Network coding provides a promising alternative to traditional store-and-forward transmission paradigm. However, due to its information-mixing nature, network coding is notoriously susceptible to pollution attacks: a single polluted packet can end up corrupting bunches of good ones. Existing authentication mechanisms either incur high computation/bandwidth overheads, or cannot resist the tag pollution proposed recently. This paper presents a novel idea termed “padding for orthogonality” for network coding authentication. Inspired by it, we design a public-key based signature scheme and a symmetric-key based MAC scheme, which can both effectively contain pollution attacks at forwarders. In particular, we combine them to propose a unified scheme termed MacSig, the first hybrid-key cryptographic approach to network coding authentication. It can thwart both normal pollution and tag pollution attacks in an efficient way. Simulative results show that our MacSig scheme has a low bandwidth overhead, and a verification process 2–4 times faster than typical signature-based solutions in some circumstances.