Quantum
Cryptography
The uncertainty principle of
quantum physics builds the earliest foundations for quantum cryptography. With quantum computers of
future being expected to solve discrete logarithmic problem and the popularly
know cryptography methods such as AES, RSA, DES, quantum cryptography becomes
the foreseen solution. In practice it is used to establish a shared, secret and
random sequence of bits to communicate between two system let’s say Alice and
Bob. This is known as Quantum
Key Distribution. After this key is shared between Alice and
Bob, further exchange of information can take place through known cryptographic
strategies.
Based On Heisenberg’s
Uncertainty Principle:
BB84 and variants –
A single photon pulse is passed through a polarizer .Alice can use a particular
polarizer to polarize a single photon pulse and encode binary value bits to
outcome of a particular type (vertical, horizontal, circular etc) of polarizer.
On receiving the photon beam, Bob would guess the polarizer and Bob can thus
match the cases with Alice and know the correctness of his guesses. If Eve
would have been trying to decode then due to poarization by Eve’s polarizer
would have caused discrepencies in match cases of Bob and Alice and thus they
would know about eavesdropping. Thus in such a system if Eve tries to eavesdrop
it will get into notice of Alice and Bob.
·
The
B92 protocol has only two polarization states unlike four in original BB84.
·
BB84
has similar protocol SSP that uses 6 states to encode the bits.
·
SARG04
is another protocol that uses attenuated lasers and provides better result than
BB84 in more than one photon systems.
Based On Quantum
Entanglement:
E91 and Variants –
There is a single source that emits a pair of entangled photons with Alice and
Bob receiving each particle. Similar to BB84 scheme Alice and Bob would
exchange encoded bits and match cases for each photon transferred. But in this
scenario the outcome of results of match cases of Alice and Bob will be
opposite as a consequence of Entanglement principle. Either of them will have
complement bits in bit strings interpreted. One of them can then invert bits to
agree upon a key. Since Bell’s Inequality should not hold for entagled
particles thus this test can confirm absence of eavesdropper. Since practically
it is not possible to have a third photon in entanglement with energy levels
sufficient for non detect ability, thus this system is fully secure.
·
SARG04
and SSP protocol models can be extended to Entangled particles theory.
Possible Attacks In
Quantum Cryptography:
·
Photon Number Splitting (PNS) Attack –
Since it is not possible to send a single photon thus a pulse is sent. Some of
the photons from a pulse can be captured by Eve and after matching of bits by
Alice and Bob, Eve can use the same polarizer as done by Bob and thus get the
key without
- being detected.
Faked-State Attack –
Eve uses a replica of Bob’s photon detector and thus captures the photons
intended for Bob and further passed it to Bob. Though Eve knows about the
encoded bit, Bob thinks that he received it from Alice.
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