To manage the secret keys, a good key management system is essential. Key management deals with taking the responsibility to manage the crypto keys during their life cycle in the crypto system. It provides a complete paradigm to handle the activities of the crypto keys, including key generation, distribution, storage, backup, archive, recovery, revocation, and destruction [10]. Figure 2 depicts a view on general key management states. The description of key management states are discussed in Table 1.  
Figure 2: Key management states. _Our Contribution:_ In this paper, by considering the generic architecture of IACS, a comprehensive key management infrastructure is designed (Figure 8). The CKMI design enables interoperability and flexibility for securing the industrial applications. The core contributions of the paper is discussed as follows:  
1. The paper handles all the key management states in one design. NIST Special Publication 800-57 summarizes the key management states as follows: key generation, device registration, key establishment, key storage, device addition, key revocation, key update, key recovery, key archival, key de-registration and destruction. To the best of our knowledge, none of the schemes handle all key management states in one design. This motivates us to propose a comprehensive key management scheme, which covers all the key management states under one design. 2. The proposed design shall be applicable for diverse industries. We have considered generic industrial automation networks. The network consists of three levels: field network, control network and plant network. The bottom level is field network; middle level is control network; and plant network is on the top of the hierarchy. Most of the industrial network follows the same hierarchy. In this paper, key management is designed in such a way that it can be applied for major industrial networks to secure the communications of industrial devices. 3. The proposed design supports secure communication between the same and different levels of IACS devices. It supports secure communication among same level devices and also across different level devices. The proposed design considers all the communication scenarios: 1. Among the field level devices 2. Among the control level devices 3. Among the plant level devices 4. Across field level and control level devices 5. Across control level and plant level devices
4. The proposed design does not adopt the same security mechanism for all levels of IACS. IACS includes various devices: field level, control level and plant level devices. The resource constraints at different levels of the automation system are different. The devices at each level differ in their capacities such as storage, communication, computation, accessibility, power requirement and latency. Field network devices are resource constrained devices. Hence, considering the parameters such as low latency, low jitter and computation and communication cost is important. At the top level of the pyramid, the devices are resource efficient when compared to field network devices. Hence, the requirement of real time properties and parameters such as delay and jitter has relaxed