## 5 Communication Dependability
### Cybersecurity for IEC61850 Multicast Communication in Wide-Area Applications
#### 5.2.1 Secured Messages  
In order to create secured messages, IEC61850-90-5 introduces KDC, which provides security-related information for the securing (digital signature and encryption) of R-SV/R-GOOSE messages. IEC 61850-90-5 recommends a security mechanism based on Group Domain of Interpretation (GDOI) [37], which is a group-key management security framework. This framework is suitable for IP-multicast applications such as wide-area applications in which common security policy and keying materials are shared among group of participants. There are two types [37] of participants in the GDOI framework: Group Controller/Key Server (GCKS) and Group Member (GM). GCKS defines and distributes security policies and keying materials between the GMs. The GMs are authorized members of a secure group, which can send/receive IP packets related to the group. The GDOI framework can be implemented for securing R-SV/R-GOOSE messages in which GMs are IEDs (e.g., PMU or PDC) with GDOI support, and GCKS is KDC, as shown in Figure 7 below. In Figure 7, there are two possible security architectures for implementing KDC: centralized and decentralized. While a single KDC is used for all GMs in centralized architecture, multiple KDCs are used in decentralized architecture to increase their availability in case of a lack of one KDC. Decentralized architecture prevents single-point-of-failure and reduces key storage complexity, but it requires additional work regarding the administration and maintenance of the KDCs. The type of security architecture is selected based on the size of the network and the utility security policy. Figure 7 shows a decentralized architecture in which multiple KDCs are distributed in the substations. KDC functions as GCKS in the GDOI security framework. GDOI security mechanisms consist of two phases: Mutual Authentication and Authorization phase, and the Periodic Security Policies and Key Update phase. Table 4 summarizes the different phases of a GDOI mechanism. \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Served Messages**} & **Secured Messages** & Security by encryption and digital signature of R-SV/R-GOOSE messages via the use of security keys provided by KDC in the Group Domain of Interpretation (GDOI) framework. \\ \cline{2-4}  & **Secured** & Private WAN & Security by GET VPN (Figure 14) \\ \cline{2-4}  & **Communication Path** & Public WAN & Security by IPsec Tunnel (Figure 15) \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Security solutions for R-SV/R-GOOSE communication over WAN. In GDOI phase 1, each substation KDC receives authentication requests from the local IEDs and registers authenticated IEDs to a specific group whose aim is to communicate via IP-Multicast. Phase 1 is based on the Internet Security Association and Key Management Protocol (ISAKMP) [38]. In phase 1, KDC authenticates the substation IEDs with a cryptography technique (for example, pre-shared key, certificate-based, etc.). This can be freely selected by the developer, but the selected technique must provide [37] peer authentication, confidentiality, and message integrity. For instance, the authors of [19] used Diffie-Hellman public key cryptography for GDOI phase-1 authentication, and the authors of [30] explained their certificate-based authentication steps. After successful authentication of the IEDs, KDC distributes the group security policy and keying material to the IEDs over an authenticated and encrypted session [26]. In GDOI phase 2, there are two types of exchanging security policy and keying material: GROUPKEY-PULL and GROUPKEY-PUSH. While the IEDs initiate communication in GROUPKEY  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**GDOI** & \multicolumn{2}{c}{**Phases**} & \multicolumn{1}{c}{**Description**} & \multicolumn{1}{c}{**Key Type**} \\ \hline \multirow{4}{*}{
\begin{tabular}{c} **Pulse** \\ **Pulse**  
PULL, KDC periodically initiates communication to distribute security updates to the IEDs in GROUPKEY-PUSH, as will be explained below.