Dropbox-style File Sharing Encryption

TL;DR

This guide explains how to implement broadcast encryption for a file sharing service like Dropbox. This allows you to encrypt files so only authorized users can decrypt them, even if they share the same storage location. We’ll focus on practical steps using symmetric key cryptography and user group management.

1. Understanding Broadcast Encryption

In traditional encryption, a file is encrypted with a single key, shared between sender and receiver. Broadcast encryption lets you encrypt a file once with a key that only a subset of users can decrypt. This is useful when many users have access to storage but only some should be able to read specific files.

2. Key Management: User Groups

1. Create User Groups: Divide your users into groups based on their permissions. For example, ‘Project A Team’, ‘Finance Department’, ‘Executive Access’.
2. Group Keys: Generate a unique symmetric key for each group. Store these keys securely – never expose them directly to the end-users. Use a Key Management System (KMS) or Hardware Security Module (HSM).
3. User-to-Group Mapping: Maintain a database mapping users to their respective groups. This is crucial for determining decryption eligibility.

3. Encryption Process

1. Identify Recipients: When a user uploads a file, determine the list of authorized recipient groups.
2. Key Combination: Combine the group keys of all authorized recipients into a single ‘encryption key’. A simple method is XORing the keys together.

   # Python example (using bytes for security)
   import hashlib
   
   def combine_keys(group_keys):
       combined_key = b''
       for key in group_keys:
           combined_key ^= key  # XOR operation
       return combined_key
   

3. Encrypt the File: Encrypt the file using the combined encryption key and a standard symmetric encryption algorithm like AES (Advanced Encryption Standard) in CBC or GCM mode.

4. Decryption Process

1. Retrieve User Groups: When a user attempts to download a file, retrieve their group memberships from the database.
2. Fetch Group Keys: Obtain the symmetric keys for each of the user’s groups from your KMS/HSM.
3. Attempt Decryption: The user tries decrypting the file using each of their group keys individually.
4. Success Condition: If any one of the user’s group keys successfully decrypts the file, decryption is successful. This works because the encryption key was created by XORing the group keys together; XORing with the correct key reverses the process.

   # Python example (AES decryption)
   from Crypto.Cipher import AES
   import hashlib
   
   def decrypt_file(encrypted_file, key):
       try:
           cipher = AES.new(key, AES.MODE_CBC) # Or use GCM for authenticated encryption
           decrypted_file = cipher.decrypt(encrypted_file)
           return decrypted_file
       except ValueError as e:
           return None  # Decryption failed
   

5. Security Considerations

• Key Rotation: Regularly rotate group keys to limit the impact of potential key compromises.
• Secure Storage: Protect your KMS/HSM and database containing user-to-group mappings with robust security measures (access controls, encryption at rest).
• Algorithm Choice: Use strong, well-vetted symmetric encryption algorithms like AES with appropriate key lengths (e.g., 256-bit keys).
• Initialization Vectors (IVs): Use unique and unpredictable IVs for each encryption operation to prevent replay attacks.
• Authenticated Encryption: Prefer authenticated encryption modes (like GCM) which provide both confidentiality and integrity checks, preventing tampering with the encrypted file.

6. Scalability

For large user bases, consider:

• Caching: Cache frequently accessed group keys to reduce KMS/HSM load.
• Distributed Key Management: Distribute key management responsibilities across multiple servers for redundancy and scalability.
• Optimized Database Queries: Optimize database queries to efficiently retrieve user-to-group mappings.