## 4 Proposed Key Management Infrastructure Design for IACS
### Key Establishment Phase
The key establishment process follows the same procedure as described in the control level device key establishment process (Section 4.3). * Typically, the higher level devices of the automation networks have more relaxed constraints in terms of storage, communication and computation capacity, processing speed, etc. as compared to the lower level devices. Therefore, plant-level devices are responsible for initiating communication between plant level and control level devices. Figure 6: Secure Communication between field and control level devices. Use of symmetric method (matrix method) between Plant and Control level devices involves less computation and communication cost. To handle multiple applications and to reduce the impact of network compromise, the grouping of devices can be done, as discussed in Section 4.11 (Figure 6). ## 5 CKMI Design and Its Features

Figure 7 depicts our overall design of the CKMI. It has plant level network (PN), control level network (CN) and field level network (FN) and all the key management operations. The proposed CKMI design features are summarized as follows:

* Handles all common key management operations: The CKMI server plays a key role in all key management operations such as Key generation (Algorithm 1), Device registration/Device joining (Algorithm 2), Key establishment (Algorithm 3), Key update (Algorithm 6), Key recovery (Algorithm 7), Device addition (Algorithm 4), Key revocation (Algorithm 5), Key storage, Key archival and Key de-registration and destruction. * The proposed design handles secure communication between the same and different levels of IACS devices. * The higher is the level of the network in the hierarchy in IACS (from a field to plant network), the more the constraints are relaxed for the devices in terms of latency, computation and communication overhead, storage, etc. According to the constraints, the appropriate crypto mechanism is considered for each layer of IACS. * Based on the resource configuration details of the devices, a suitable crypto mechanism is employed for the devices. A symmetric crypto system is employed for field and control level devices. An asymmetric crypto system is employed for plant level devices. * Our design follows a centralized key management aspect. It offers the following benefits:
* It provides a place to define unified security policies. * It provides a higher degree control of crypto keys and hence provides operational efficiency. * It provides transparency in auditing, storage and backup processes with clear separation of tasks. * A single point of failure of CKMI server can be handled with a secondary CKMI server. * Only communications between devices authenticated by the CKMI server will occur in the network. For any device to communicate with another device, they must participate in the secret key generation. With suitable pre-shared parameters and message exchanges, the same secret key is generated on both the devices. * The secret key is never transmitted over the network during the key establishment phase. Thus, the chances of exposure of secret keys are reduced. * The hierarchical industrial architecture and separate crypto systems for each level of devices avoid the disruption of service due to key management in the case of device failures. In other words, device failures will not affect the comprehensive key management infrastructure operations, although the communication with that device is lost. * The proposed scheme minimizes the number of communications for setting up the keys, which is generally one of the most resource consuming operations in these types of networks. * Scalability is efficiently achieved at all levels of IACS. * Device addition is supported. This will not disturb the existing communications, and no modification is required in the existing devices. * Device deletion, i.e., key revocation is handled to stop the communication between the deleted device and the existing devices. * Replay attack is eliminated using nonces during key establishment.