Broadly, satellites have been used for some decade in communication. During this communication

electromagnetic waves are used as signal carriers of information to transmit it from ground

terminal to the satellite and satellite back to the ground. If the information relayed via the

satellite is unsecured, it may be eavesdropped on. The people everywhere need to communicate

with a trustable communication. To solve this security problem and communicate only trustable

information we need to use satellites that relay encrypted information.

Traditionally public key cryptography or asymmetrical cryptography has been used often in compu-

tational of mathematical functions which does not provide efficient solution (Blumenthal, 2007).

This proves that it cannot guarantee the security of information and can be easily intercepted by

an adversary who wants to get the information from two parties that are communicating. Public

key cryptography requires two keys, one for encrypting and another for decrypting but the main

problem of key exchange is even the same for secret key cryptography. By using Shor’s algorithm

(Ekert and Jozsa, 1996) had been demonstrated that most of the existing classical cryptographic

infrastructure may be vanquished by a quantum computers (Liao et al., 2018).

Unlike public key cryptography, quantum key distribution as one of the best task known in quan-

tum cryptography offers the strongest encryption of information as a solution of key exchange

problem existing in public key cryptography. Quantum cryptography provides the way for trans-

mitting information securely through a public communication channel using principles of quantum

mechanics such as qubits ( quantum bits ). Obviously quantum cryptography is a new field of

interest and attractive, it is secure and even in the coming of quantum computer. In quantum

cryptography we use qubits to ensure secured communication, its security foundation is rooted

from the fact that each qubit is carried by a single photon and if there is any attempt of mea-

suring the photon, will suddenly alter that photon and real time intrusion is detected by two

communicating parties. From the theory of quantum cryptography that was first introduced by

Stephen Wiesner (Elboukhari et al., 2010) optical fibre and terrestrial free space optical experi-

ments were done based on quantum key distribution. Communication via optical fibre have had

enormous effects on the photon transfer, it is hardly affected by the weather, background light

and suffer from fibre’s attenuation loss. These problems bring photon’s restriction to a distance

of few hundred kilometers (Hosseinidehaj et al., 2017). Apart from fibre channel, terrestrial free

space optical channel is more advantageous, for during communication results with lower losses

(Peng et al., 2005), this is because the atmosphere has negligibly turbulence and low absorption.

With the result from this last experiment gives an important point for communicating towards

satellite based quantum cryptography.

Based on the idea of quantum key distribution that guarantee secured unconditional communica-

tion worldwide and from previous were done experiments that restrict the usage of some protocols

for long distances, the Chinese came up with an idea of making a quantum satellite based on

quantum key distribution (QKD) which is immune against all attacks from quantum computers

and can communicate over long distance with higher communication efficiency than the other

terrestrial free space and fibre channels.

On August 16, 2016, a satellite named Micius, built by the Chinese Academy of Sciences (CAS)

was launched into space with booster, with the capability to transmit data in the form of images

and a video stream secured by quantum cryptography from the satellite to the ground stations

(i.e downlinks ) between China and Austria over a distance of 7600 kilometers and these data

are virtually impossible to be hacked by the fourth party (Liao et al., 2018). The method used to

secure the communication is quantum key distribution (QKD) using cryptographic BB84-protocal.

Micius satellite has been launched to be operating on a Low Earth Orbit(LEO) using decoy-state

quantum key distribution for encryption to ensure that the message is undecipherable during

communication (Liao et al., 2017). Quantum key distribution takes photons and use them for

transmitting data to different ground stations and generating a strings of random bit also known

as secret key that is to be shared between the two ground stations. It could be argued that the

state of the art in quantum cryptography is now more advanced than classical cryptography. Even

though quantum cryptography is not as on large scale as classical, secured communication and key

exchange between two communicating parities is possible. Currently, quantum computers have

been made and are more power than classical computers due to their performance to solve hard

problem in a minute. No matter how hard computation problem is quantum computer can break

it easily. Thus this makes classical computers become obsolete. By then, quantum cryptography

came us solution of key distribution problem by allowing cryptographic key exchange between

two remote parties with a complete security as it is indicated by the laws of quantum physics.