We consider a K-receiver wiretap broadcast channel where Kw receivers are weak and have cache memories and Ks receivers are strong and have no cache memories. We derive an upper bound on the secrecy rate-memory tradeoff under a joint secrecy constraint and under decentralized caching. In contrast to previous works, prefetching in our scheme is purely decentralized and receivers randomly sample from a random key stream available at the transmitter and from the files in a library. For small cache sizes, the performance of our scheme improves with increasing length of the random key stream. For moderate and large cache sizes, a small key stream suffices to perform close to the information-theoretic limit of the system.