Ro.boot.vbmeta.digest

ro.boot.vbmeta.digest is a property related to Android's Verified Boot (VB) mechanism, specifically referring to the digest (a type of hash) of the vbmeta partition. The vbmeta partition contains metadata about the verified boot state of the device, including hashes of other partitions that are verified during the boot process.

6. Attack Resistance and Limitations

• Resistance: An attacker cannot change the reported digest without either:
  1. Re-signing vbmeta with a valid OEM key (impossible without private key), or
  2. Modifying the bootloader/ kernel to spoof the property (requires physical compromise or exploit).
• Limitations: If the bootloader is unlocked, the digest will either be:
  • Omitted, or
  • Set to a default dummy value, or
  • Reflect a self-signed custom vbmeta. Therefore, ro.boot.vbmeta.digest alone is insufficient; it must be paired with ro.boot.verifiedbootstate (green, yellow, orange) for full context.

4. Anti-Rollback Detection

Malware often attempts to downgrade a device to an older, vulnerable OS version. AVB stores rollback indexes in vbmeta. ro.boot.vbmeta.digest

• The bootloader checks the new index against the last known good index stored in persist or fuses.
• If a rollback is prevented, the boot fails. However, if a rollback succeeds (due to cleared fuses), the ro.boot.vbmeta.digest will change to match the older vbmeta.
• Security auditors can query this property to detect if a device has been rolled back to a vulnerable state.

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8. Relationship to Other Properties

| Property | Relation | |----------|----------| | ro.boot.verifiedbootstate | green (locked) / yellow (unlocked) / orange (corrupted) | | ro.boot.vbmeta.device_state | locked / unlocked | | ro.boot.avb_version | e.g., 1.1, 2.0 – AVB spec version | | ro.boot.boot_hash (legacy) | Older digest for boot image only – not as comprehensive as vbmeta digest | Resistance: An attacker cannot change the reported digest

Part 2: The Property – ro.boot.vbmeta.digest

During the boot sequence, the bootloader verifies the vbmeta image. Once verification passes, the bootloader extracts the hash of the VBMeta structure itself (or the hash of the entire verified chain) and passes it to the kernel via the Android bootloader control block. The kernel then exposes this as the read-only system property: Re-signing vbmeta with a valid OEM key (impossible

ro.boot.vbmeta.digest

For developers

• Verify boot integrity after flashing custom images
• Debug verified boot failures (mismatch with expected digest)
• Build system validation – check that flashing succeeded correctly

Part 5: The Technical Anatomy (For Developers)

If you want to manipulate or understand the digest, use Google's avbtool (part of AOSP).

Comparison with original

To verify authenticity:

# Extract digest from original vbmeta image
avbtool info_image --image vbmeta.img | grep "Digest:"

10. Important notes

• Do not try to set this property manually – it's read-only from bootloader
• On unlocked devices, the digest may still be present but not enforced
• Pixel devices use this extensively for Verified Boot 2.0 (AVB)
• Some custom ROMs may zero out this value to bypass checks