—Global Navigation Satellite System (GNSS) signals serve as a worldwide timing reference in numerous technological sectors. Yet GNSS receivers are vulnerable to so-called spoofing attacks that can manipulate the time reference. We illustrate the need for a probabilistic security model in the context of authenticating a timing signal as opposed to the traditionally non-probabilistic security models of message authentication and cryptography. Our primary contribution is establishing the necessary conditions for timing assurance in the context of securityenhanced GNSS signals. In addition, we formulate a probabilistic framework for timing assurance that combines cryptography and statistical signal processing across multiple network layers.