The problem a locked vault can't solve
Picture a safe in your hallway. However good the lock, the safe itself makes a statement: something valuable is in here. Anyone who sees it can demand it be opened — a thief, an abusive partner, an official at a checkpoint. Encryption on its own is that hallway safe. The contents are protected; the existence of the contents is broadcast.
Deniable storage removes the safe from the hallway. Done right, there is no visible lockbox, no count of lockboxes, and no way to establish one is present. The demand "open the rest" fails — not because you resisted it, but because "the rest" cannot be shown to exist.
How a deniable vault works
Three properties have to hold together. First, every vault is independent: each is encrypted with keys derived from its own PIN, pattern or passphrase, so opening one tells you nothing about any other. Second, there is no registry: no index, list, or counter of vaults is stored anywhere — the software itself doesn't know how many exist. Third, ciphertext looks like noise: an unrevealed vault is indistinguishable from unused random data, to a person scrolling and to forensic tooling alike.
The result inverts the usual unlock ritual. In Sealby, a PIN doesn't "unlock the app" — it chooses a vault. The right PIN reveals its vault; any other input opens a fresh, empty vault — identically. Even a mistyped PIN is indistinguishable from opening a vault that was always empty, because they are the same thing.
A short history
The idea is older than smartphones. Desktop tools — most famously TrueCrypt and its successors — offered hidden volumes: an encrypted container inside the free space of another, with two passwords revealing two different realities. Researchers formalised the property as deniable encryption. What changed recently is where the stakes live: the most personal device is now the phone in your pocket, and it is the thing people are actually pressured to unlock.
What deniability is not
Honest limits, stated plainly. Deniability protects data at rest against inspection. It cannot protect you from someone who watches you enter the PIN, from confirming the vault exists yourself, or from plaintext copies elsewhere — a synced photo library, a chat thread, a backup made before the vault. And it is not an excuse to break the law; it is a way to keep private life private under pressure that shouldn't decide it.