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The IEEE 802.15.4 standard specifies an M-ary spread spectrum system with public and fixed spreading sequences. We propose instead to use secret and dynamic, random spreading sequences to obfuscate communications. Through theory and experiments with a prototype IEEE 802.15.4 transmitter and receiver that have been adapted to use random and dynamic codes, we quantify the benefits and performance implications of our approach. We first give a theoretical approximation of the performance loss when secret random codes are used instead of the IEEE 802.15.4 codes. We identify then three transitional regions for IEEE 802.15.4 communication above thermal noise i.e., when the incoming power level at the attacker is above thermal noise. We derive the obfuscation gain for signal detection and signal interception attacks respectively in these regions. Our implementation shows the feasibility of our approach. It is also used to evaluate the performance loss and time requirements to perform attacks on the system under realistic wireless conditions.