Quantum
Cryptography
Cryptography
 Art of writing messages so that no one other than intended
receiver can read it.
 Encryption – Performing mathematical operations to hide
message content.
 Decryption – Reverse of encryption to get original message
 Plain Text – A clear unencrypted message.
 Cipher – An Algorithm to encrypt plain text and convert it
into seemingly nonsensical cipher text.
 Cipher Text – The finally encrypted text
Some Well known Methods
 Public Key and Private Key Systems – Uses a Public Key to
encrypt and Private key is used to decrypt message.
 One Time Pad(OTP) – The OTP is the random key , same
length as message, applied once and only once.
Theoretically IMPOSSIBLE to decipher the message
without the key.
 Some Other methods are RSA, DES, Double DES , Triple
DES, Digital Signatures etc.
Quantum Computing
 Superfast Computing. Based on the principles of Quantum
Theory .
 Photons will be the data carriers.
 Building Blocks are “QUBITS”. A QUBIT can be 0 or 1 at the
same time.
 Operations are performed on QUBITS and their
properties.
 Can factorize 250 digit number in finite time.
 Finding Key of is just a matter of time.
 Present encryption techniques will become useless.
Important facts
 The properties of photons are tied together so tightly that
measuring one property will cause change in other thus
disturbing the whole system
 In Quantum World A particle can be in any state until it is
observed. Once observed It gets settled in that state.
 The concept of uncertainty states that One cannot have
the knowledge about two properties of a photon at the
same time . For example Heisenberg uncertainty principle.
 The concept of Quantum Entanglement states that
photons are so closely related that it is impossible to
describe One without another.
 An attempt to measure the property of one photon will
immediately Affect the other Photon.
 It is like One Photon talking to Another photon in no time.
Quantum Cryptography
 The property of photons used in Quantum Cryptography is
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“Polarization” .
Light Waves moves in every plane.
Polarization is the act of selecting light waves of a
particular plane.
In Linear polarization Light can be polarized either
horizontally or vertically.
In circular polarization light waves are made to move in
circular fashion.
 Linear and circular polarization are bound with uncertainty .
 Measurement of linear polarization will destroy measurement of
circular polarization.
1. Light travels in all planes. 2. Horizontal Polarisation—isolating
light that travels horizontally
3. Vertical Polarisation
4. Circular polarisation
Artur Ekert’s Card Trick
 Given by Artur Ekert in 1991 to explain quantum
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cryptography.
There is a whole bunch of sealed envelops with red and
blue cards, marked with 0 and 1, in them.
two machines to read the cards. Red and blue machine.
Red machine understands red card and blue machine
understands blue card.
Red card with 1,in red machine will output 1.
Similarly for Blue cards with blue machine.
If Red card with 0 is inserted in blue machine , it will throw
a random guess of 0 or 1.
 Let us say there are two people Alice and Bob who want s to share
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the secret key safely.
We hand over one sealed envelop to each.
They will get the same results if they use the same color
machine.
This happens due to the concept of quantum entanglement.
When Alice‘s blue machine says the card is blue and 1 written
on it, then both the cards are blue. And so blue machine of Bob
will process the card as blue and yield the same result.
For sharing key they only tell each other what machine they
used for every card and not the card.
 Due to Quantum Entanglement they will get same result
every time they use same machine.
 To verify they will tally some of the values.
 The tally values and different machine values are discarded.
 The remaining left is the KEY.
The measurements that Alice (top row) and Bob (bottom row) make. They
get the
same result when they make the same measurement.
Now that they’ve made their measurements, they only keep the
results where they both used the same machine
They pick random results and compare them to see if there were
errors
Finally, the Key!
Final Words
 The Concept of quantum cryptography has been around
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since the 1970’s .
Working models have been set up in educational institutes.
Can be implemented on fiber optics Network, which is not
very common.
Infrastructure for quantum cryptography system is
Expensive.
Companies like New York’s MagiQ and Geneva’s
idQuantique are already providing quantum cryptography
solutions to bank’s and military.
Thank
You