Many Wireless Sensor Networks (WSNs) employ security schemes based on the symmetric encryption, which requires the establishment and management of cryptographic keys. This paper presents a new key management scheme based on Random Key Pre-Distribution. The novelty of the proposed approach is the introduction of a keyed transformation with a Transitory Master Key, which is exploited by the nodes in order to increase the number of different keys employed in the network. The presence of a larger pool of keys provides more robustness and connectivity than previously proposed Random Key Distribution schemes. Furthermore, random pre-distribution allows recovering secure communication also when the master key has been compromised.