Trusted Platform Module (TPM)

One of the things that allows us to have both full disk encryption and also do some machine verification as it starts up is the trusted platform module, or TPM.

What the TPM is and what it does

For the most part, your TPM is just a chip. It's a chip that does processing, processing of cryptography, versus your CPU, which does all of the other processing. What it does is it offloads some of the cryptography processing onto a separate chip.

Here are some of the things that your TPM can do. Number one, it can generate random numbers, which is actually a pretty critical step when it comes to cryptography. It also can generate cryptographic keys, so we can generate keys using this. It can also do key storage and management. It can do cryptographic computations, things like encrypting and decrypting. It can do system integrity verification checks. And it can also do something called remote attestation.

Some of the things that this allows us to do are things like secure boot, or do disk encryption. It allows us to do credential protection, device health attestation, and things like virtual smart cards.

But there are two main functions that this TPM is really designed to do. One is for platform integrity: during the boot process, it goes and does some checks to verify things, to make sure that we have trusted hardware, to make sure we have trusted software. And then also that disk encryption — it really facilitates that at an early stage here. So these are the main functions of what the TPM performs.

The key storage problem

Let's discuss some of the problems that TPM actually solves for. The first one is key storage. Let's say we have some sort of file, or some sort of something on this disk, that needs to be encrypted. Maybe it's the whole disk itself, but in this case let's just use a file that needs to be encrypted. We could memorize the password to get into it — that would be one way — or memorize the key. We could use some sort of password manager, but that's not actually a great solution in many cases, especially when you're talking about full encryption of our disks, so full disk encryption.

So what the problem is is that we're storing the key. We could potentially be storing the key on the same device that gives it access into that device, and that's problematic. If it's the system that just is stopping you from accessing the key, all we have to do is pop this into another machine — maybe it's like a Linux machine — and then gain access to this key and then gain access to the file. So this can be really problematic.

This is where that trusted platform module comes into play. This is an extra chip that can do things like store the keys, or, even if it's not storing the key, it can help protect these keys. One of the things that this TPM has on it is a storage root key. This storage root key is embedded on the chip and never leaves the chip, so it can't be handed out to any other device. It is stored, and essentially it's burned in, on this TPM chip.

Now, if you have a key that you want encrypted, all you need to do is pass this key off to the TPM. The TPM will encrypt this, and now you can store this key anywhere else. If you ever need to gain access to this key, all you need to do is send it back to the TPM and then it can decrypt it. So this is great, because now you can't just take this disk and move it to another computer and gain access to this key. Instead, the only thing that can decrypt this key is this TPM on this machine. So what this machine will do is send the key, when you try to access this, send it to the TPM. The TPM will decrypt that and then you can use it to unlock whatever it is that you're trying to unlock. And so this is one of the real strengths to the trusted platform module.

Platform configuration registers

But we still have another issue here. That is, what if we just then take this disk right here, and take this module, and we just move it to another machine? Now when we move it to another machine, we can do the same thing. The other machine can grab a hold of this key, pump it into the TPM, and grab a hold of it to unlock whatever the files are that we are unlocking. So this is the second problem here.

This is where we can use platform configuration registers. You can think of these PCRs as being like a fingerprint of the machine. What it does is it takes certain data from the machine and creates a fingerprint out of that, and that fingerprint can be used as part of this encryption process. So now, in order to get the key back out, not only do you have to have the specific TPM chip that you have, but you also have to know what the fingerprint of the machine is and feed that information into this TPM in order to get the key back out.

What happens essentially is we take the key and the fingerprint. We feed it into the TPM. It generates then our key, and it doesn't matter then where that key is stored at. Now, in order to get that key back out, what will have to happen is that this key will be fed into the TPM — the encrypted key into the TPM — with the fingerprint, and then it will generate the key, or will decrypt it to create the key, and now we have access to this disk or this file.

The way these PCRs would work is that we could actually select what criteria it is, and there's lots of different criteria out there, like the firmware or the UI settings or other different types of settings, the boot manager. There's different things that can go into this fingerprint, so we can create a fingerprint of the machine.

Platform integrity and full disk encryption

Remember what the main functions of this TPM are designed to do. It's designed to make sure that we have a good boot process, that the hardware is trusted and the software is trusted. That gives us platform integrity, so it makes sure that nothing is changed on these systems. From the hardware perspective, it's got this fingerprint and can measure things up against the fingerprint. Same thing with the software: we can take certain aspects of the software and make sure that those measure up as well.

And then it does that full disk encryption. So this is a way to be able to store the key, grab the key with this fingerprint, and then be able to have access, or decrypt this disk. And that's where TPM plays into this full disk encryption and allows us to be able to facilitate verifying these disks and then decrypting these disks. So that's a big part of TPM.