Quantam Cryptography

29 11 2012

Cryptography as we know today has evolved over generations and has come a long way from Caesar cipher to modern Ciphers like AES. Modern ciphers are known to be cryptographically strong and some of them have not yet been broken. Stability of these modern ciphers and crypto graphical techniques has not satiated the hunger of modern cryptologist. We keep hearing of new innovations and breakthrough in the field of Cryptology. One such innovation of modern times has been the birth of Quantum Cryptography. First time I heard of “Quantum Cryptography” was when my professor happened to mention about it in my Information Security class. It captured my imagination instantly and I just wrote the words down in my notebook to dig more into it. And as I researched more and more about the topic, my fasciation for it kept growing.

Quantum Cryptography has its roots in physics, using properties of photon to achieve security. Quantum Cryptography is based on Heisenberg Uncertainty Principle which states that it is impossible to determine certain properties such as position of a photon without changing some other properties such as velocity. Since QC (Quantum Cryptography) relies on the characteristics of photon to transmit messages securely through the channel, anyone trying to detect one of the properties of photon to launch an attack will eventually end up disturbing some other properties of the photon, which would be easily detected by receiving party. Once the receiving party observes that some of the properties of the incoming photon have been disturbed, they can be very well sure that an attacker in the middle was trying to eavesdrop on the message and receiving party can drop this message [1].

So the question is how photons are analogous to bits we have in classical cryptography.  Light waves are transmitted as very minute massless particles known as photons. When this light wave composed of photon passes through a filter, depending upon the type of filter used photons will come out of the filter aligned in certain direction. Say for example we have stream of photon passing through a vertical filter (|) the emerging photons will be aligned in vertical direction and this can be noted down as 1 bit. Likewise if we use a horizontal filter (–), photons will be aligned in horizontal direction and we can note this state as 0 bit. Diagonal filter (X) is another filter commonly used to note the state of a photon. This alignment of photon in a particular direction is known as polarization. Thus a sender such as Alice can send stream of photon polarized (effectively a stream of 0’s and 1’s) and receiver Bob can detect these state of photon [2].

This form of communication using photon has been used by cryptographers to exchange secret keys and enable them to send encrypted data on the network. BB84 protocol developed by Bennett and Brassard in the year 1984 is one of most popular Quantum Key Distribution protocol [3]. BB84 protocol uses polarization of light wave to exchange a secure key between Alice and Bob. Alice sends a stream of photon by polarizing them. Bob detects the photons using any of possible filters and notes the stream of 0’s and 1’s based on photon alignment. Finally by using classical channel Bob and Alice can verify which bit’s were received correctly by Bob and whether there was any eavesdropping by Eve. Secret key that they will finally come up will be a subset of bit’s exchanged between the two [4].

Popularity of Quantum Cryptography

Quantum Cryptography has already found commercial use and a number of companies are selling QC products. Companies like ID Quantique and MagiQ Technologies are already in this business [5][6]. In Geneva, Switzerland in the year 2007, votes were cast during parliamentary elections using secret key exchanged using Quantum Key Distribution.  This was one of the first public uses of Quantum Cryptography [7]. Apart from this, a lot of popular research is undergoing in this field in various institutions around the world. Los Alamos National Lab researchers claim to make smart phones secure using quantum cryptography [8].

Weakness/Limitations of Quantum Cryptography

Although Quantum Cryptography promises to be more secure than its Classical counterpart but it still has some weaknesses.  As the technology is getting more popular and finding more commercial uses, it is becoming a target for attackers. One of the popular attacks on quantum cryptography in recent times has been the side-channel attack [9]. Apart from certain weakness there are certain limitations associated with this model. Both the receiver and sender needs to have end to end fiber channel between them. Also quantum cryptography focuses more on providing confidentiality and integrity once authenticity has been achieved using classical methods. There is no guarantee of availability i.e. protection against DOS attacks. One major weakness with Quantum Cryptography is failure to detect even secure exchanges due to noise or interference in the transmission medium [10].

Though Quantum Cryptography offers a promise to provide more secure connections than Classical Cryptography but it still has a long way to go before it becomes a real threat to Classical Cryptography. The technology is still in its infancy and a lot of research is still needed to make it more accessible and deployable in organizations. This is not just a competition between Quantum and Classical Cryptography but a competition between Physics and Math. Whatever is the outcome of this competition, ultimate winner will be the organizations looking for more secure means of communications. Finally, I can say that the rate at which Quantum Cryptography is being adopted it’s still a long way before it becomes as pervasive as Classical.


  1. Vittorio, Salvatore. “Quantum Cryptography: Privacy Through Uncertainty”. ProQuest  – Discovery Guides. October 2002. Web. 22 October 2012. <http://www.csa.com/discoveryguides/crypt/overview.php&gt;.
  2. See 1
  3. Bennet, Charles and Brassard Gilles. “Quantum Cryptography: Public Key Distribution and Coin Tossing.” Internation Conference on Computer, Systems and Signal Processing. Bangalore, India: IEEE, 1984.
  4. See 3
  5. MagiQ Home Page. N.p, n.d. Web.  28 October 2012. <http://www.magiqtech.com/MagiQ/Home.html&gt;.
  6. Quantum Cryptography. N.p, n.d. Web. 28 October 2012. <http://swissquantum.idquantique.com/?-Quantum-Cryptography-&gt;.
  7. Josh, Clark. “”How Quantum Cryptography Works”. howstuffworks. ” n.d. Web 24 October 2012. <http://science.howstuffworks.com/science-vs-myth/everyday-myths/quantum-cryptology.htm&gt;.
  8.  Michael, O’Connell. “”Scientists use quantum cryptography to create ‘un-crackable’ phone security”.” federalnewsradio .  23 January 2012. Web. 27 October 2012. <http://www.federalnewsradio.com/241/2717409/Scientists-use-quantum-cryptography-to-create-un-crackable-phone-security&gt;.
  9. Lee, Chris. “”Quantum cryptography: yesterday, today, and tomorrow”.” arstechnica .  17 September 2012.Web. 27 October 2012. <http://arstechnica.com/security/2012/09/quantum-cryptography-yesterday-today-and-tomorrow/3/&gt;.
  10. 10. See 1



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