The encryption key is like an additional password that you create when you register for an account in the Read Your Body app. It's what makes your backups so extra private and secure. The key can be as many characters as you like and any combination of letters, numbers, special characters.
You need to be able to remember your encryption key to restore your precious charting data from your account if your phone is ever lost, stolen or broken. For this reason every two weeks in the app you'll see a yellow banner at the top of your data screen prompting you to check that you can enter your key correctly.
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If you can't remember your encryption key and you see this yellow banner inside the app then there's no need to worry! Simply reset your key and continue charting :)
Menu > Account > Password & encryption key > Reset key
Your existing data in the app will not be affected by resetting your key. When you reset your key then a fresh backup will immediately be made with this new key.