To maybe clarify what dasil003 means, state-of-the-art encryption for this requires that everyone has a pair of keys, one of which they keep a secret - their Private Key, and one of which they tell all of their friends, acquaintances, everyone who might ever want to communicate with them - their Public Key.

And then if you want to talk to them, you encrypt your message with their Public Key. Then only they can decrypt it with their Private Key (assuming no one else their Private Key).

But if a hacker / NSA agent / script kiddo pretends to be your friend and tells you their Public Key, then they'll be able to decrypt your messages with their Private Key. You might never know that they aren't your friend.

You'll have to ensure that you actually have the Public Key of your friend by some other method. For example meet them in person to exchange Public Keys, or read it out over the phone, if you know what their voice sounds like on the phone.