NAVIGATION MESSAGE AUTHENTICATION FOR NEXT GENERATION GNSS

Abstract

Since the early days of GNSS, one of the main issues has been the need to protect the final user from attackers trying to emulate the data sent by the real system, the so-called spoofers. This can be done in particular adding the system the function of navigation message authentication, that represents the subject of this paper. The paper introduces a general approach to the problem, to come to a detailed real-world in-orbit case, that is the Galileo Navigation System. This kind of authentication feature is in fact foreseen for the evolution of the system towards its own second generation, evolution that is currently ongoing. Recent research (performed by Wiser and TASI in the frame of internal R&D activities) within the GNSS evolution study for Galileo Second Generation as well as GPS evolution, are investigating the possibility to have a more robust and reliable signal through the broadcasting of navigation message authentication bits. The paper describes a possible solution to the problems wherein the authentication bits are included in a dedicated channel that is added on top of the current signal-in-space format using IBOC multiplexing. This represents an alternative solution to the standard idea of reserving some bits of the current navigation message for authentication. The advantage of the dedicated channel is relatively clear: the bit-rate dedicated to authentication is considerably larger than that resulting from the reserved bits in the current message, thus providing a higher security level, and lower time to first authenticated fix (TTFA)/time between authentication (TBA). This allows in particular to easily meet the latest recommendations stating that (at least) a security equivalent to a symmetric algorithm with 128-bit key is needed. Concerning the authentication algorithm, the present study proposes a combination of the TESLA protocol with a public-key protocol (such as ECDSA). This allows to improve on a weakness of TESLA that requires time synchronization: albeit loos, it cannot guaranteed at receiver start-up, and in addition the TESLA root key shall be authenticated anyway. The optimal combination is found based on a trade-off between security strength, bandwidth overhead, authentication error rate, and time between authentication. Our proposal may pave the way towards an Open Service Navigation Message Authentication (NMA) solution to be broadcast on future GNSS signal channel using IBOC multiplexing solution. The proposed NMA criteria are based on the mutual combination of Elliptic Curve Digital Signature Algorithm (ECDSA) and TESLA authentication protocol. NAVITEC 2018 The solution foresee the use of ECDSA in a first phase when time synchronization between the sender and the recipient is not available, to switch afterwards TESLA when time synchronization is achieved. The paper concentrates on a specific solution wherein iBOC multiplexing on the E1 channels is supplemented with NMEA. This feasibility assessment is just a case study to show the generality of the approach and does not preclude any other implementation for future GNSS signals. The paper is organized as follows: first a summary of the authentication protocols are provided, focusing on the TESLA one, and the IBOC multiplexing scheme for GNSS evolution signals. Then a proposed solution is detailed, together with associated performance, and finally the conclusions are draw