Supervisory Control and Data Acquisition (SCADA) systems are responsible for automatic control of several critical infrastructures like power grids. An important requirement of SCADA systems is to provide cryptographic protection of its communications. Many key management schemes have been proposed to provide cryptographic protection. But cryptographic modules are seen as more appropriate solution as it provides cryptographic protection of keys. Recently I has proposed a draft standard SCADA cryptographic module(SCM) which provides direct link security. The goal of this research is joint design of a comprehensive key distribution scheme for SCADA systems, and the architecture of simple SCM to support key distribution and mutual authentication of messages exchanged between any two SCADA entities. Furthermore, unlike the I module, our SCM will be transparent to the type of SCADA communication protocol employed, and will not be restricted to the Bump in the Wire (BITW) mode of operation.