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Command-Based Assertions

Although not strictly an assertion, it's possible to restrict a policy so that it can only be used for a particular command. For example, you can restrict a key so that it can be used for signing but not to certify other keys. If this is done, then the policy can only be used to do that one particular command. Generally this isn't done as a single assertion, but it could be. By declaring in a key's policy that it can only be used for signing, the key is prevented either from certifying another key or from itself being certified. This is because when a key is certifying or being certified, it needs to provide an authorization that can't be provided.

To create such a policy assertion, you create a policy variable of size equal to the hash algorithm and initialize it to zero. It's then extended with the value TPM_CC_PolicyCommandCode || the command code to which the policy is to be restricted. [1] If you're using a trial session to create this policy, you execute TPM2_PolicyCommandCode, passing it the handle of the trial session and the command code.

Usually, if you're restricting a TPM entity like a key to only be used in a single command, you also want to authenticate use of that key for that command. This requires that more than one restriction be placed on the key, which is the subject of multifactor authentication.

Multifactor Authentication

The TPM knows how to authenticate using assertions. It also can be told to require more than one of them. For example, it may be asked to specify that both a fingerprint and a smart card be used to provide authentication to log in to a PC.

Policies, as you'll see, build together in a way similar to the way PCRs are extended. They start with an initial value of all zeroes (the number of zeroes depends on the size of the hash algorithm used to create the policy). When a policy command is invoked, the current policy value is extended by appending a new parameter to the old value, hashing the result, and then replacing the old value with the result of this calculation. This calculation is called extending in PCRs. A logical AND in a policy is accomplished by extending the new assertion into the policy. Just like a PCR, the policy is initialized to all zeroes before the first assertion, but later assertions build on the value created by the previous assertion.

This means if you're using a trial session to build this kind of policy, you start and end exactly the same way—you just add more commands in the middle to correspond to the various ANDed assertions.

  • [1] The TPM_CC listing table is found in part 2 of the specification.
 
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