## 4 Proposed Key Management Infrastructure Design for IACS
### Key Establishment Phase
### Key de-Registration and destruction

CKMI server removes all the traces of keying materials from local storage, backup and archive when the usage of keying materials is no longer required. In the case of field level devices, the keying materials are polynomial key establishment parameters. In the case of control level devices, the keying materials are matrix key establishment parameters. In the case of plant-level devices, the keying materials are ECDH key establishment parameters. ### Across Field Level and Control Level Devices

Along with supporting secure communications between the same level devices, the proposed scheme handles secure communications between different level devices. For secure communication between the control and field level devices, our design uses a polynomial method. * Workflow During the device registration phase (Section 4.2), along with storing the keying materials of matrix method, polynomial method parameters (a polynomial share and the ID used for generating the polynomial share) are also stored in the control level devices. The control device uses matrix method parameters to communicate devices belonging to the same level network. The control device uses polynomial method parameters to communicate with field level network devices. The key establishment process follows the same procedure as portrayed in the field-level device key establishment process (Section 4.3). * Let us consider the scenario shown in Figure 6. The control level devices are represented by CD1, CD2,... CDn. The field level devices are represented by FD1, FD2,..., FDn. The devices CD1, FD1,..., FDi are considered as Group 1. The devices CD3, FDi+1,..., FDn are represented as Group 2. In this scenario, the field devices FD1, FD2,... FDi need to communicate only with the control device CD1. The field devices FDi+1, FDi+2,... FDn need to communicate only with the control device CD3, i.e., intergroup communication is not allowed. Hence, the CKMI server
can use different polynomial equations for Group 1 and Group 2. In the key distribution phase, along with storing matrix method parameters, the device \(CD1\) is also stored with one polynomial share of the polynomial equation, which is considered for Group 1 devices. Using the polynomial share, the device \(CD1\) can communicate with field devices (_FD1_, _FD2_,... _FD1_) and vice versa. Similarly, \(CD3\) is also stored with one polynomial share of the polynomial equation, which is considered for Group 2 devices. Using the polynomial share, the device CD3 can communicate with field devices (_FDi+1_, _FDi+2_,... _FDn_) and vice versa. * If similar applications are running in the industrial plant, the CKMI server can use the shares of the same polynomial equation for different groups. It enables communication between different groups of devices under different control level devices. * If applications are different between the groups, it is better to use polynomial shares of different polynomials for each group of devices. This avoids the communication between different group devices, which are running different applications. * Use of different polynomial equations for each group also reduces the impact of a network compromise. This is because, if any device is compromised, the impact will be on the compromised group. Other groups are safe and, hence, can continue their operations. ### Across Control Level and Plant Level Devices

The matrix method is used to support secure communication between the control level and plant level devices. * Workflow During the device registration phase, along with storing the keying materials of ECDH method (Section 4.2), matrix method parameters (a secret row and a seed value) are also stored in the plant level devices. The plant device uses ECDH method parameters to communicate with devices belonging to the same level network. The plant device uses matrix method parameters and vice versa to communicate with control level network devices.