The elevated concern of cyber security throughout the power industry has created a need to protect communications to and from substations. This trial use standard defines a cryptographic protocol known as Substation Serial Protection Protocol (SSPP) that can protect the integrity and optional confidentiality of asynchronous serial communications typically used by substation equipment. SSPP is primarily intended to protect serial SCADA communications, but can be applied to other serial communications - such as the maintenance ports of IEDs. SSPP is largely independent of the underlying communications link and protocol (e.g. Modbus, DNP3), and is appropriate for serial communications over leased lines, dial-up lines, multi-drop links radio, power line carrier, fiber optic, etc. SSPP is suitable for implementation in new equipment or for deployment in bump-in-the-wire devices retrofitting protection to existing systems.