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.

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