Safety in the Internet is an extremely important issue. However, the war between cybercriminals and security companies is still continuing. New technologies provide new methods of Internet data protection, and thanks to the efforts of scientists and engineers those methods enter the market quickly. This is how quantum physics makes criminals’ lives harder.

The use of a wide variety of electronic services exposes us to the possibility of someone stealing our information during data transmission. In many cases this is not only a privacy-related problem but a problem related to the safety of our finances as well. Regardless of our activity on the Internet, it is essential to ensure the highest possible level of security. The encryption of all Internet-transmitted data is necessary to achieve this. The biggest problem is that even if the data is encrypted, the key needs to be transmitted between computers. It is the most sensitive part of the whole process. However, there is an innovative method allowing for safe transmission of the key, and the guarantee is not a company certificate but the laws of quantum physics. The safety of these methods is guaranteed by the basic laws of physics, not by the limited technical capacities of a person intercepting data (e.g. computing capacity), as happens in case of classic methods.
This unique method is called Quantum Key Distribution (QKD). Among its many types, the most common are the BB84 protocol and the E91 protocol. I would like to focus on the use of the BB84 protocol.

The name of the BB84 protocol comes from the names of its creators – Charles Bennett and Gilles Brassard, and the year of invention – 1984. This protocol uses the transmission of photons, light particles, as in common telecommunication, with a difference that here a single photon is a single bit of code. Photons are sent over long distances using fibres.

To put it in simple terms, each photon can be found in one of the four states (polarisations), and the states are in pairs: 0° and 90° (horizontal/vertical base) and 45° and 135° (diagonal/anti-diagonal base). In each pair, one state is determined as logical 1, and the other as 0. For different pairs of states, different reading bases are used (the state read by an inadequate base gives a completely random measurement result). The sender is usually named Alice (A), the recipient is named Bob (B), and the person who intercepts the transmission is named Eve (E). Alice sends Bob the key; during the key transmission Alice randomly assigns a horizontal/vertical base or a diagonal/anti-diagonal base; then Bob also randomly assigns a base and measures the state of the photons. Then Bob and Alice compare the bases they used, using a public channel.

Only measurements with compliant bases are valid. Thanks to the differences in measurement for the compliant bases, Alice and Bob can detect Eve. The laws of quantum physics do not allow Eve to hide that she has been intercepting data.

However, due to the faults in fibre transmissions, it is necessary to take account of errors and correct them.
Classic algorithms, which can compare the keys with great accuracy, are used for that purpose. The only weak point is the fact that in the process a number of bits are lost, which reduces the relative speed of key transmission.

The BB84 protocol already has a number of practical applications and is used for commercial purposes. It can secure bank transactions, for now only in some countries and banks. That protocol is also used by some governmental agencies to secure the most confidential information. It is used to secure elections and to create secure networks. Unfortunately, the QKD is still not a widely used method due to its high cost and low transmission speed (about 1Mb/s for distances ~50km). However, quantum hackers have already been emerging and the war is being taken to a completely new level. Nevertheless, it is no longer as easy as it used to be for cybercriminals, since the laws of physics constitute a massive barrier that provides, at least in theory, complete security.