A/B Users are more secure than Private Space

# TL;DR

From a forensic or security perspective, despite the additional authentication layer for Private Space, data is more vulnerable than a separate, second user.

# Private Space

Private Space uses a file-based encryption model where user-specific data transitions from BFU-restricted to AFU-accessible once the primary user unlock occurs. Here is the breakdown:

BFU (Before First Unlock): Device just rebooted, no authentication yet. Only Device-Encrypted (DE) storage is accessible. Credential-Encrypted (CE) data remains locked.

AFU (After First Unlock): User has authenticated once post-boot. Both DE and CE storage are accessible with derived keys.

Private Space uses file-based encryption and stores its data in the CE partition. Once the device reaches AFU state:

The encryption keys for Private Space are available in memory

Private Space itself still requires separate authentication to access

The underlying storage is decrypted and accessible to the system

# Second User

A second user profile remains in a BFU-like (Before First Unlock) state regarding its own data encryption keys until that specific user authenticates, even if another user (the primary owner) has already authenticated and put the device in AFU.

Here is the breakdown:

Per-Profile Key Derivation: Android File-Based Encryption (FBE) generates unique credential-encrypted (CE) keys for each user profile. When User A unlocks the device, only User A's CE keys are derived and loaded into memory.

Isolation: User B's CE keys remain locked and inaccessible. The file system permissions prevent User A's session from reading User B's private data or deriving User B's keys without User B's specific authentication credential.

Shared Storage Exception: The only data accessible to both users while User B is "locked" is the shared storage area (e.g., /sdcard if not strictly partitioned, though modern Android isolates this too). Private spaces and individual app data for User B remain encrypted and effectively in a BFU state relative to User A's session.

Therefore, the device being in AFU because of User A does not transition User B to AFU. User B stays secure and encrypted until they personally unlock their profile.

# Conclusion

Choose a second user profile for robust security against physical seizure and Private Space for everyday convenience where absolute isolation is less critical than usability.

# Sources

Android Open Source Project (AOSP) – Official documentation on the architectural implementation of Private Space and its relation to user profiles: https://source.android.com/docs/security/features/private-space

GrapheneOS Forum – Community technical analysis comparing the encryption isolation of Private Space versus standard second-user profiles: https://discuss.grapheneos.org/d/16569-android-15-private-space-please-explain

Privacy Guides Community – Discussion on the security implications of User Profiles vs. Work/Private Spaces for digital forensics resistance: https://discuss.privacyguides.net/t/android-private-space-vs-work-profile/21101

Next Star Systems – Comparative guide on file access and encryption boundaries between hidden folders, private spaces, and multi-user setups: https://nextstarsystems.com/blog/private-space-vs-hidden-folders-when-to-use-each-on-android

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