In the context of EU operations abroad, a diplomat from the European External Action Service (EEAS) must design a secure digital communication strategy to coordinate joint missions with the defense ministries of third countries. Considering the EU's digital security framework and the functions of the EEAS described in its official documents, what is the most critical measure to guarantee the integrity and confidentiality of sensitive data shared on digital communication platforms, aligning with the principles of the Common Security and Defence Policy (CSDP) and EU digital governance?