[Android Software/Hacking General [Developers Only]] phhusson:
Hello,

I've tested the full verified boot stack of Nexus 5X, to better understand the various implications, especially for my SuperUser project ( http://xdaforums.com/android/software-hacking/wip-selinux-capable-superuser-t3216394/ ).

First of all, I'd like to say that they took time, but with this bootloader, Google did a really nice piece of security, without excluding alternate ROMs. (No doubt they got their inspiration from Chrome project, which are(were?) ahead on the matter).

This means that we can have alternate ROMs, even possibly roots, without compromising our devices! (of course assuming root/ROMs do their part of the job)

Everything is explained in https://source.android.com/security/verifiedboot/verified-boot.html and https://source.android.com/security/encryption/index.html

For users who haven't got their hand on a Nexus 5X/6P yet, here are the various screens you might see on boot when booting a non official ROM: https://source.android.com/security/verifiedboot/verified-boot.html#user_experience

The bootloader has two states: locked or unlocked, nothing new here. locked won't let you flash, unlocked will.

But then, there are 4 boot state that can be shown (or not to the user):

- Green boot state: the device is locked and running Google's ROM. It's the only boot state which is not displayed by bootloader

- Orange boot state: the device is unlocked.

- Yellow boot state: the device is locked, running an alternate ROM

- Red state: This shouldn't happen, it means the boot.img has been badly signed. Could mean an attack is undergoing.

Yellow state has two different possible screens depending on if the boot.img is signed. The difference is, if it is signed, an ID is displayed: this is the fingerprint of the public key used to signed the boot.img.

This means that we, the users, have a way to always be sure we booted the

boot.img made by the ROM developer

.

The same way we know we are on a Google ROM when there is no warning screen, we know that we are indeed on the ROM we are expecting.

But wait, there is more! The bootloader gives to the TEE/Trustzone the public key of the boot.img, and the TEE/Trustzone changes its answers based on the public key.

The TEE/Trustzone answers are used to generate the key of the /data encryption.

This means that in locked mode, with a signed boot.img, a trusted keystore-owner (doesn't have to be Google), full disk encryption enabled, and dm-verity enabled, even if an attacker is capable of overwriting any part of eMMC through either a privilege escalation, or physical attempt, trying to phish you to prompt your password, then

the attacker won't get access to your data

!

The TL;DR is, it is now possible to be as secure as original Google's ROM with a custom ROM (you still have to trust Google though) thanks to verified boot features.

Here are my advices to fully benefit from this feature:

- KEEP Full Disk Encryption (forceencrypt)

- KEEP keystore/keymaster/qseecomd

- Lock bootloader back

- Use custom security keys, including verity key, and keep them safe.

- Sign boot.img (this is default AOSP behaviour I think)

- Use a different security key for recovery (so recovery won't ever access to your userdata)

- Untick "OEM unlocking" if you feel safe about your ROM, to have Factory Reset Protection working (ie anti-theft feature)

For unknown quality ROMs:

- When first booting a new ROM/security key, try to memorize the fingerprint. If you don't memorize it all, try to memorize characters are random position, not the beginning.

- When the device asks for you deciphering password, check you booted the proper fingerprint. Do that especially if you have unexpected reboots.

Even though I consider this is a really great step forward to having secure alternate ROMs, there are some downsides:

- There is no "verified" mode where you can flash anything, but it does apply the security policy. This makes TWRP/Flashfire/CWM/a proper OTA system needed to be able to flash again without erasing all data

- The fingerprint is 6bytes/48bits, this is by far lower than most security standards. Also, some algorithms make ascii-art from fingerprints to help user recognize matching fingerprint.

- Changing ROM is complicated: you either need to resign all ROMs with your own key, to factory reset or "transmit" in clear the cipher key

Despite the the downsides, I hope to convince some ROM devs, and users to go to lock mode and check they implemented my various advices.

Please note that this security works only on Nexus 5X/6P, it doesn't work on N6 or N9.