Using the Trusted Platform Module to protect your keys

There was a big hype when the Trusted Platform Module (TPM) was introduced into computers. Briefly it is a co-processor in your PC that allows it to perform calculations independently of the main processor. This has good and bad side-effects. In this post we focus on the good ones, which are the fact that you can use it to perform cryptographic operations the same way as in a smart-card. What does that mean? It simply means that you can have RSA keys in your TPM chip that you can use them to sign and/or decrypt but you cannot extract them. That way a compromised web server doesn't necessarily mean a compromised private key.

GnuTLS 3.1.0 (when compiled with libtrousers) adds support for keys stored in the TPM chip. This support is transparent, and such keys can be used similarly to keys stored in files. What is required is that TPM keys are specified using a URI of the following forms.

tpmkey:uuid=c0208efa-8fe3-431a-9e0b-b8923bb0cdc4;storage=system
tpmkey:file=/path/to/the/tpmkey

The first URI contains a UUID which is an identifier of the key, and the storage area of the chip (TPM allows for system and user keys). The latter URI is used for TPM keys that are stored outside the TPM storage area, i.e., in an (encrypted by the TPM) file.

Let's see how we can generate a TPM key, and use it for TLS authentication. We'll need to generate a key and the corresponding certificate. The following command generates a key which will be stored in the TPM's user section.

$ tpmtool --generate-rsa --bits 2048 --register --user

The output of the command is the key ID.

tpmkey:uuid=58ad734b-bde6-45c7-89d8-756a55ad1891;storage=user

So now that we have the ID of the key, let's extract the public key from it.

$ tpmtool --pubkey "tpmkey:uuid=58ad734b-bde6-45c7-89d8-756a55ad1891;storage=user" --outfile=pubkey.pem

And given the public key we can easily generate a certificate using the following command.

$ certtool --generate-certificate --outfile cert.pem --load-privkey "tpmkey:uuid=58ad734b-bde6-45c7-89d8-756a55ad1891;storage=user" --load-pubkey pubkey.pem --load-ca-certificate ca-cert.pem --load-ca-privkey ca-key.pem

The generated certificate can now be used with any program using the gnutls library, such as gnutls-cli to connect to a server. For example:

$ gnutls-cli --x509keyfile "tpmkey:uuid=58ad734b-bde6-45c7-89d8-756a55ad1891;storage=user" --x509certfile cert.pem -p 443 my_host.name

An easy to notice issue with TPM keys is that they are not mnemonic. There is only an UUID identifying the key, but no labels making the distinction of multiple keys a troublesome task. Nevertheless, TPM keys provide a cheap way to protect keys used in your system.