If you have an iPhone 3GS or later, an iPod touch 3rd generation or later, or any iPad, you can protect the contents of your device using encryption. That means that if someone gets physical access to your device, they will also need your passcode to decrypt what's stored on it, including contacts, texts, iMessages, photos, and more.

Enabling encryption on your iPhone is really easy:
 

Step 1: Launch the Settings app on your iPhone or iPad.

Step 2: Select Touch ID & Passcode from the list of available options.

Step 3: Now simply tap Turn Passcode On and enter in a passcode of your choice. A longer alphanumeric passcode is recommended but a six-digit PIN code will do as well. Avoid four-digit PINs as your passcode. You can change this from Passcode Options in Set Passcode screen.

From iOS8, however, Apple bumped up its encryption in response to the PRISM scandal. Now, Apple can’t access any data at all, only the person with the passcode can.

"Unlike our competitors, Apple cannot bypass your passcode and therefore cannot access this data," the company state in their privacy policy. "So it's not technically feasible for us to respond to government warrants for the extraction of this data from devices in their possession running iOS 8."

Apple have gone even further with iOS 10.3. They have introduced the next-generation Apple File System (APFS) with native encryption support, which replaces the age-old HFS+ file system, a feature first introduced with macOS Sierra.