The three pillars of a secure communication are privacy, integrity, and authentication.
Privacy
A secure conversation should be private. In other words, only the sender and the receiver should be able to understand the conversation.
Integrity
A secure communication should ensure the integrity of the transmitted message. This means that the receiving end must be able to know for sure that the message he is receiving is exactly the one that the transmitting end sent him.
Authentication
A secure communication should ensure that the parties involved in the communication are who they claim to be. In other words, there should be a protection against malicious users who try to impersonate one of the parties in the secure conversation.