By allowing the mixture of information at the source and intermediate nodes, network coding benefits network protocols with increased throughput and higher reliability. However, such mixture makes network coding systems suffering pollution attacks, in which malicious nodes inject corrupted packets into the information flow. Previous solutions are either computationally expensive or too ineffective to limit pollution attacks with arbitrary collusion among malicious nodes. In this paper, we propose an efficient authentication scheme, STNC, which allows intermediate nodes to efficiently detect corrupted packets by using space and time properties of network coding. Our work is an innovative space and time based solution to frustrate pollution attacks with arbitrary collusion among malicious nodes, and the security of STNC scheme relies on the asymmetry of time in packet verification. We also present security analysis and simulations of our scheme, and results demonstrate the practicality and efficiency of STNC scheme.