What is Quantum Cryptography (Key Distribution and Quantum Mechanics)

Security is guaranteed by the laws of nature and makes codes truly unbreakable through one thing: quantum cryptography.


Because this is the only known method for transmitting a secret key over distance that is secure in principle and based on the laws of physics, quantum encryption needs to be explained “clearly” if at all we have to keep the privacy of our data.

The primary focus of encryption should be to maximize security for the data awhile delivering the best possible value to users. Who doesn’t like to keep their data private?

So, What is Quantum Cryptography?

Quantum cryptography is the science that applies principles of quantum mechanics to encrypt messages in a way that it is never read by anyone outside of the intended recipient.

If you have ever bought something online at all, a data encryption scheme is an absolute must. Your trust is based on math, simple math that’s easy to do in one direction but extremely difficult to do in reverse. This is what protects your credit card information from thieves.

Quantum cryptography uses the laws of physics to develop a cryptosystem that is unhackable, one that is completely secure against being compromised without knowledge of the sender or the receiver of the message.

The security model of quantum cryptography is different from traditional cryptographic systems, it relies more on physics, rather than mathematics.

The drawbacks of the most popular encryption schemes used today is that, they can be undone only by factoring a huge random number, a “key” unlocking encoded information, into two prime numbers.

Although today this can be seen very difficult, with enough computing power, anyone could break the key.

In search of greater security, experts in atoms and other particles and cryptologists are exploiting the laws of quantum mechanics to send messages that are have been proofed unhackable.

The field of quantum cryptography has only been in existence in the past few decades.

How it works in theory, quantum cryptography is based on the usage of individual particles and waves of light (photons) and their intrinsic quantum properties to develop an unbreakable cryptosystem.

The idea being, it is impossible to measure the quantum state of any system without disturbing that system.

Photons are used as the information carriers in optical fiber cables.

The strength of quantum cryptography is in, ability of photons to simultaneously exist in more than one place or more than one state of being.

This is as a result of weirdness of reality of the particles making up our universe. These particles choose how to behave only when they bump into something else or when their properties are measured.

The Two Key Aspects of Quantum Cryptography

There are two main “principles” that make this secure:

  • Quantum Key distribution
  • Quantum Mechanics

The encryption process may vary from one cryptosystem to another, but quantum key distribution and quantum mechanics are the two key starting or focal points.

Quantum Key Distribution: Exchanging Keys securely

Quantum key distribution, also called QKD applies the strange behavior of particles that make up our universe to exchange keys securely.

A quantum key is used to encode and send the information needed to decrypt the message in the properties of light particles.

Eavesdroppers trying to get the key must make measurements of those particles. As a result of these measurements, there is change in the particles behavoir.

This introduces errors that can be detected and alert users that a key has been compromised and should not be used to encode information.

There are many variations of QKD, some make use of long-distance quantum connection called entanglement to protect information.

Entanglement allows two particles to behave like a single entity, no matter how far apart they are.

Messing with one particle, its partner instantly reacts, even at the opposite end of the universe. This is used to detect the presence of eavesdropper.

Here are examples of quantum systems:

  • In 2004, researchers in Vienna used entangled photons to transfer 3000 Euros deposit into their bank account, making this the first quantum transaction ever.
  • In 2013, commercial QKD systems were already being used in United States, R & D a non-profit Battelle installed a fiber optic network protected by encrypted photons.
  • In 2007, ID Quantique developed a system that used quantum technology to protect the results of an election in Geneva.

Quantum cryptography at work.

Although it’s yet to become a standard for commercial use, it’s getting there fast.

Entanglement can allow for extra security compared to other QKD schemes. The latter requires the device being used to be trusted.

On the other hand, entanglement opens door to device-independent cryptography that remains secure even on untrusted equipment.

The privacy of an entanglement-based system, can be verified using a set of statistics that describe how similarly the particles behave, called Bell inequalities.

A brief history of quantum cryptography:

In the 1960s, a professor at Columbia University developed the idea of a quantum money, that would be impossible to counterfeit. Each note would have trapped particles whose properties could be verified by banks.

Today there several quantum random number generators that can spit out numbers not based on computer algorithms but really random quantum fluctuations.

There is also on-going developments for secure ways of sending data from a normal computer to the quantum computers of the future.

Quantum Mechanics: How it Works in Theory

Lets assume that two people, Alice and Bob, want to exchange a message securely. Alice initiates the message by sending Bob a key, which will be the mode for encrypting the message data.

This is a random sequence of bits, sent using a certain type of scheme, which can see two different initial values represent one particular binary value (0 or 1).

Lets assume that this key is a stream of photons traveling in one direction. Each of these photon particles represents a single bit of data ( 1 or 0).

As they travel linearly, the photons also vibrate in a certain manner. The vibrations can occur in any 360-degree range across any axis.

