The purpose of this study is to realize a practical method of providing cyber security for power system communications. Considering communication requirements needed for Smart Grids, power system communications can be classified into five main categories: real-time protection, supervisory control and data acquisition (SCADA), remote access, demand management system (DMS) and advanced metering infrastructure (AMI) . This paper describes a concept and method of effectively applying information-theoretic cryptography to these power system communications. From the requirements of real-time communication and long-term maintenance of such systems, we explore a novel scheme based on unconditionally secure authentication codes (A-codes) having short authenticators (authentication tags) and a feasible number of keys, and have developed a data format for the scheme. The efficiency of the scheme is illustrated using several examples and experimental results.