Ultralong Fiber Laser for Secure Communications

The aim of this research is to explore fundamentally new secure information technologies in optical engineering and to develop a practical framework for the design of secure communication systems based on using an ultralong fiber laser as a transmission medium.

One of the key challenges in modern developments of information technologies is to ensure reliable and Highly secure communications for public, business and government activities. The main Achilles’ heel of many of the contemporary encryption methods is that they require the two parties (Alice and Bob) to share a secret key before the secure communication can take place and in many practical scenarios this requirement is difficult to be realized. This obstacle, known as the key-distribution problem, has attracted much attention over several decades and much work has been devoted to resolve it. In particular, substantial efforts were focused on physical-layer based cryptographic protocols such as quantum key distribution, lasers synchronization in the chaotic regime, speckle pattern based optical one-way functions, and more recently – symmetry based key generation systems utilizing thermal fluctuations or ultralong fiber lasers (UFLs). Many of these physical layer encryption/key distribution schemes suffer from practical constraints which severely limit the achievable performances in terms of data-rate, range and cost. In addition, most of these schemes (including QKD) have been proven to be breakable by taking advantage of the non-ideal nature of the components comprising the scheme. The UFL key distribution scheme, which is the youngest member in this family, exhibits the greatest potential in terms of range, key-rate and cost. Very recently, UFL based secure and error-free key distribution was demonstrated over a 500km long link with key-rate of 125 key-bits per second – substantially outperforming all other physical-layer KDS. In addition, it was shown that the probability of a prospective eavesdropper, employing a variety passive attack strategies, to recover a key-bit can be reduced below 55% (where 50% indicates unbreakable scheme).

The UFL-KDS consists of a long fiber laser with Alice at one end and Bob on the other. Each of the two parties controls the reflection spectrum of one of the laser end mirrors while the laser cavity serves as a communication link between them. To exchange a bit, each party randomly selects a bit and encodes it (using his/her end mirror) into the laser. The lasing characteristics allow the two parties (and a potential adversary) to determine only whether they chose identical or opposite bits. Since each party knows his/her own bit they can deduce the other party’s choice and to exchange a bit. An adversary can only deduce whether Alice and Bob succeeded in exchanging a bit, but not to determine its value. The low cost and superior performances (compared e.g. to QKD and KJLN) of the UFL-KDS render it highly attractive for practical implementation. Nonetheless, the resilience level of the scheme to cryptographic attacks should be studied more thoroughly in order to identify its potential vulnerabilities and limitation and to develop appropriate protective mechanisms. The preliminary analysis demonstrated the resilience of the UFLKDS to a wide variety of passive attacks. However, the system might be vulnerable to other attack strategies, in particular active attacks. Unlike passive eavesdropping where the adversary tries to extract information without altering or modifying it, actives attacks may involve external injection of (optical) power into the systems, attempts to impersonate a legitimate user (e.g. man-in-the-middle attack) etc. Active attack strategies are generally more diverse than passive ones and may require reinforcing of the UFL-KDS security. The main objectives of the proposed exploratory program are to investigate the resilience of the scheme to a variety of active attacks and to obtain quantitative metrics to information of the key that might be obtained by a potential adversary. The research will include both theoretical and experimental efforts to study the system resilience to eavesdropping and for developing appropriate countermeasures.

Although the main physical platform of the proposed research is fiber optics, the scheme can be implemented for diverse applications such as on-chip and board to board secure communication using dielectric integrated waveguides and semiconductor optical amplifiers. Such applications are in particular attractive because of the technological maturity and relatively low cost of such components, as well as the compatibility with conventional processes for CMOS technology.