Lets assume that photons vibrations have the following states:


The angle of this vibration is known as the polarization of the photon. Then we have to introduce a polarizer into the equation. So now, Alice has a polarizer that can transmit the photons in any one of the four states.

She can choose either of the following polarization filters:

  • Rectilinear (UP/DOWN and LEFT/RIGHT)

Alice swaps her polarization scheme between rectilinear and diagonal filters for the transmission of each single photon bit in a random manner.

Thus, the transmission can have one of two polarizations represent a single bit, either 1 or 0, in either scheme she uses.

When receiving the photon key, Bob must choose to measure each photon bit using either his rectilinear or diagonal polarizer.

Sometimes he will choose the correct polarizer and at other times he will choose the wrong one. Both Alice and Bob select each polarizer in a random manner.

What happens with the photon when the wrong polarizer is chosen?

The Heisenberg Uncertainty principle states that it’s not known exactly what will happen to each individual photon, because by measuring its behavior, we alter its properties.

This means measuring one property prevents us from quantifying the other.

What is we make a guess, what happens with them as a group?

Suppose Bob uses a rectilinear polarizer to measure UPLEFT/RIGHTDOWN and diagonal polarizer UPRIGHT/LEFTDOWN. In this case photons will pass through in a changed state.

This means half will be transformed to UP/DOWN and the other half to LEFT/RIGHT. Note we can not know which individual photons will be transformed into which state.

Bob measures some photons correctly and others incorrectly. Alice and Bob then establishes a channel of communication don’t have to be secure. Alice then proceeds to advice Bob as to which polarizer she used to send each photon bit but not how she polarized each photon.

For example she could say that photon number 25 was sent using the rectilinear scheme, but she will not say whether she sent an UP/DOWN or LEFT/RIGHT. Bob then confirms if he used the correct polarizer to receive each particular photon.

Alice and Bob then discard all the photon measurements that Bob used the wrong polarizer to check.

They are left with a sequence of 0s and 1s that is half the length of the original transmission. This forms the basis for a one-time pad – the only cryptosystem that, if properly implemented, is proven to be completely random and secure.

Suppose we have an eavesdropper, Eve, who attempts to listen in, has the same polarizers that Bob does and must also randomly choose whether to use the rectilinear or diagonal one for each photon. Eve like Bob has the same problem, in half the time she will choose the wrong polarizer.

But bob has the advantage of speaking to Alice to confirm which polarizer type was used tor each photon.

This is useless to Eve, as half the time she used the wrong detector and will misinterpret some of the photons that will form that final key, making it useless.

There also another level of security inherent in quantum cryptography – intrusion detection. Alice and Bob would know if Eve was eavesdropping on them.

Suppose Alice transimits photn number 52 as an UPRIGHT/LEFTDOWN to Bob, but for that one, Eve uses the rectilinear polarizer, which can only measure UP/DOWN or LEFT/RIGHT photons accurately.

In that case, Eve will have transformed that photon into either UP/DOWN or LEFT/RIGHT, as that is the only way the pgoton can pass.

If bob uses his rectilinear polarizer, then it will not matter what he measures as the polarizer check Alice and bob go through above will discard that photon from the final key.

But if he uses the diagonal polarizer, a problem arises when he measures its polarization; he may measure it correctly as UPRIGHT/LEFTDOWN, but he stands an equal chance, according to the Heisenberg Uncertainty Principle, of measuring it incorrectly as UPLEFT/RIGHTDOWN.

Eve’s use of the wrong polarizer will warp that photon and will cause Bob to make errors even when he is using the correct polarizer.

To discover Eve’s nefarious doings, they must perform the above procedures, with which they will arrive at an identical key sequence of 0s and 1s – unless someone has been eavesdropping, whereupon there will be some discrepancies.

They must then undertake further measures to check the validity of their key. It would be foolish to compare all the binary digits of the final key over the unsecured channel discussed above, and also unnecessary.

Let us assume that the final key comprises 2,000 binary digits. What needs to be done is that a subset of these digits be selected randomly by Alice and Bob, say 100 digits, in terms of both position (that is, digit sequence number 8, 72, 65, 509 etc) and digit state (0 or 1).

Alice and Bob compare these – if they match, then there is virtually no chance that Eve was listening. However, if she was listening in, then her chances of being undiscovered are one in countless trillions, that is, no chance in the real world.

Alice and Bob would know someone was listening in and then would not use the key – they would need to start the key exchange again over a secure channel inaccessible to Eve, even though the comparisons between Alice and Bob discussed above can still be done over an insecure channel.

However, even if Alice and Bob have concluded that the their key is secure, since they have communicated 100 digits over an un-secure channel, these 200 digits should be discarded from the final key, turning it from a 2,000 into a 1,900 bit key).

Thus, quantum cryptography is a way to combine the relative ease and convenience of key exchange in public key cryptography with the ultimate security of a one-time pad.


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