The art and science of concealing the messages to introduce secrecy in
information security is recognized as cryptography.
Word Cryptography comes from greek word:
Cryptography=Krypto + graphene
“Secret Writing”.

WHAT IS QUANTUM?

A quantum is a minimum amount of any physical entity involved in an
interaction.
The fundamental notion that a physical property may be "quantized" is
referred to as "the hypothesis of quantization".
Word Quantum comes from Latin ‘quantus’ means “how great”.

?

Science of exploiting quantum mechanical properties to
perform cryptographic tasks.
Method of coding information based on quantum mechanics, which is said
to be unbreakable.
. It uses current knowledge of physics to develop a cryptosystem i.e. not able
to be defeated .

History
Proposed first by Stephen Wiesner in 1970’s.
Uses Heisenberg's uncertainty principle formulated in 1927.

Quantum Key Distribution
Process of using quantum communication to establish a shared key between
two parties without a third party learning anything about that key, even if
Eve can eavesdrop on all communication between Alice and Bob.
If Eve tries to learn information about the key being established, key
establishment will fail causing Alice and Bob to notice.

How Quantum Cryptography Works?
 QKD uses quantum mechanics to guarantee secure communication.
 Enables two parties to produce a shared random secret key known only to
them, which used to encrypt and decrypt messages.
In quantum computing, a qubit or quantum bit is a unit of quantum
information.
The state of a qubit can be in a ‘superposition' of 0 and 1 simultaneously.

QUANTUM PROPERTIES

a)
• A single photon is emitted from a light source and passes through a linear
polarizer, in this case - horizontal. That process creates a qubit with
horizontal polarization.

b)
• When the horizontally-polarized photon passes through a
horizontally/vertically-oriented polarizing beamsplitter, it always retains its
horizontal polarization.

c)
• If that horizontally-polarized photon passes through a diagonally-oriented polarizing
beamsplitter:
-There is a 50% probability of finding the photon at one of the exits.
-The photon will only be detected at one of the exits.
-The polarization of the photon will have changed to the corresponding diagonal polarization.

Spin values

BB84 Protocol
• Relies on quantum indeterminism and no-cloning theorem.
• Can be used between Alice and Bob to “negotiate” a key through a quantum
channel + a classical channel.
• Key is generated on-the-fly.

The BB84 Protocol’s steps
1) Key transmission through the quantum channel .
2) Error correction
3) Key distillation

Eve Possible Attacks
Cloning the photon
Intercept-resend
Intercept the public classical channel
Spoofing attack through the public channel

Why Quantum Cryptography?
• only known method for transmitting a secret key over distance that is secure
in principle and based on the laws of physics.

Why Quantum Cryptography So Secure?
• The quantum no-cloning theorem states that an unknown quantum state
cannot be cloned.
• In a quantum system, which can be in one of two states, any attempt to
measure the quantum state will disturb the system. A quantum message that
is intercepted and read by an eavesdropper will reproduce in a confused and
distorted way and useless to the intended recipient of the message.
• The effects produced by measuring a quantum property are irreversible.

Weaknesses and Limitation of Quantum
Cryptography
Only works along unbroken and relatively short fiber optic cables.
Doesn’t solve authentication problem.
Doesn’t address some of the weakest links in data security such as human
corruptibility and key storage.
Relatively high cost.

Advantages
Virtually Un-hackable
Simple to use
Less resources needed to maintain it
Used to detect eavesdropping in Quantum Key Distribution

Disadvantages
The signal is currently limited to 90 miles.
Could replace a lot of jobs.

Conclusion
Due to the high cost of implementation and the adequacy of current
cryptological methods, it is unlikely that quantum cryptography will be
in widespread use for several years.