## 3 Background and Related Works
### HIP DEX Distribution Scheme
#### 4.1.2 New HIP DEX Secure Lightweight Association Establishment Phase  
On the issue of the Secure Lightweight Association (SLA) establishment phase, the peers establish the HIP association for exchanging data over a secure communication channel. The HIP DEX SA may be concluded in two steps. Firstly, the initiator and the responder agree together publicly on an Elliptic Curve. Then, both the Initiator and the Responder choose random values \(x\) and \(y\), respectively, and also publicly choose a point P from the curve. These two exchanges do not need any secure channel between the initiator and the responder. The most expensive computational part of all elliptic curve cryptosystems is the scalar multiplication in the form of \(k\,\times\,P\), where \(k\) is an integer and \(P\) is a point, from the curve, called the generator. For this reason, we propose to delegate all scalar multiplication operations to the collaborator node (6 BR in our solution). By definition, the node initiating a HIP DEX is the Initiator, and the peer is the Responder. This distinction is negligent once the handshake is taken place and the communication is operational. We propose, in this paper, a modification of the standard's HIP DEX SA to alleviate the overhead of scalar multiplication in the HIP peers during the computation of the Host Identities (HI). To our knowledge, we are the first to propose such a solution in HIP DEX and with a minimal number of collaborative third parties (only one). The modification of the proposed SA is shown in Figure 7. The new proposed authentication process is as follows:  
1. The process starts with sending a trigger packet, II, by the initiator to the responder;
2. After receiving II and a successful mutual authentication with the CTTP, the responder sends the R1 packet containing the puzzle and (\(y\), \(P\)) parameters to the CTTP;
3. The CTTP performs the scalar multiplication operation \(PK_{resp}\) = \(y\,\times\,P\) where the resulting \(PK_{resp}\) is the responder's public key (HI). Then, the CTTP forwards R1 packet to the responder;
4. The initiator sends the puzzle solution, the random value \(x\), and MAC in the I2 packet to the CTTP;The CTTP computes \(PK_{init}\) = \(x\)\(\times\)\(P\) that represents the initiator's public key (HI) such that the CTTP stores the generator point P received in step 2. The CTTP, also, computes the secret's initiator as S = \(PK_{resp}\)\(\times\)\(x\). Then, CTTP forwards the received I2 packet, including the computed S, to the responder;
6. On receiving I2, the responder checks the puzzle solution and does not need to calculate the secret key because it has already been obtained from the CTTP. In HIP DEX the secret key calculated by the initiator and the responder are the same;
7. The fourth message R2, also, has the MAC value, the key wrap parameters F(S,y) and it finalizes the handshake. In this paper, we propose a modification of the SA phase so that we minimize the number of exchanged messages between the collaborator and the HIP-peers that deal with the minimization of the communication costs. Besides, we detail in this work the mutual authentication process between the collaborator (CTTP) and the HIP-peers (originally only with the responder node). Mutual Authentication Responder/CTTP  
To perform an efficient and light mutual authentication between the CTTP and the responder, we propose to implement a One Time Password (OTP) authentication process regarding the lower capabilities of both the HIP-peers. Likewise, the solution given by authors in [33] regarding the Micro-Macro IoT Authentication Paradigm, we propose an authentication scheme based-OTP between two sensor nodes as the followings:  
1. The adopted communication mechanism is the Direct Sensor Access (DSA) between the CTTP and the HIP-peer (Responder);
2. The first step is about the reception of the hashed password, that will be used as an ID by all the sensor devices in the HIP IoT domain.