How Ghost encryption works, in plain English

Ghost is the part of Simplify1 for the things you would never put on a leaderboard: recovery, medication, a habit you are trying to break. Ghost items count toward your own grade like everything else. What makes them different is where they can be read: on your phone, and nowhere else.

Here is how that works, without the jargon.

A key that lives on your phone

Simplify1 doesn't use passwords. You sign in with Apple, Google or a code sent to your email, so there is no password to build a key from. Instead, your phone makes a random key of its own, a long secret number, and keeps it in the phone's secure storage: the Keychain on iPhone, the Keystore on Android. The key belongs to your account, so two people who sign in on the same phone have two separate keys.

That key is never sent to us.

What happens when you tick a Ghost item

Ghost items are numbered rather than named: Ghost category 1, Ghost category 2, and so on. You know which is which. Anyone glancing at your screen doesn't.

Each Ghost item, and each day's tick on it, is sealed on your phone with AES-256-GCM, a standard and widely used form of encryption, under your key. What leaves the phone is the scrambled form. Our servers keep that scrambled form, the date it belongs to and the time it was saved. That is all: we store only the scrambled form and dates. Our staff can't read it, our admin tools can't open it, and if we were ever ordered to hand it over, the scrambled form is all we would have to give.

Before the Ghost screen opens, your phone asks you to unlock it with Face ID, a fingerprint or your passcode, so someone holding your unlocked phone can't simply scroll to it.

Where Ghost shows up, and where it doesn't

Ghost moves your own daily grade. It is left out of everything anyone else can see: your public score, leaderboards, groups, friends, competitions, your accountability partner's view and your employer's. Reminders never name a Ghost item on your lock screen. The in-app coach leaves Ghost out of what it reads. And the anonymous usage events the app sends say nothing about Ghost at all, not even that you logged something there.

The recovery phrase

Because the key exists only on your phone, a new phone can't read your old Ghost history on its own. That is what the recovery phrase is for.

You choose a phrase of at least three words and twelve characters. Your phone turns it into a second key using Argon2id, a method built to make guessing slow and expensive, and uses that to lock a copy of your Ghost key. We store the locked copy, a random value called a salt, and the settings used. The phrase itself never leaves your phone, and nothing we hold can open the locked copy without it.

On a new phone you type the phrase, the locked copy is opened on that phone, and your Ghost history is readable again.

The honest trade-off

If you lose your phone and never set a phrase, or you forget it, your Ghost history is gone. Not hidden, not waiting on a support ticket: gone, because nobody, including us, holds anything that can open it. The app says so in those words when you set Ghost up, and keeps asking you to set a phrase until you do.

When you delete your account, your Ghost data and the locked copy of your key are deleted with it.

Where Ghost runs

Ghost is on the iPhone and Android apps only. It relies on the phone's secure storage, so the web app at simplify1.org/app does not include it. Ghost is part of Premium, which you will be able to buy in the phone apps at launch. You can read the short version on our Ghost page, and the details in our privacy policy.