A distributed Wireless Sensor Network (WSN) is a collection of n sensors with limited hardware resources and multihop message exchange capabilities. Due to the scarceness of resources, the distributed paradigm required, and the threats to the security, a challenging problem is how to implement secure pairwise communications among any pair of sensors in a WSN. In particular, storage memory and energy saving as well as resilience to physical compromising of a sensor are the more stringent requirements. The contributions of this paper are twofold: (1) we describe a new threat model to communications confidentiality in WSNs (the smart attacker model); under this new, more realistic threat model, the security features of the previous schemes proposed in the literature drastically decrease; (2) we provide a new pseudo-random key pre-deployment strategy that assures: (a) a key discovery phase that requires no communications; (b) high resilience against the smart attacker model. We provide both analytical evaluations and extensive simulations of the proposed scheme. The results indicate that our pseudo-random key pre-deployment proposal achieves a provably efficient assignment of keys to sensors, an energy preserving key discovery phase, and is resilient against the smart attacker model.