KEY SYNC

Secure Device-to-Device Key Transfer

Transfer your encryption keys to a new device using QR codes. X25519 ECDH key exchange + AES-256-GCM encrypted transfer.

06 / 08

Groups × E2EE

1,000-member groups, fully E2EE.

Encrypted with the same Sender Keys protocol used by Signal and WhatsApp. We deliberately don't raise the cap — design integrity over scale.

SIGNAL FOUNDATION SENDER KEYS

1,000

MAXIMUM MEMBERS PER GROUP

Across all plans — no upgrades to the limit

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Sender Keys protocol

Official libsignal implementation, optimized for groups

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Forward Secrecy

Past messages stay encrypted even if future keys leak

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Low latency

Sender-side encryption is O(1) — fast at any scale

Source Device

On the device that has your keys: Settings → Key Sync → "Transfer Keys". A QR code is displayed (valid for 5 minutes).

Target Device

On your new device: After login, select "Scan QR" and scan the QR code from the source device with your camera.

Transfer Complete

Keys are securely transferred via encrypted channel. SHA-256 checksum verifies data integrity. Session auto-expires.

Transfer Protocol

Source Device                        Target Device
     │                                    │
     │  1. Create Session (Firestore)     │
     │  2. Generate X25519 Ephemeral Key  │
     │  3. Display QR Code               │
     │          ─── QR Scan ───►          │
     │                                    │  4. Parse QR → sessionId + publicKey
     │                                    │  5. Generate X25519 Ephemeral Key
     │                                    │  6. ECDH → Shared Secret
     │                                    │  7. HKDF-SHA256 → AES Key
     │          ◄── Firestore ──          │  8. Write encrypted response
     │  9. ECDH → Same Shared Secret     │
     │  10. Encrypt keys (AES-256-GCM)   │
     │          ── Firestore ──►          │
     │                                    │  11. Decrypt → Import keys
     │                                    │  12. SHA-256 Checksum verify
     │  13. Cleanup session               │
     └────────────────────────────────────┘
              Session TTL: 5 minutes

Keys Transferred

Key	Algorithm	Purpose
Identity Key Pair	X25519	Identity verification / PQXDH
Signing Key Pair	Ed25519	Message signatures (AEGIS XEdDSA)
Signed PreKey Pair	X25519	X3DH session establishment
Registration ID	—	Session identifier

One-Time PreKeys are NOT transferred. They are regenerated on the new device to avoid key competition between devices.

Platform Support

Platform	QR Scan	Key Storage	Alternative
iOS	Supported	Keychain	—
Android	Supported	Android Keystore	—
macOS	Camera dependent	Keychain	Regenerate Keys
PWA (Web)	Camera permission dependent	Web Crypto API	Regenerate Keys

Regenerate Keys: If QR scanning is unavailable (PWA/macOS), choose "Regenerate Keys" to create new encryption keys. New sessions will be established with all contacts, but previous messages cannot be decrypted.

Multi-Device Usage

Important considerations when using the same account on multiple devices.

Same Account, Multiple Devices

• Each device stores independent encryption keys
• Key Sync required to decrypt messages from original device
• One-Time PreKey conflicts possible with simultaneous devices
• Device limit: Essential 1 / Premium & Intelligence 3

Different Account, Same Device

• All encryption sessions fully reset
• Previous account messages permanently undecryptable
• No key collision between different accounts
• Warning dialog shown before account switch

Arc vs Signal: Multi-Device Model

Feature	Signal	Arc V2
Device Model	Primary/Linked	Independent key sets
Device Addition	Via Primary	Direct registration + Key Sync
Key Distribution	Primary distributes to Linked	Peer-to-peer QR exchange
Device Authentication	Implicit trust	Ed25519 signature verification
Primary Failure	All Linked devices orphaned	No impact (independent)