4. Alice and Bob are employees residing in two dispersed branches, D1 and D2, of the same company. They want to secure all the communications between them as follows:

• They want to ensure the authenticity of their IP packets when those packets are routed in the company intranet (i.e., Alice can verify that a packet is indeed from Bob, and vice versa);

• They want to ensure confidentiality of their IP packet data (including IP source addresses) when those packets are routed in the external network between D1 and D2. For example, the communications are transferred via nodes: D1 —N1—N2—N3—N4—D2. When N2 receives the packet from N1, it doesn’t know that the packet is from D1.

Design a security solution for the above scenario without changing the header of an IP packet when it is delivered at different sections of the network and highlight the differences compared to normal solution without the above protections. For simplicity, we assume that A and B are using unique and public IP addresses and they have a shared secret key before the communication.