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Certified Policies

One last thing you can do with policies is prove that a policy is bound to a particular entity. When ink is used to sign a contract, the signature that is formed is irrevocably tied to the person signing it via a biometric that represents the way the person's muscles and nerves are formed. That is what produces the characteristic swirls of a signature.

Electronic signatures have never been tied to a person in the same way. Typically, electronic signatures have been tied to a password (something a person knows) or a smart card (something a person has), or sometimes (usually in addition to the others) a biometric. Biometric devices can break, so in most implementations, there is always a backup password that can be used if the biometric doesn't work. (Interestingly, the ink signature has a similar problem, because people can break their hands.)

With the TPM 2.0, it's possible to tie the use of a key directly to a biometric and prove that it's so tied. First a non-duplicatable key is created, with its authValue set so that a password isn't useful for authorization. This means only the policy can be used to authorize use of the key. The policy is set to only allow use of the key when authorized by a biometric reader, using TPM2_PolicySign and a policyRef that is produced and signed by the biometric reader when it matches the person. This produces a key that can only be used to sign something if the biometric reader is convinced the person is who it thinks the person is. We call this key A.

Next a credentialed non-duplicable restricted signing key is used with TPM2_Certify to produce a signature over the name of key A. This signature binds the public portion of key A (which is in the name), the authValue (which are in the name), and the policy (which is in the name). By checking the credential of the restricted signing key, an attesting agent can verify that the certificate produced by TPM2_Certify is valid. Then, by hashing the public data of the key, the agent can verify that the name is correct. This then validates that the only way the key could be used for signing is by satisfying the policy, not by a password. The policy is then examined and, using the public key of the biometric device, is validated to be satisfied only if the user swiped their finger over the reader.

In this way, the electronic signature with the key becomes tied to the fingerprint biometric. In a similar way, producing certificates binding policies to keys can be used to prove to an auditor that the policies being used for keys meet a corporate standard for security. This in turn satisfies the last of the problems that EA was created to solve.

Summary

This chapter has examined the new enhanced authorization in the TPM 2.0, which can be used to authorize any entity in the TPM. You have seen that EA policies can be used to create logical combinations (AND and OR) of multiple kinds of assertions—everything from passwords and smart cards to the state of the TPM or the state of a remote machine. You have looked at examples of using EA for multiple users, multifactor authorization, and the means to create policies that allow flexible management. Many examples demonstrated the ways these commands can be used to solve varied problems. Then you saw how such policies can be satisfied. Finally, you saw how a key can be bound to its policy. EA in the TPM 2.0 is one of the most complex but also most useful new capabilities in the TPM 2.0 design.

 
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