This document proposes a key exchange mechanism for wireless sensor networks that reduces the number of keys each sensor node must store. It presents a protocol where each sensor node stores (n+1)/2 keys, rather than the n-1 keys required in the original protocol, where n is the number of sensor nodes. The protocol uses a "key sender" that distributes keys to sensor nodes. It proves this approach is optimal by showing any secure keying protocol must store at least (n-1)/2 keys per node. The protocol provides mutual authentication and secure data exchange between neighboring sensor nodes using these shared keys.