How can client-side encryption be applied for a user with multiple devices

Question

I’m working on an app that encrypts user data locally before syncing it to the cloud. How can I securely manage encryption keys so the same encrypted data is accessible across multiple devices without compromising security?

Answer 1

Using client-side encryption for users who own several devices requires a strong system to safely distribute and control encryption keys among devices. The important actions and techniques are:

1. Master Key and Key Derivation

• Master Key Storage: The user generates a master key (MK) derived from a strong password (e.g., using PBKDF2, Argon2). The master key encrypts the data encryption keys (DEKs), not the data directly.
• Key Derivation: Each device derives the same master key locally using the user’s password and consistent parameters (e.g., salt, iteration count).

2. Secure Key Backup and Synchronization

• Key Backup: Encrypt the DEKs with the master key and store them securely in the cloud.
• Key Sync: When a new device joins, the user logs in and derives the master key to decrypt and access the DEKs from the backup.

3. End-to-End Encryption

• Data is encrypted locally on the device using the DEKs before syncing to the cloud.
• DEKs are never uploaded in plaintext, ensuring the cloud provider cannot access user data.

4. Device Registration

• Each new device is "registered" by authenticating the user and downloading the encrypted DEKs.
• Use a device-specific encryption key for additional protection during transmission.

5. Rotating Keys for Security

• Regularly rotate DEKs to limit the impact of potential compromises.
• Re-encrypt data with new DEKs while securely updating them across devices.