I would like to know how to encrypt the phone to stop access to my data

What to do before selling or giving away your iPhone, iPad, or iPod ...

Just erase it, the person buying your phone will not have access to your data.

Erasing the iPhone will be fine. If you want, you could even go so far as to Use iTunes on your Mac or PC to restore your iPhone, iPad, or iPod to factory settings - Apple Support. These aren't spin disk hard drives though. There is little chance of recovering any amount of data once the iPhone has been erased.

The data on your iPhone is already encrypted as long as you have a screen lock passcode enabled on it. What happens when you choose erase all content and settings is the decrypt key is deleted permanently. Without that key, the binary data stored on your iPhone is meaningless gibberish to everyone, including yourself, as the sole means of decrypting it has now been permanently deleted. This way, even the most sophisticated data recovery software would not be able to recover anything more than encrypted hashes of data that would be unintelligible.

This method has been in place since the iPhone 3Gs and iOS 4 and has been enhanced since then on newer devices as the hardware has advanced and allowed even more complex encryption.

radiowatch wrote:

I am going to trade-in my iPhone 5 for an iPhone 6sPlus. Before doing this, I would like to know how to encrypt the phone to stop access to my data. I have been told that simply erasing the data does not actually remove it…

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The contents of your phone are already encrypted using strong encryption. If you go to Settings/General/Reset and choose Erase all content and settings this deletes the encryption key, which essentially makes all of your content unreadable, even though it is still present in the phone's storage. It's essentially random gibberish. If you want to be extra-sure restore iOS using iTunes and DFU mode.

Your sarcasm is unwarranted. If you are skeptical, then why did the FBI spend almost a million dollars to break into one phone?

If you doubt, then smash it to bits with a hammer.