The last years brought many novel challenges for the Internet of Things (IoT). Low capital and operational expenditures, massive deployments of devices, reliability and security are among the most crucial ones. The recently introduced Low-power wide area (LPWA) technologies provide one possible way of addressing these challenges. In the current paper, we focus on one of the most mature LPWA technology, namely Sigfox. We provide a brief security assessment of this technology and highlight the main security imperfections. Notably, we also consider the recent changes introduced in the last revision of the Sigfox specification released in the fourth quarter of 2017. Importantly, this paper discusses the highlighted issues and compares three selected cryptographic encryption solutions (AES, ChaCha and OTP) in respect to the main IoT triad of performance, security and cost. We investigate the encryption solutions and characterize their energy consumption in a real-life implementation. The results herein presented are useful for understanding the cost of enabling security aspects and enable selecting the most efficient encryption protocol.