For our iOS app, we've used a new feature that Apple has only recently made available: Login with AppleID. When you download the app, you will be asked to login using your AppleID. This is a seamless process and means that you don't ever have to remember another username/password for OnForm.
When you login with AppleID, you'll be asked if you want to share your email address, Apple allows you to choose to either share your email directly or to share a "relay" email so that we can't see your actual email address.
Once you've logged in, you'll never have to login again. It's that easy, and it's very secure, since Apple manages all the storage of your authentication.