So you want to send a short, instant, text-based dispatch to another human. The options are endless -- iMessage, Slack, Instagram, WhatsApp, Skype, Snapchat -- but their security is variable. Short of whispering words into another person's ear, it's difficult to guarantee that no one else will ever be eavesdropping. For anything you wouldn't want to be seen by your ISP or used against you in a court of law, end-to-end encryption is necessary. It works by giving every user of an app a public key and a private key. Messages sent to you are encrypted with your public key and can only be opened with the private key. To anybody without your private key -- including the app company or a government that comes for the data later -- the text is indecipherable.