Figure 4 shows the secure negotiation protocol of the supported security mechanisms. Three entities are involved in the negotiation procedure: E1 is the onboard entity (e.g. a passenger terminal), E2 is the ground entity (e.g. a server), and the corresponding SMP (c.f. system architecture). E1 requests a secure connection with E2, SMP initiates the negotiation in
order to establish the SSPD by sending a Request_SSP message containing a random number Nonce1 to avoid replay attacks. The SSPD structure storage is the following: