A key characteristic of Smart Cities is the ability to reduce conflicts between different agents coexisting in a dynamic system, such as vehicles and pedestrians. This paper describes a system to augment the awareness of vehicle drivers regarding the presence of pedestrians in nearby crosswalks. To address this need, the system interconnects crosswalks to Road Side Units (RSUs), and RSUs to vehicles, in order to spread to vehicles the information about pedestrians in crosswalks. To prevent false information spreading, RSUs authenticate the alert messages they broadcast and all vehicles can validate them. This poses strong security requirements, such as non-repudiation of alert messages, as well as strong real-time requirements, such as minimum message propagation delays among vehicles approaching a crosswalk of interest. Because of this, the system minimizes the usage of asymmetric ciphers, which are fundamental to assure non-repudiation but increase performance penalties, and uses mostly symmetric ciphers for source authentication. To do so, we designed a new protocol, called Nimble Asymmetric Cryptography (NAC), which is useful to broadcast authenticated, implicit messages.