## 5 Analysis of HWMP Security Protocols
### One Example of the Security Scheme for IEEE 802.11s  
To illustrate how the security requirements (Table 9) and analysis of HWMP Security Protocols (Tables 10-12) are utilized in the security protocol design, in this section, we present a simple security scheme.  
From Table 9, we can identify that the end-to-end authentication and integrity are common security service requirements for the NMF in the PREQ, PREP, RANN, and PERR frames. A digital signature scheme is a solution satisfying these security requirements. The originator of the messages attaches a digital signature for the NMF of each message. By verifying the signature, not only a destination node but the intermediate nodes on the message transmission path can authenticate the origin of the message and confirm the integrity of NMF. Thus, with these security services for the NMF, we can protect against a path diversion attack and an impersonation attack, as shown in Tables 7 and 8.  
From Table 9, we can identify that a forgery detection method is a common security service requirement for MF in the PREQ, PREP and RANN frames. Because the most important information in MF of these messages is the metric, in this example we introduce a new field called the previous node metric (PNM) to detect illegal modification of the metric by an intermediate node. This field carries the metric value of a node one hop ahead to provide to the node one hop behind along the transmission path.  
By utilizing the existing IEEE 802.11i key hierarchy protocol with CBC-MAC (cipher block chaining message authentication code) on the MF of the message, the protocol is able to provide point-to-point integrity. For the broadcast messages (PREQ or RANN), GTK will be used to generate the CBC-MAC, while PTK will be used for the unicast message (PREP). The MF of the message will also be encrypted, just as in the current protection mechanism. In short, the current security mechanism can be used to protect the MF alone instead of the entire message.  
Equations (17-19) provide the aforementioned security features for PREQ, RANN and PREP, respectively. \(Enc_{GTK}(MF)\) and \(Enc_{PTK}(MF)\) indicate that the MF will be encrypted using the GTK and PTK, respectively. \(CBC-MAC_{GTK}(MF)\) and \(CBC-MAC_{PTK}(MF)\) indicate that the CBC-MAC will be generated by using the GTK and PTK, respectively. \(Sign_{P_{0}}(\,)\) represents signing with node \(i\)'s private key. An ID-based signature scheme is useful for the WMN because it does not require public key certificate management and can drive the public key from signer's identity information.  
\[\text{PREQ}:Enc_{GTK}(MF),NMF,CBC-MAC_{GTK}(MF),\ Sign_{P_{P_{ resource}}}(NMF),Sign_{P_{0}}(metric),Sign_{P_{0}i-1}(PNM) \tag{17}\]  
\[\text{RANN}:Enc_{GTK}(MF),NMF,CBC-MAC_{GTK}(MF),\ Sign_{P_{P_{ resource}}}(NMF),Sign_{P_{0}}(metric),Sign_{P_{0}i-1}(PNM) \tag{18}\]  
\[\text{PREP}:Enc_{PTK}(MF),NMF,CBC-MAC_{PTK}(MF),\ Sign_{P_{ resource}}(NMF),Sign_{P_{0}i}(metric),Sign_{P_{0}i-1}(PNM) \tag{19}\]  
A source node initiates a PREQ, RANN, or PREP message with a signature for the NMF \(Sign_{P_{ resource}}(NMF)\) to provide the end-to-end integrity. The source node's public key will then be used to verify the signature at the immediate nodes and the destination node. After receiving \(Sign_{P_{0}}(metric)\) and \(Sign_{P_{0}i-1}(PNM)\), the \(i+1\)_th_ node then verifies these signatures and checks whether \(metric>PNM\) and the difference value is larger than a threshold value. If the verification is successful, it can convince that the _metric_ value of the \(i\)_th_ node is not forged.  
Except for \(Enc_{GTK}(MF)\), \(Enc_{PTK}(MF)\) and NMF in Equations (17-19), the other message fields can be attached to each message as extended fields, such as SAODV.