Comment by Thaxll
2 days ago
TPM is really badly implemented. When you upgrade your firmware, OS, everything can go south.
Just upgrading your firmware with bitlocker enabled can brick your PC.
2 days ago
TPM is really badly implemented. When you upgrade your firmware, OS, everything can go south.
Just upgrading your firmware with bitlocker enabled can brick your PC.
Windows uses full disk encryption with keys from the TPM by default.
Nobody says "disable disk encryption right away incase the tom forgets the keys". The vast majority of TPM's manage to not forget the keys.
They may not say "turn off bitlocker", but people definitely recommend backing up the recovery keys, and windows allows you to back up the key to microsoft because they know people won't actually back them up. Not sure if that happens by default, but they provide a variety of options for the recovery keys because there is definitely a non-zero chance you need them. There were several stories of this happening with the windows 10->11 upgrade push, where people were auto-updated and then scrambling to decrypt their hard drives.
If windows is encrypted with keys from the TPM anyways, then tailscale doesn't need to encrypt a second time.
Windows also bit me in the ass with this feature, but tailscale not enabling encryption wouldn't have helped one iota.
Local software could be stealing plaintext secrets from your encrypted disk. Physical access is not the only attack vector.
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I'm curious. If the motherboard with the TPM dies, you're basically locked out of your data right? Keys backed up on MS server or not.
No, the backed up keys (MS server, file, printed) give you full access, they contain the full encryption key.
4 replies →
> TPM is really badly implemented. When you upgrade your firmware, OS, everything can go south.
Could you elaborate ? Firmware/OS should not affect TPM contents ? Otherwise e.g. TPM-reliant Windows installs would break ?
In addition there are cloud scenarios where your VM has a TPM and you want to e.g .stop a malicious actor poaching your VM and running it elsewhere.
Having the tailscale TPM tied to your cloud hypervisor prevents the "lift and shift" attack.
Everytime I have to upgrade my MB firmware it breaks bitlocker and I have to either use restoring keys from microsoft website or disable bitlocker encryption before the upgrade.
https://www.reddit.com/r/MSI_Gaming/comments/15w8wgj/psa_tpm...
You cant reliably store secrets in tpm and expect it to work after an os update. Windows is using workarounds during windows update to avoid breaking bitlocker.
https://learn.microsoft.com/en-us/windows/security/hardware-...
You are correct. Updating the firmware or the OS does not actually erase the TPM. What is really going on is that the TPM register holds a value that is like a hash. Each time you measure the system state you update the register with a hash of the previous value and the measurement. When you ask the TPM to hold a key you specify which register value is used to encrypt the key. Later when you use the key it will fail if the TPM cannot decrypt the key. This can only happen if the TPM register has the wrong value, which can only happen if someone has tampered with the system. But voluntarily upgrading the BIOS or the OS looks exactly like tampering.
The correct procedure is to unlock the keys, copy them out of the TPM, perform the upgrade, reboot to remeasure the system state, then finally store the keys back into the TPM.
Wouldn't you want TPM to brick the machine if the firmware was modified? If something or someone modified your firmware, do you want the TPM key to remain intact? Its something you need to be aware of when upgrading firmware, disable encryption that relies on TPM or make a backup copy of the key.