Mobile computing technology is reaching every corner of our lives. Smart phones, tablets, laptop computers are just a few examples of the most known applications. Recent advances in the ultra-low power technologies enabled the development of even smaller, more mobile, autonomous devices. Wireless Sensor Networks (WSNs), Smart Dust, and Radio Frequency Identification (RFID) are several examples of this trend and have been applied to a large number of areas and will be more and more popular for various applications. Security is a critical factor to many applications due to the impact on privacy, trust and control, and is also important for many applications powered by the ultra-low power devices. Ultra-low power devices are highly constrained in terms of resources, such as they have insufficient computing and storage capabilities. Therefore, it is a challenge to implement security affordable and efficient, and meet the security requirements. This dissertation presents a suite of cryptographic mechanisms, including a cryptographic hash function, a construction of one-way hash chains, a dynamic access control, and a secure data transmission protocol to offer affordable, efficient but necessary security protection to ultra-low power devices to meet their network security requirements. More specifically it is to provide data confidentiality, data integrity, authentication, and access control in their data transmission, for secure data unicast, secure data broadcast, and secure data multicast.