Automation and especially the advent of the computer have provided cryptography with complex mathematical algorithms. In World War II, the British gained fame for their skill in breaking German codes: this weapon used by the allies is called cryptanalysis. It must have been this duel in the sphere of espionage that inspired scientists to adopt the names Alice and Bob, instead of mathematical symbols such as “A” and “B” to identify the origin and reception of the message, putting Eva on the wire to assess the risks of interception.
Now living in a time where information is synonymous with power, Alice and Bob have become more sophisticated, and so has Eve. From the privacy of bank statements to the encryption of Internet purchases, everything requires defense against potential interference. Professor Antonio Vidiella Barranco, from the Department of Quantum Electronics at the Gleb Wataghin Institute of Physics (IFGW) at Unicamp, explains that encryption has reached a refinement that, in practice, makes it impossible to break a message, given that current computers would take years to perform the mathematical operations required to decipher a code. In theory, however, quantum computers are already being perfected and would be capable of carrying out such operations in seconds, much to Eva's delight. That's why physicists, engineers, mathematicians and computer scientists are searching for a new security key.
Against the risk posed by such computational power, quantum cryptography was developed, which would be immune to security flaws. Antonio Vidiella, one of the few researchers working on the subject in the country, believes he has devised a new protocol involving laser beams that has numerous advantages over others studied in the world. And he is in a hurry to finish an article for publication, before Eva hears us. “The work was only presented at the 27th Condensed Matter Physics Meeting in Poços de Caldas, at the beginning of May. This is a concrete proposal, which still needs to be improved and tested in the laboratory regarding safety issues, but which can perfectly be developed in Brazil. Our objective is to generate a protocol to be effectively used, such as in the transmission of government data or in any operation that requires confidentiality”, says the researcher.
Story – To make the new proposal understandable, Vidiella goes over at least the basics of recent cryptography. He explains that, at the beginning of the 20th century, a way was found to transmit a message in an indecipherable way, using binary code, basically a random sequence of zeros and ones, which would be the key. “Alice performs an operation involving her message with this random sequence, forming the encrypted message that is transmitted to Bob. It has been proven that if there is a sequence of numbers with the same size as the message, it is impossible to decipher it, as long as the key is not known”, says the IFGW professor. However, how does Alice get the key to Bob so he can decrypt the message? “The distribution of the key among stakeholders was the problem that arose,” he notes.
Alice and Bob, in this case, would have to meet to share the key, running the risk of interception. If Alice chose to use a messenger, he could well be bribed by Eva. A website for the Association of Mathematics Teachers has an illustrative story in which Alice and Bob live isolated and can only communicate by mail. But they know that the postman reads all the letters. Alice thinks about sending the letter inside a safe, locked with a padlock. But how do we get the key to Bob so he can open the safe? Simple: Bob receives the safe and locks it with a second padlock, to which he has the key. He sends the safe with the two padlocks by mail. Alice removes a padlock with her key and returns the safe. Bob just has to use his key to open his lock, while the postman watches ships.
Public key – This little story inspired the creation of the RSA system, by three young Americans, Whitefield Diffie, Martin Hellman and Ralph Merkle, and is widely used today. According to the idea, in order for Bob to receive a message, he first chooses a secret key, from which he constructs a public key. Alice uses this public key to encrypt the message and send it to Bob, who in turn decrypts the message using his private key. Eva, even having intercepted the public numbers, will still see ships. The keys are not exchanged, but both Alice and Bob end up being able to open the safe, without Eva being able to do so.
We then have the public key system, which serves banks, the Internet and many other services. “By putting some theorems behind this logic, we arrive at combinations of prime numbers that no computer today is capable of factoring quickly. This encryption is very convenient, because I can send messages to several people”, observes Vidiella. Even this encryption, however, presents a potential weakness, which is uncertainty regarding its security. According to the researcher, the quantum computers in development would have a very high parallelism capacity, being able to factor huge numbers into prime factors with a speed of just a few seconds. It is in the search for an alternative that fits your work.
BB84 Protocol – “Twenty years ago, physicists Bennett and Brassard proposed a new form of cryptography, quantum. This protocol was called BB84. In it, security is based on the laws of nature, on natural limitations. The values of physical quantities cannot always be accessed with absolute precision. This is evident in quantum mechanics, where from the beginning it was obvious that access to information from microscopic systems, in general, was limited. The idea is to apply these ideas to transmit secure information. In fact, quantum cryptography is a way to distribute keys”, explains Vidiella.
For this distribution, the quantum properties of light would be used. Alice generates light with certain characteristics, transmitting it to Bob, who performs certain measurements. Alice knows the parameters to encode the key information she used and Bob writes down what she measured. Alice and Bob then establish additional communication, through a public channel, discarding a series of bits and distilling the key that will be shared. As Eve may be intercepting the light transmission, this will cause interference that will certainly be noticed by Bob. “It turns out that the new protocol requires a very special light source, a single-photon source, generated by a stimulated atom or quantum dots. We have already come close to it, but there is a need to obtain controlled light, in which it is possible to generate one photon at a time, which is not easy to do”, he says.
Simple idea – Another problem is how to propagate this light, whether through air or optical fiber. Groups from Switzerland, which used an attenuated laser, managed to transmit this quantum key within a radius of 100km via optical fiber. Over the air it is more complicated, but there are already transmissions of up to 10km. “I realized that it would be interesting to move away from proposals researched abroad, such as the attenuated laser, and look for other protocols that would adapt to the laser light source that we have already mastered and that is easier to generate and control”, recalls the professor, who decided dive into the topic together with doctoral student Luiz FM Borelli.
The idea, which emerged surprisingly naturally given so much effort by the scientific community, is to make use of laser polarization, as the direction of the electric field is perfectly controllable. “I can send a beam horizontally, vertically, however I want. The receiver can easily measure this polarization, where the key would be encoded. Furthermore, the data transmission rate using single-photon sources is low. It's like sending one bit and then another, a limitation when dealing with large volumes of information. With the laser the rate is much higher”, explains the researcher. Antonio Vidiella hopes to complete the theoretical work in a month, with the caveat that the issue of security is more delicate. “Every protocol is susceptible to attacks. It is an embryonic proposal, with great prospects, but we need to confirm, both theoretically and experimentally, that security will be good enough”, he concludes.
VLet's enter the world of Alice, Bob and the spy Eva, well-known characters in cryptography who disturb the minds of researchers absorbed in developing message transmission systems with complete security, whether over the air or via optical fibers. Cryptography, the science of transforming original texts into unreadable information, is as old as writing itself. There was a time when Julius Caesar, Roman emperor, sent encrypted messages on horseback, where one letter had to be replaced by another that was three positions ahead in the alphabet, in the naive conviction that, if the messenger was captured, the enemy would not be able to Read the tactical order given to win the battle. 
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