52 Things: Number 48: What is the purpose and use of a TPM?

This is the latest in a series of blog posts to address the list of '52 Things Every PhD Student Should Know To Do Cryptography': a set of questions compiled to give PhD candidates a sense of what they should know by the end of their first year.

Before examining the point of this question (namely what the purpose and use of a TPM is) it's worth trying to understand the problem a TPM is designed to overcome. The problem is really one of trust. Trusting what? Well, primarily the memory and software running on a computer. These things can be directly accessed by the operating system and so secret information (such as cryptographic keys) can be accessed by an attacker who has access to the machine at the operating system level. If these keys are being stored directly in memory and being accessed by software, it could be fairly trivial for an attacker to read off the memory location where the keys are being stored and then compromise security.

One way around this problem is make sure that keys are never stored directly in the computers memory which can be accessed by software. Given that the keys are required for secure applications they must at some point be presented in a state that can be used by the software so how could this be possible? Well, one way is to protect the secret keys stored in memory by wrapping them using a key that the software does not have access to. By having a separate piece of hardware for instance that has a key burned into it and which is able to perform certain cryptographic operations with that key. This piece of hardware could therefore be employed by the software to do various things with this secret key that is stored on the hardware to do things such as wrap keys to be stored in memory, but never have access to this key directly.

This is essentially what a TPM does. A TPM has an RSA key pair called the Storage Root Key (SRK). The private part of this key is kept secret from everything and everyone. Using this private key, other keys (that software uses) can be wrapped (often called “binding”) using the SRK, protecting them from disclosure. In addition to simply wrapping keys, TPMs can also wrap keys and tie them to certain platform measurements. This type of key can only be unwrapped when those platform measurements have the same values that they had when the key was created. This process is known as “sealing.” TPMs can also be used for cryptographic key generation and perform other cryptographic tasks one of which is know as remote attestation, which creates a hash key summary of the hardware and software configuration allowing a third party to verify that the software has not been changed.

The real point to understand here is that by pushing security down to the hardware level and ensuring that it is given over to a separate piece of hardware that has it's own firmware and circuits that can't be altered from the outside, the system is not exposed to software vulnerabilities and is therefore more trustworthy.

So what is the purpose of a TPM? To overcome the problem of trusting (or rather not trusting) software to be completely reliable.

What is the use of a TPM? We mentioned a number of them. First of all was binding, which essentially wraps a key using the private key of the SRK. The second was sealing which also ties the wraped key to a particular platform measurements. And thirdly we looked at remote attestation and noted that TPMs can also be used for other cryptographic functions such as key generation.