Cross-Device Transfer

UltraLocked supports two cross-device transfer mechanisms. Encrypted Export / Import (this article) is the general-purpose one — produces a portable encrypted file that travels over any channel. Receive via QR is the air-gapped optical alternative for in-person transfers.

How the .ultralocked Format Works

When you export, UltraLocked builds a single binary file with this structure:

1. Public header (96 bytes) — magic bytes, format version, KDF parameters, salt, manifest nonce, manifest size. Used as additional authenticated data (AAD) by every subsequent crypto operation.
2. Encrypted manifest — JSON with item metadata (id, name, type, size, dates, TTLs, per-item nonce). Encrypted with a manifest key derived from your passphrase via HKDF.
3. Encrypted item records — one ciphertext block per item, each with its own unique key and the item's UUID baked into the AAD.

Master key derivation uses Argon2id, the Password Hashing Competition winner, with parameters tuned for ~1 second of work on a recent iPhone. Per-item keys are HKDF-SHA256 derived from the master key. Encryption is AES-256-GCM.

The id-bound AAD on each item is critical: it means an attacker who somehow obtains both the bundle and your passphrase still can't swap one item's ciphertext into another item's slot — the auth tag rejects. Every item's place in the bundle is cryptographically pinned.

Exporting

1. Open Settings → Data → Encrypted Export…
2. Pick which vault items to include.
3. Optionally add an export label (a free-text string like "iPhone backup 2026-04" — you'll see it on the receiving device after passphrase verification).
4. Type a passphrase twice. Watch the strength meter and use a passphrase you'll remember — there is no recovery.
5. Tap Encrypt and Export.
6. When ready, tap Share and choose AirDrop, Files, Messages, email, or any other transport.
7. When done, tap Done — Securely Delete Local Copy. The temporary .ultralocked file on your device is securely overwritten and deleted.

Export is a Premium feature.

Importing

1. Receive the .ultralocked file on the destination device (e.g., AirDrop accepts directly into Files; or save from email).
2. Open Settings → Data → Import .ultralocked File…
3. Pick the file from the file picker.
4. You'll see filesystem-only info first (filename, size, modification date) — the bundle's contents are still sealed.
5. Enter the same passphrase.
6. After verification, you see the manifest preview: item names, sizes, expiration timers, the optional label, and the export date.
7. Choose which items to import. Each is decrypted from the bundle and re-encrypted under the destination device's Secure Enclave-anchored keys.
8. At the end, optionally toggle on Securely delete source file so the .ultralocked doesn't linger on disk.

Import is never subscription-gated. Recovery must always work, regardless of subscription state.

Self-Destruct Timer Preservation

Items with a TTL (self-destruct timer) keep their original expiration on import. The receiving device honors the timer based on the source's creation timestamp, not the moment of import — so a "1 day to live" item doesn't get reset to a fresh day when transferred.

Choosing a Transport

Same-user transfers (your own iPhone to your own iPad):

• AirDrop — fastest, most private. Direct device-to-device over Bluetooth + Wi-Fi, encrypted at the Apple-platform layer in addition to UltraLocked's encryption. No internet required.
• Files / iCloud Drive — useful when devices aren't physically together.
• Email / Messages — works but the file sits on those services until deleted. The bundle is still encrypted, but consider lifecycle.

Security Tradeoffs

.ultralocked is a passphrase-derived format. The strength of the bundle is the strength of your passphrase — if you use "password" as the passphrase, the cryptography is irrelevant.

That's a deliberate tradeoff. The on-device vault uses Secure Enclave-anchored keys that are hardware-protected and unextractable. Cross-device transfer cannot use the Secure Enclave directly because each device has a different Enclave. So we fall back to a passphrase-based format, with industrial-strength KDF parameters to make brute force expensive.

Use a long, unique passphrase. We recommend 6+ random words.