As an EU official assigned to a digital cooperation mission abroad, you must design a secure communication protocol for the European External Action Service (EEAS). The protocol must ensure the integrity of confidential data and the authenticity of the sender in insecure network environments, aligning with the EU's digital security principles. Which of the following technical measures is the MOST effective for ensuring the confidentiality of data in transit and the authenticity of the sender in a communication between two EEAS offices?