A Locket for Later

What it does

You pick some photos or a video, write a note if you want to, and choose a date. Could be next month. Could be your daughter's eighteenth birthday, eleven years from now.

Half-Locket turns all of it into a single file. You send that file however you already send things — message it, email it, put it on a memory stick in a drawer. It does not matter, because until the date arrives the file is unreadable. Not "password protected". Not "we promise not to look". Genuinely unreadable, including by us, including by you.

On the day, the person you sent it to opens it in their browser and it becomes photos again.

The bit that sounds made up

Here is the fair objection: to lock something you need a key, and if a key exists, somebody has it. And if somebody has it, then "cannot be opened until June" really means "we're choosing not to open it until June" — which is a promise, not a lock.

So a locket is not locked with a key that exists.

It is locked with a key that hasn't been made yet — using nothing but the key's address.

Picture a clock tower

Somewhere in the world there is a clock tower. Instead of chiming, it announces a number. It does this every three seconds, forever, and each number is different and impossible to guess in advance. Once announced, a number is public — printed on the side of the tower for anyone to read.

The tower is not run by one person. It is run by a couple of dozen separate organisations in different countries, and each of them holds only a fragment of the mechanism. No one of them can make the tower announce tomorrow's number today. They would have to nearly all agree to cheat, at once, in secret.

Now the strange part. There is a kind of mathematics that lets you build a lock which opens only to announcement number 12,345,680 — and lets you build it today, without knowing what that number will turn out to be. You are not using the number. You are using its place in the queue.

That is the whole trick. When the tower reaches your announcement, the number becomes public — and the number is the key. Not a key someone hands over. A key the world simply arrives at.

Why the key is torn in half

A date that absolutely cannot be moved is a strong thing to build, and on its own it is slightly frightening. People change their minds. A message written for a fifteen-year-old is not always one you still want a twenty-five-year-old to receive.

So we tear the key in two before anything is locked.

Not like snapping a physical key, where each piece is half the teeth and a determined person could file the rest. Torn in a way where each piece, on its own, is pure noise. Hold one half and you have learned precisely nothing: every possible key is still equally likely. Only both halves together mean anything at all.

Time can't be talked out of it. You can still change your mind.

That second half is your veto. Cancel a locket before its date and the server erases its half for good. From that point on the file cannot be opened by anyone — not the person you sent it to, not us, not you. The pieces to reassemble it no longer exist.

Three moments

1. You seal it

   Choose files, a note, a date. Your device does the scrambling and hands you back one file. Nothing has been uploaded.

2. It waits

   You send the file. If they open it early, all they get is a countdown. You can call it off any time before the date.

3. It opens

   On the day, the two halves come back together on their device and the photos appear. They can save them, and after that they're ordinary photos.

What we can see

Almost nothing, and it is worth being exact about it rather than just saying "we care about privacy".

For a locket that uses our server, we hold four small facts and one small piece of data: the date it opens, a scrambled fingerprint of your cancel code, when it was made, whether it has been cancelled — and the thirty-two bytes that make up the second half of the key.

That is everything. We do not have the file. We do not have the photos, the video, the note, the filenames, or how many there are. The entire conversation between your device and our server is about a hundred characters long, whether your locket holds one photo or two hours of video.

Things we won't pretend about

Every honest description of a tool like this has a section like this one, and a tool that doesn't have one is worth less trust, not more.

• Once it's open, it's open. Cancelling works before the date. After the recipient has opened the locket, they have the photos — the same as if you'd texted them. Nothing can reach into someone's phone and take a picture back.
• Saved photos are just photos. When the recipient saves what's inside, those files sit on their device unprotected and back up to whatever cloud they use, like anything else they save.
• Your cancel code lives in your browser. Clearing your history, switching phones, or simply not visiting for a while can lose it — browsers tidy up after themselves. That's why the app offers you a small file to keep as a spare. Keep it somewhere you'll find it in ten years.
• A borrowed or infected device sees what you see. If something has already got into the phone or laptop doing the opening, no website can protect what's on that screen. That is true of every app, and we'd rather say it than imply otherwise.
• Some facts travel with the file. Anyone holding a locket can see its size and the date it opens — that's what tells their app when to count down to. They cannot see anything inside it.