I'm a big fan of the Signal app from Open Whisper Systems for sending and receiving encrypted communication to other Signal users. If you use the disappearing message feature for Signal on a Mac, the messages don't really disappear and are recoverable according to security researcher Alec Muffett. Objective-See has an excellent blog post that has the technical details of what's going on if you are really interested. In simple terms, if you use Signal on a Mac the messages are copied by the operating system in order to accommodate the notification system, which Signal uses. This means that there is an unencrypted version of the messages stored in another area of the machine disk that is recoverable even if you intended the message to disappear.
So what's the lesson here? Don't use Signal on a Mac if you want to keep your messages private. Use the Signal app on Android or iOS or Windows if you don't want the "disappearing" messages to be recoverable.
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Digital Forensics/Information Security/Information Technology