Home Computer Science A Practical Guide to TPM 2.0
Public Key Certification
Certification is part of an asymmetric-key protocol and solves the following problem: How do you trust the public key? After all, it accomplishes nothing to verify a digital signature with a public key if you don't know whose public key you're using. Secrecy won't be preserved if you encrypt a message with the attacker's public key. Establishing trust in a TPM public key includes knowing that the key really came from whom it was supposed to come from—in this case, a TPM.
The solution is to create a digital certificate. A certificate includes the public part of the key being certified plus attributes of that key. The certificate is then signed by a certificate authority (CA) key. It's possible that the CA public key is in turn certified by another CA key, forming a hierarchy (a certificate chain). At some point, this certificate chain terminates at a root certificate. The root public key must be conveyed to a verifier out of band and is declared trusted without cryptographic proof.
The X.509 standard describes a widely used certificate format. The TPM, as a limited-resource device, neither creates nor consumes X.509 certificates. The TCG Infrastructure work group does specify some X.509 certificate formats, and the TPM typically stores them. This storage is simply for provisioning convenience, pairing a certificate with its key, not to achieve any security goal.
In the TPM space, there are several certification processes:
• The TPM vendor and platform manufacturer may provision the TPM with TPM vendor and platform endorsement keys (EKs) and corresponding certificates before shipment to the end user.
The TPM vendor certificate asserts, “This endorsement key is resident on an authentic TPM manufactured by me.” The platform manufacturer certificate asserts, “This key is resident on a TPM that is part of my platform, and this platform supports certain TPM features.” These certificates typically use X.509 format.
• If the TPM keys (and their corresponding certificates) just described exist as signing keys, they can be used to certify other keys as being resident on the TPM and having certain properties. The TPM 2.0 specification provides commands to create certificates. These TPM-generated certificates do not use X.509, which is too complex for the limited on-chip resources of the TPM.
Essentially, digital certificates rest on the integrity of the CA. The CA is considered to be a neutral party that can be trusted by two parties: the parties that create the certificates and those that use them. A CA's functioning is similar to an escrow agent that mediates fund transfers in a real-estate transaction. If the CA is worthy of trust, all is good. If not, all bets are off.
When a TPM manufacturer produces a certificate for a TPM, the manufacturer faces a quandary. What algorithm should be used for the key? The manufacturer doesn't know if the end user will want RSA-2048, ECC2-56, ECC-384, or some other algorithm. And it also needs to know what hash algorithm and symmetric algorithm should be used in the creation of the key. To solve this problem, the TPM is designed to allow the creation of many keys derived from a single large random number, using a key-derivation function, as described earlier. You see in the chapter on hierarchies how this is used to provide many certificates for multiple algorithms without using up space in the TPM.
By examining a sample use case, you've seen all the major security operations and concepts that are used in the rest of the book to explain the creation and use of the TPM. This isn't surprising, because the TPM was designed with use cases in mind, and one of the major ones was e-commerce. By starting with the attacks cryptographic operations need to defend against, you saw why cryptographic algorithms are chosen from well-vetted internationally recognized algorithms and how key strengths are chosen. You reviewed the concepts of confidentiality, integrity, electronic identity, and nonrepudiation and how they relate to the standard classes of algorithms: symmetric, asymmetric, hash, and HMAC. Finally, you learned about some specific new features in the TPM specification that use those algorithms: extend, tickets, and certificates. You're ready to consider all the use cases the TPM was design to solve.
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