The book "Energy & Time Performance Metrics in Wireless Sensor Networks" offers a comprehensive analysis of the challenges and solutions in the field of wireless sensor networks (WSNs). These networks consist of a multitude of resource-constrained nodes distributed in various environments, often in remote and hard-to-reach areas. The authors highlight the security issues exacerbated by the specific characteristics of WSNs, such as their wireless nature, limited battery life, and node mobility. The book introduces the concept of the "Cryptography Assistant," a powerful device capable of implementing complex cryptographic algorithms. The focus is on the energy and time trade-off of encryption algorithms and file communication, which is crucial for developing effective security solutions in WSNs.