SealdayA time lock nobody can bypass, including us
Sealday encrypts a capsule on your iPhone and locks its key to a future round of drand, a public randomness beacon run by independent organizations. The key for that round does not exist until the open date. Changing your phone clock cannot open it, and Sealday has no servers that could.
Coming soon to iPhone
How does Sealday keep a capsule closed?
When you seal, the app encrypts your content with AES-256-GCM using a random key made on your iPhone. That key is then locked with tlock, a scheme that ties encryption to a future drand round. Anyone can encrypt to a future round, but nobody can decrypt until the beacon publishes that round's signature.
Sealday uses the published drand quicknet network and the open-source tlock library without modifications. There is no home-made cryptography in the app.
What is drand?
drand is a network of servers that publish fresh, verifiable random numbers on a fixed schedule. It is operated by the League of Entropy, a consortium of independent organizations, so no single company decides when a number appears. Sealday reads one value per open date. See the League of Entropy on Wikipedia for background.
What happens on the open date?
Sealday asks the drand relays, one after another, for the signature of the right round. It checks the signature against the network key bundled in the app, so a relay cannot fake it, then uses it to open the capsule. A fetched signature is cached.
The request contains only a round number. It carries no content and no identifier, though the relay operator can see your IP address like any web server can.
Which algorithms and libraries are used?
Only standard, published ones, used without changes. Sealday does not contain home-made cryptography. The table lists each job and the tool behind it.
The signature of the right round is checked against the quicknet network key bundled in the app, so a relay cannot hand over a forged value.
| Job | What is used |
|---|---|
| Encrypting your content | AES-256-GCM, a random 32-byte key per capsule and a random 96-bit nonce per file |
| Locking the key to a date | tlock, which is identity-based encryption on the BLS12-381 curve, in the age file format, on drand quicknet |
| Checking the beacon | A BLS signature check against the embedded network key |
| Making random numbers | The iOS system random generator |
| Libraries | tlock-js, drand-client and the noble cryptography packages, unmodified |
Can I open my own capsule early?
A capsule for yourself has a spare key in your iPhone Keychain, used for one emergency exit. Break the seal asks you to type OPEN EARLY, then waits 24 hours before opening, and the capsule carries an "Opened early" stamp forever.
A capsule you send to someone else has no spare key. Nobody can open it early, not the recipient, not you and not Sealday.
| Capsule | Early opening | Spare key |
|---|---|---|
| For future me | Yes, after a 24-hour wait, marked forever | In your iPhone Keychain |
| Sent to a friend | No one can | None |
| Table entry | No one can | None |
Where does the spare key live?
In the iOS Keychain, set to be available only while the phone is unlocked and only on that device. It does not sync to other devices, it is not part of a backup and it is not in a vault file. That is why a restored capsule opens on its date but has no early exit on a new phone.
Deleting a capsule removes its spare key, and Delete all data wipes every spare key on the iPhone.
What are the honest limits?
The lock holds as long as drand keeps publishing. If the beacon network ever shut down permanently, capsules you sent could not be opened. Your own capsules would still open on that iPhone with the spare key, after the open date has passed and the beacon has been unreachable for 72 hours.
Dates run from tomorrow to twenty years ahead. Sealday shows a note when you pick a date more than ten years out, because long horizons depend on the network, your backups and your device surviving.
The privacy policy lists what the app sends over the network, and how time-lock encryption works explains the idea behind drand in plain language.
If drand is unreachable on open day, Sealday shows a retry screen and tries again when you return. It never promises a time.
What does sealing look like, step by step?
Sealing takes one deliberate gesture and a couple of seconds of work.
- You review the summary: date, contents and recipient.
- You press and hold the wax seal for 2.4 seconds while haptics rise.
- During the hold, Sealday encrypts the files and locks the key to the drand round for your date.
- The seal stamps, the envelope closes and the date appears on the card.
How is this different from a promise?
Most "future message" websites hold your text on a server and promise not to show it before the date. That promise depends on the company staying honest, staying solvent and not being breached. A time lock removes the promise: the key is missing until the date, for the company and for you.
The idea is old. Researchers described time-lock puzzles in 1996, and you can read about them on Wikipedia. drand makes the same goal practical by using a public beacon instead of a long computation.
Frequently asked questions
Can I trick the lock by changing the date on my phone?
No. The key for a future date is not published yet, so there is nothing for a changed clock to open.
Does Sealday see my capsule?
No. Content is encrypted on your iPhone and Sealday has no servers that receive it.
What if the drand network is down on the open day?
Sealday shows a retry screen and tries again when you come back to the app. For your own capsules, a spare-key option appears after 72 hours without the beacon.
How far ahead can I seal?
From tomorrow up to twenty years, with a note for dates beyond ten years.
Where is the spare key for my own capsules stored?
In the iPhone Keychain, on that device only. It is not synced, not in backups and not in vault files.
Does Sealday write its own cryptography?
No. It uses AES-256-GCM and the open-source tlock-js and drand-client libraries without modification.