There are lots of different ways to send a message to a friend, but some offer more privacy than others. Kevin Mitnick, one of the world's most famous hackers and author of the book "The Art of Invisibility," explains what you should look for when choosing an app with the most security.

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Following is a transcript of the video:

Hi my name is Kevin Mitnick, and I'm the world's most famous hacker.

If you want secure communications, especially when you're texting friends using messenger services. You know, there's Facebook Messenger, there's WhatsApp. The key thing to look for out there are messaging apps that support end-to-end encryption.

Now what does that actually mean?

That means if Alice is communicating with Bob, the encryption key to actually encode and decode those communications are stored on Alice's device and Bob's device. In other words, nobody in the middle has it.

For example, if you use iMessage, it uses end-to-end encryption, but Apple still has the keys. Which means they could be forced to disclose those to law enforcement, to the National Security Agency. Who in the heck knows?

One of the most well-known apps, that's been tested academically in the security community, is an app called Signal. And Signal is absolutely free, and it supports secure voice calls and secure text calls. And that way, if you simply download the app, it makes it extremely difficult even for a nation-state to monitor what you're doing.

What they would actually have to do is compromise your device. To steal the key. Which the NSA could do because they have lots of money.

The key is to look for apps and to verify that they're using end-to-end encryption with what they call PFS. PFS stands for Perfect Forward Secrecy. So what that means, if your key is ever compromised, that key cannot be used to decode your communications from prior sessions.

Because usually you would think you could steal the key, and you can go decode the communications forever. But with PFS, it could constantly changes so that wouldn’t work.