From what I understand he was not formally arrested at the time, just interrogated at the border. I am not familiar with US laws but from what I understand the device was not (yet?) considered evidence in an investigation. I imagine backing up might not be as easy an option without tearing down the device as the memory chip is no the device mainboard, so not something you can necessarily do on the side of the road or at a border without a formal warrant/investigation.

This is really an interesting case. Hope to see a ruling here. The edge cases are really interesting as well, like the fake pin on the back of the phone. I also wonder generally about the 'destruction' of data Wouldn't the government need to prove that there is no backup, because just making it more difficult (like hiding) would probably not call for the paragraph. Unfortunately for the rest of us the defence is based on more proven grounds. I guess only plausible deniability is helpful: I e.g. would love to see my trusted android space being empty/recreated on false password.

> I'm surprised they didn't back up the device first.

There's no use in taking an image of the SSD and restoring it after a wipe. Data needed to derive the key encryption keys was wiped from the secure element. Separately from that, there's also additional data on the SSD needed to derive the key encryption keys which the TEE hardware will no longer consider valid.

Entering the duress PIN/password does the following:

* wipes TEE hardware keystore * wipes secure element hardware keystore * wipes the secure element other than Factory Reset Protection data which isn't used by GrapheneOS at this time * wipes the encryption metadata on the SSD with special wiping commands

The secure element data is inside the secure element's internal storage. TEE keystore keys are normally stored encrypted on the SSD with a hardware-based anti-rollback storage system to prevent reusing deleted keys. The secure element keystore is a lot more secure.

The most important data that's wiped is the Weaver table on the secure element. Weaver is the secure element rate limiting feature we describe in our post. It's implemented by the OS passing a dedicated hash of the initial key derived from the user's PIN/password. If it's the valid hash, the secure element provides a token needed alongside the initial derived key for the final key derivation. If it's not valid, it's a failed attempt wasting one of 20 attempts and triggering a hardware enforced delay.

It also isn't possible to simply image the SSD and then try to brute force the PIN/password due to the secure element rate limiting and the final key derivation done in the TEE.

It isn't the same as a typical disk encryption approach on a desktop, although GrapheneOS supports using a strong passphrase to avoid depending on the secure element rate limiting. GrapheneOS makes it convenient to use one via 2-factor fingerprint+PIN secondary unlock in After First Unlock state. It's the same as regular fingerprint unlock but with a PIN needed to complete it where incorrect ones count towards the limit of 5 attempts (reduced from 20 by GrapheneOS).

My trick there is not travelling to the US.

I carry a burner phone when travelling most of the time anyway. It has access to email only, 99% of which is in offline folders anyway.

The alleged charges seem nonsense - destruction of property to prevent seizure? First no property was destroyed, and second they could still seize it. But US courts are a coin toss so it may still work for the government.

It sounds like he did give them the password, but it was the password to wiping his phone and not unlocking it. I'm surprised they didn't back up the device first.

The duress password does not wipe the phone. It wipes the encryption keys from the secure element. The phone's storage is the backup, but it is worthless, unless law enforcement has an attack against AES that does not require a brute force attack (unlikely).

I'm not sure it would have been that easy for them to back it up.

Depending on his settings.

You can disable the usb port entirely if you like, so that it is only possible to charge the device by switching it off. Or enable charging only when unlocked etc.

Or if his device had rebooted I don't think it would be possible to extract anything.

So, technically, the border agents wiped the phone. I wonder how specific their request was of Tunick when they asked for the password.

Backup is useless without security chip.