TPM+PIN is pretty great, is a shame it's nearly never used.
Bypassing disk encryption on systems with automatic TPM2 unlock
81–90 of 136 posts
Re: Bypassing disk encryption on systems with automatic TPM2 unlock
#82Earlier quoted context omitted.
Depends on the use case. If boot requires a password, the computer can never lose power or be rebooted without human presence. That’s not always practical.
You can reboot your full-disk-encryption server while you sleep. Obligatory plug: https://www.recompile.se/mandos > Disclosure: I am a co-author of Mandos.
Thank you for this. I will almost certainly be deploying that.
Re: Bypassing disk encryption on systems with automatic TPM2 unlock
#83This is mitigated if your initramfs + kernel are measured into the TPM, right? Edit: never mind, I think it's still vulnerable.
Your edit is correct. The kernel and initramfs are exploited unmodified to boot a tampered root filesystem, which allows userspace tools to extract data from the TPM, as the PCRs used to protect data like the LUKS passphrase have not changed.
Re: Bypassing disk encryption on systems with automatic TPM2 unlock
#84Earlier quoted context omitted.
> 3. the automated tpm unlock fails, prompting for a password. > 4. type in our password. In a serious security conscious setup this should be a big red flag to investigate. Any unexpected boot password prompt.
yes of course - but in this case the "unexpected" prompt is presented to the attacker, not the user.
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Re: Bypassing disk encryption on systems with automatic TPM2 unlock
#85I don't understand why anyone would use passwordless disk encryption. It just seems inherently vulnerable, especially with the threat model of physical compromise. Entering a password on boot isn't even that much work
It's always felt like the best way to lock myself out of my own machine. While I don't really keep any important data on it that's not backed up, I also don't build my workstation image from a config file or anything, so some time will be lost. The thing that's always bugged me is that before AMD had a TPM built into the CPU, the TPM was just a poorly-secured dongle hanging off the motherboard (at least with Asus mot…
I sympathize with your points, especially rebuilding the box from scratch.
But all the “passwordless” schemes I’ve seen support at least an additional “master key” which you can type in.
So if you’re ok with the security tradeoffs of passwordless tpm, it’s only an added convenience on top of your approach.
Re: Bypassing disk encryption on systems with automatic TPM2 unlock
#86You can mitigate this by including PCRs that sign the kernel and initrd, however it means whenever you update you need to unlock manually. On Redhat-based distros this can be done with PCRs 8 and 9, though IIRC this may change on other distros. Also AFAIK there is no standard way to guess the new PCRs on reboot so you can't pre-update them before rebooting. So you either need to unlock manually or use a network decry…
At least for PCR 7, it's well specified and documented how the digest is generated. You can dump the component digests of a PCR using `tpm2_eventlog`, and I've written a tool that can be used to populate the requisite data structures for hashing.
Re: Bypassing disk encryption on systems with automatic TPM2 unlock
#87I don't understand why anyone would use passwordless disk encryption. It just seems inherently vulnerable, especially with the threat model of physical compromise. Entering a password on boot isn't even that much work
It's always felt like the best way to lock myself out of my own machine. While I don't really keep any important data on it that's not backed up, I also don't build my workstation image from a config file or anything, so some time will be lost. The thing that's always bugged me is that before AMD had a TPM built into the CPU, the TPM was just a poorly-secured dongle hanging off the motherboard (at least with Asus mot…
This was actually a problem for me on my current gaming PC build!
I had switched to a new AM4 mobo a few years back and decided to spring for a pluggable TPM chip (since the CPU I have doesn't come with TPM onboard). Plugged it in, set everything up pretty seamlessly in windows, no fuss, no muss, boot drive's encrypted transparently. The lack of a password was a bit jarring at first, but it's a gaming PC, so if things go pear-shaped it's not the end of the world.
Fast forward six months and my PC suddently refuses to boot; turns out the pluggable TPM thing was defective and stopped working (without any warning that got surfaced to me).
It was just my boot drive, and reinstalling windows isn't a huge hassle, but it definitely cemented my mixed feelings about passwordless FDE. Had that been the drive I use for my photo library, or my software projects, or work-related documents (tax slips, employment contracts, whatever), that would've been devastating.
It's actually made me rethink the strategy I use for my laptop's backups, and I think I'm in a better place about that now.
Re: Bypassing disk encryption on systems with automatic TPM2 unlock
#88I don't understand why anyone would use passwordless disk encryption. It just seems inherently vulnerable, especially with the threat model of physical compromise. Entering a password on boot isn't even that much work
I believe with software-based encryption (LUKS and the like), the decryption key ends up being stored in RAM, which will always present a risk.
And TPM-based solutions don't have this problem. Could be wrong though.
Re: Bypassing disk encryption on systems with automatic TPM2 unlock
#89Hah, my gist from 2022 is on this post as a "broken guide". I'd disagree in saying it is broken. It works just fine and I flat out say at the top to not use it if you are worried about a cold boot attack on your hardware.
Re: Bypassing disk encryption on systems with automatic TPM2 unlock
#90Earlier quoted context omitted.
It's always felt like the best way to lock myself out of my own machine. While I don't really keep any important data on it that's not backed up, I also don't build my workstation image from a config file or anything, so some time will be lost. The thing that's always bugged me is that before AMD had a TPM built into the CPU, the TPM was just a poorly-secured dongle hanging off the motherboard (at least with Asus mot…
This was actually a problem for me on my current gaming PC build! I had switched to a new AM4 mobo a few years back and decided to spring for a pluggable TPM chip (since the CPU I have doesn't come with TPM onboard). Plugged it in, set everything up pretty seamlessly in windows, no fuss, no muss, boot drive's encrypted transparently. The lack of a password was a bit jarring at first, but it's a gaming PC, so if thing…
Don't all AM4 CPUs feature fTPM which is a firmware-based TPM? Bitlocker at least accepts this as secure enough to boot Windows 11.