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Wrong Type

In TPM 1.2, keys are basically signing keys or decryption/storage keys. Stepping through the function that performs the type check should uncover the error.

TPM 2.0 adds the concept of restricted keys, which introduce two new error cases. First a restricted key might may be used where only a nonrestricted key is permitted. Second, the user may try to change an algorithm, but restricted keys are created with an algorithm set that can't be changed at time of use.

There is also far more variability with respect to algorithms than TPM 1.2 has, where there were just a few padding schemes. Even PCRs have variable algorithms, which may lead to failures during an extend operation.

In addition, TPM 2.0 NV space has four types (ordinary, bit field, extend, and counter). This will undoubtedly lead to errors such as trying to write ordinary data into a bit-field index.

Bad Size

Asymmetric key operations are limited in the data size they can operate on. A common bug is trying to sign or decrypt (unseal) data that exceeds the capacity of the key and algorithm. For example, an RSA 2,048-bit key can operate on somewhat less than 256 bytes. The “somewhat” accounts for prepended data that includes padding and perhaps an object identifier (OID).

Policy

TPM 2.0 introduces policy authorization, which is very flexible but may prove hard to debug. Fortunately, the TPM itself has a debug aid, TPM2_PolicyGetDigest. Although you can't normally look inside the TPM or dump the contents of internal structures, this command is an exception and does exactly that.

Recall that an entity requiring authorization has a policy digest, which was precalculated and specified when the key was created. The value is computed by extending each policy statement. At policy-evaluation time, a policy session starts with a zero session digest. As policy commands are executed, the session digest is extended. If all goes well, the session digest eventually matches the policy digest, and the key is authorized for use.

However, in this chapter, the presupposition is that all isn't well. The digests don't match, and the authorization fails. We anticipate that the debug process will again consist of “divide and conquer.” First determine which policy command failed, and then determine why.

Some policy commands, such as TPM2_PolicySecret, are straightforward, because they return an error immediately if authorization fails. Others—deferred authorizations like TPM2_PolicyCommandCode—are harder to debug because failure is only detected at time of use.

To determine which policy command failed, we suggest that you save the calculations used to calculate the policy hash. That is, the first hash value is all zeroes, there is an intermediate hash value for each policy statement, and there is a final value (the policy hash). Then, at policy-evaluation time, after each policy command, use TPM2_PolicyGetDigest to get the TPM's intermediate result. Compare the expected value (from the policy precalculation) to the actual value (from the TPM). The first miscompare isolates the bug to that policy statement.

One author's dream is that a premium TSS Feature API (FAPI) implementation will perform these steps. It has the policy, an XML document, so it can recalculate the intermediate hashes (or even cache them in the XML policy). It could implicitly send a TPM2_PolicyGetDigest after each policy evaluation step. This way, the evaluation could abort with an error message at the first failure rather than waiting until time of use, where it can only return a generic, “it failed, but I'm not sure where” message.

Determining why it failed strongly depends on the policy statement. Debugging the “why” is left as an exercise for you, the reader.

Summary

This chapter has described many of the best-known methods we've found for debugging TPM applications. Hopefully these will give you a good start in debugging, and you'll go on to discover even better techniques.

 
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