In this paper, the secure low-power Internet of Things (IoT) transmission methods for encryption and digital signature are presented. The main goal was to develop energyefficient method to provide IoT devices with data confidentiality, integrity, and authenticity. The cryptograph energy efficient and security algorithms modifications for IoT domain were made. The novelty in our solution is the usage of encryption method popular in the image processing in the domain of the Internet of Things. Proposed modification improves immunity for the brute-force and plain-text attacks. Furthermore, we propose the modifications for hash calculation method to transform it into digital signature calculation method that is very sensitive to input parameters. The results indicate low energy consumption of both methods, however it varies significantly depending on the architecture of the devices.