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0 pointsFoxboron1y ago0 comments

> For us, the benefits of TPMs and measured boot for personal use are a lot more obscure. You'll sometimes hear people claim it protects against 'evil maid attacks' where an attacker repeatedly gets physical access to your laptop. The truth is it provides no such protection.

TPMs give you fine and adequate protections in many scenarios, even physical ones.

They also provide you with better protection for private key material.

I'll even give you an example:

https://github.com/Foxboron.keys

The last key is a TPM key from my `ssh-tpm-agent` project: https://github.com/Foxboron/ssh-tpm-agent

Here is the private key: https://paste.xinu.at/9fc2YJQuUCbg1Sa/

I don't remember if the key has a PIN (it was for a presentation/demonstration), but if it has it's like 4 digits long.

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michaelt1y ago

> TPMs give you fine and adequate protections in many scenarios [...] my `ssh-tpm-agent` project

I agree that's adequate, in the sense that keeping the an SSH key as a password-protected file on disk is adequate, and having it be a password-protected secret in the TPM is no less secure than that.

But the whole point of binding a key to hardware is to be secure even if a remote attacker has gotten root on your machine. An attacker with root can simply replace the software that reads your PIN with a modified version that also saves it somewhere. Then they can use the key whenever your computer is online, even if they can't copy the key off. And although that's a bit limiting, once they've SSHed to a host as me once they can add their own key to authorized_keys in many cases.

That's why Yubikeys and U2F keys and suchlike have a physical button.

TPMs would be a lot more useful if the spec had mandated a physical button for user presence.

FoxboronOP1y ago

> But the whole point of binding a key to hardware is to be secure even if a remote attacker has gotten root on your machine. An attacker with root can simply replace the software that reads your PIN with a modified version that also saves it somewhere. Then they can use the key whenever your computer is online, even if they can't copy the key off.

It protects against extraction, not usage on the machine itself. Of course they can use the secret on the compromised machine.

> And although that's a bit limiting, once they've SSHed to a host as me once they can add their own key to authorized_keys in many cases.

Assuming they can edit the file.

> That's why Yubikeys and U2F keys and suchlike have a physical button.

The TPM spec has a policy setup to account for some fingerprint reader that can be used to authenticate. I haven't been able to figure out how/what/whys of the implementation here but this is very much a thing.

michaelt1y ago

> It protects against extraction, not usage on the machine itself. Of course they can use the secret on the compromised machine.

Yes, this is why I was careful to say that the benefits are obscure, rather than saying they're entirely nonexistent.

I'll admit that's a benefit, but it seems very small benefit considering the far-reaching changes it's needed like kernel lockdown mode, the microsoft-signed shim, distro-signed initrd, the difficulties it creates with DKMS, and so on.

Whereas people who need to bind their SSH key to hardware can get a higher degree of security with a far smaller attack surface by simply spending an hour's wages on a Yubikey.

FoxboronOP1y ago

> I'll admit that's a benefit, but it seems very small benefit considering the far-reaching changes it's needed like kernel lockdown mode, the microsoft-signed shim, distro-signed initrd, the difficulties it creates with DKMS, and so on

None of this is needed to take advantage of TPMs.

> Whereas people who need to bind their SSH key to hardware can get a higher degree of security with a far smaller attack surface by simply spending an hour's wages on a Yubikey.

Yubikeys are expensive devices, and TPMs are ubiquitous. Better tooling solves this problem.

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michaelt1y ago

> TPMs give you fine and adequate protections in many scenarios [...] my `ssh-tpm-agent` project

That's why Yubikeys and U2F keys and suchlike have a physical button.

TPMs would be a lot more useful if the spec had mandated a physical button for user presence.

FoxboronOP1y ago

It protects against extraction, not usage on the machine itself. Of course they can use the secret on the compromised machine.

> And although that's a bit limiting, once they've SSHed to a host as me once they can add their own key to authorized_keys in many cases.

Assuming they can edit the file.

> That's why Yubikeys and U2F keys and suchlike have a physical button.

michaelt1y ago

> It protects against extraction, not usage on the machine itself. Of course they can use the secret on the compromised machine.

Yes, this is why I was careful to say that the benefits are obscure, rather than saying they're entirely nonexistent.

Whereas people who need to bind their SSH key to hardware can get a higher degree of security with a far smaller attack surface by simply spending an hour's wages on a Yubikey.

FoxboronOP1y ago

None of this is needed to take advantage of TPMs.

> Whereas people who need to bind their SSH key to hardware can get a higher degree of security with a far smaller attack surface by simply spending an hour's wages on a Yubikey.

Yubikeys are expensive devices, and TPMs are ubiquitous. Better tooling solves this problem.

1 more reply

j / k navigate · click thread line to collapse