According to NBC, messages between different devices are encrypted only with Rich Communications Services (RCS), which in the U.S. are all decrypted by Google.

How to lock an iPhone to prevent snooping The first step to ensuring your text messages aren’t easily accessible to be read by others is keeping your phone locked whenever it’s not in use ...

However, when an iPhone user wants to send a message to an Android user (or vice versa), the messages are encrypted using a different method, RCS, which is more vulnerable.

Your iPhone will detect when it has no cellular or WiFi service. On phones with satellite capability, you'll see a lock screen message indicating you can send and receive messages via that connection.

Apple has always had its own Messages setup but earlier this year, the company announced it would make Apple messages compatible with Rich Communications Services, the text messaging standard used ...