Summary

Your data is encrypted on your iPhone and never leaves it unless you export a backup.
There is no server, account or recovery mechanism. Only your password can open the vault.
The design uses standard primitives: Argon2id, HKDF and AES-256-GCM, through Apple CryptoKit and libsodium.

From password to key

Step	Details
Password handling	Any Unicode characters are allowed. Passwords are NFC-normalized so the same text always gives the same key. Whitespace counts.
Key derivation	Argon2id (libsodium), 3 passes over 256 MiB of memory, with a random salt.
Master key	A random 256-bit key, wrapped with AES-256-GCM under the password-derived key.
Changing password	Only the wrapped master key is re-encrypted. Your files don’t need to be touched.
Wrong password	Costs about one second of Argon2 work. Right and wrong answers take a similar time, and the app never reports a wrong password.

How data is encrypted

Every blob (each file, each thumbnail and the index of names and folders) has its own key, derived from the master key with HKDF.
Content is encrypted in 1 MiB AES-256-GCM chunks. Each chunk’s authentication covers its position and a final-chunk flag, so tampering, reordering and truncation are all detected.
File and folder names live inside the encrypted index, not in the file system.

No plaintext at rest

Photos, notes, PDFs and text are decrypted in memory only.
Video and audio stream through a resource loader that decrypts only the byte ranges being played.
Sharing and Quick Look previews need a real file. These use a temporary copy that is deleted when you close it and whenever the vault locks.

iOS hardening

The vault is excluded from backups and uses iOS FileProtection.complete, so its files are also protected by the device passcode while the phone is locked.
The vault locks the moment the app leaves the foreground.
The app switcher snapshot shows only the hedgehog.
Third-party keyboards are blocked.
The name field has autocorrect, auto-capitalisation, spell check, smart punctuation, predictions and Writing Tools turned off, so iOS never changes what is typed or learns it.
Unlocking, vault creation, imports and note saves are cancelled or discarded if the app locks while they are running. A closed session can never write to disk again.
Drag and drop carries only an item’s ID under a private type that other apps don’t accept, so nothing can be dropped out of the vault.

Encrypted backups

A backup is the vault’s own ciphertext packed into one file. It is never decrypted or re-encrypted on the way, so it is as strong as the vault itself.

Integrity. An HMAC keyed from the master key covers every entry. Any change, truncation, missing item, swap or addition is rejected.
All-or-nothing restore. The password is checked first. Data is unpacked into a staging folder, verified against the index, flushed to disk and only then moved into place.
Locking stops it. Locking cancels an export or restore in progress, so the key isn’t held in the background.
What a backup reveals. Roughly how large the vault is and how many items it holds.

What this protects against

Someone who picks up or borrows your unlocked phone and opens the app.
Someone who copies the app’s files from the device or a backup without your password.
Modified or damaged backup files.
Your data being exposed through a cloud service, because there isn’t one.

Known limitations

We think it is important to be clear about these.

The password is visible as you type it. The name field shows letters, like any name field. Be aware of who can see your screen. The field clears when the app leaves the foreground.
Recently Deleted. Photos removed from your library after import stay in the Photos app’s Recently Deleted album for up to 30 days. Empty that album too.
No recovery. If you forget the password, the vault cannot be opened, by you or by us.
Lost phone, lost vault. Without an exported backup, erasing or losing the iPhone means losing the vault.
The cover is not encryption. The games make the app look ordinary. Someone who knows about Hedgehog Bits knows a vault may be inside. Your protection comes from the encryption and a strong password. See what a disguise can and cannot do.
A compromised device. No app can protect data on a phone that is already under someone else’s control, for example through malware or a jailbreak.

Choosing a password

Any characters work: letters, numbers, spaces, symbols and emoji. A few random words make a password that is strong and easy to remember. The app shows a strength estimate while you choose it.

Questions or findings

If you believe you have found a security issue, we would like to hear about it. Please write to info@hedgehogbits.com. See also /.well-known/security.txt.