Sequential logic encryption is a countermeasure against reverse engineering of sequential circuits based on modifying the original finite state machine of the circuit such that the circuit enters a wrong state upon being reset. A user must apply a certain sequence of input patterns, i.e., a key sequence, for the circuit to transition to the correct state. The circuit then remains functional unless it is powered off or reset again. Most sequential encryption methods require the correct key to be applied only once. In this paper, we propose a novel Sporadic-Authentication-Based Sequential Logic Encryption method (SANSCrypt) that circumvents the potential vulnerability associated with a single-authentication mechanism. SANSCrypt adopts a new temporal dimension to logic encryption, by requiring the user to sporadically perform multiple authentications according to a protocol based on pseudo-random number generation. We provide implementation details of SANSCrypt and present a design that is amenable to time-sensitive applications. In SANSCrypt, the authentication task does not significantly disrupt the normal circuit operation, as it can be interrupted or postponed upon request from a high-priority task with minimal impact on the overall performance. Analysis and validation results on a set of benchmark circuits show that SANSCrypt offers a substantial output corruptibility if the key sequences are applied incorrectly. Moreover, it exhibits exponential resilience to existing attacks, including SAT-based attacks, while maintaining a reasonably low overhead.