cryptography by providing a way for two users who are in
different locations to securely establish a secret key and to
detect if eavesdropping has occurred.

11.  The most important unit of information in computer
science is the bit.There are two possible values that can
be stored by a bit: the bit is either equal to “0” or equal
to “1.”
 Quantum system with at least two states can serve as a
qubit. For example, the spin of an Atom or the
polarization of a light particle can represent the state of
a qubit.

12. Heisenberg’s Uncertainty Principle:
 It is possible to encode information into quantum properties
of a photon in such a way that any effort to monitor them
disturbs them in some detectable way.This statement is
known as the Heisenberg uncertainty principle.
Quantum Entanglement:
 The entangled particles cannot be described by specifying
the states of individual particles and they may together
share information in a form which cannot be accessed in any
experiment performed on either of the particles alone.

13.  The foundation of quantum cryptography lies in the
Heisenberg uncertainty principle, which states that certain
pairs of physical properties are related in such a way that
measuring one property prevents the observer from
simultaneously knowing the value of the other.
 In particular, when measuring the polarization of a photon,
the choice of what direction to measure affects all
subsequent measurements.

14. Polarization of
Photons

15.  A pair of orthogonal (perpendicular) polarization states used
to describe the polarization of photons, such as
horizontal/vertical, is referred to as a basis.
 A pair of bases are said to be conjugate bases if the
measurement of the polarization in the first basis completely
randomizes the measurement in the second basis. It is a
fundamental consequence of the Heisenberg uncertainty
principle that such conjugate pairs of states must exist for a
quantum system.

17.  Alice sends a sequence of photons to Bob.
Each photon in a state with polarization corresponding
to 1 or 0, but with randomly chosen basis.
 Bob measures the state of the photons he receives, with
each state measured with respect to randomly chosen
basis.
 Alice and Bob communicates via an open channel. For
each photon, they reveal which basis was used for
encoding and decoding respectively.All photons which
has been encoded and decoded with the same basis are
kept, while all those where the basis don't agree are
discarded.

18.  If the sender Alice, uses a filter in the 0-deg/90-deg basis to
give the photon an initial polarization (either horizontal or
vertical, but she doesn't reveal which), the receiver Bob can
determine this by using a filter aligned to the same basis.
 However if Bob uses a filter in the 45-deg/135-deg basis to
measure the photon, he cannot determine any information
about the initial polarization of the photon .
 If an eavesdropper Eve uses a filter aligned with Alice's filter,
she can recover the original polarization of the photon. But if
she uses a misaligned filter she will not only receive no
information, but will have influenced the original photon so
that she is unable to reliably retransmit one with the original
polarization. Bob will either receive no message or a garbled
one, and in either case will be able to deduce Eve's presence.

19.  Switzerland has been using quantum cryptography to
conduct secure online voting in federal and regional
elections.
 Secure communications with satellites and astronauts is an
increasing concern, and a company calledQuintessenceLabs
is working on a project for NASA that will ensure secure
communications from Earth with satellites and astronauts,
using quantum cryptography.
 With “Quantum encrypted” internet, our most sensitive
transmissions would be passed along in an ultra-secure
manner.This would achieve the ideal of a simultaneously fast
and secure internet.

20.  The maximum distance covered by a message using
quantum cryptography is 150km, which is very short.
 When one photon was measured in one polarization, its
entangled counterpart took the opposite polarization,
meaning the polarization the other photon would take could
be predicted.
 Exchanging information using single photon needs a
dedicated channel of high quality in order to achieve high
speed communication. It is impossible to send keys to two or
more different locations using a quantum channel as
multiplexing is against quantum principles.