In this paper, we propose SR3 (which means Secure Resilient Reputation-based Routing), a secure and resilient algorithm for convergecast routing in WSNs (Wireless Sensor Networks). SR3 uses lightweight cryptographic primitives to achieve data confidentiality and unforgeability. Security of SR3 has been proven formally using two verification tools: CryptoVerif and Scyther. We made simulations to show the resiliency of SR3 against various scenarios, where we mixed selective forwarding, blackhole, wormhole, and Sybil attacks. We compared our solution to several routing algorithms of the literature. Our results show that the resiliency accomplished by SR3 is drastically better than the one achieved by those protocols, especially when the network is sparse. Moreover, unlike previous solutions , SR3 self-adapts after compromised nodes suddenly change their behavior.