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KDF: Key Derivation Function

A manufacturer might want to ship a TPM with certificates for multiple key sizes for multiple algorithms. But a TPM has a limited amount of space to store these keys. Further, many TPM protocols require more than one secret. For example, one secret may be required to symmetrically encrypt a message, and a second may be used to HMAC the result. In order to reduce the cost of manufacturing a TPM, the TPM has the ability to create multiple keys from a single secret. This secret is called a seed, and the algorithm used to derive multiple secrets from this seed is called a key derivation function (KDF).

The TPM can use KDFs to derive both symmetric and asymmetric keys.

Because, as explained earlier, encryption doesn't provide integrity, the usual pattern is to encrypt with a symmetric key and then HMAC with an HMAC key. The question that arises is, “Does this mean there must be two shared secrets?” Not usually. The design pattern is to share one secret, a seed. From this one seed, a KDF is used to derive a symmetric encryption key and an HMAC key.

The TPM uses an HMAC as a KDF based on an algorithm specified by NIST in Special Publication 800-108. In a typical case, it HMACs some varying data using the seed as the HMAC key to derive the keys. The varying data always includes a string of bytes describing the application of the keys. This ensures that, when the same seed is used for different applications, different keys are used. This satisfies a basic crypto rule: never use the same key for two different applications. The other data also includes values unique to the operation. If you need two keys with the same description, you must use different unique values to guarantee that unique keys are created.

Authentication or Authorization Ticket

A ticket is a data structure that contains an HMAC that was calculated over some data. The ticket allows the TPM to validate at a later time that some operation was performed by the TPM. The HMAC asserts that some operation has been previously performed correctly and need not be performed again. In practice, the data is often not the actual message, which may be too large to fit in the ticket, but a digest of the message. The TPM uses tickets when it splits cryptographic operations into multiple operations with respect to time. Here, the HMAC key used to generate the HMAC is not a shared secret, but a secret known only to the TPM.

For example, the TPM uses a ticket when it calculates a hash over a message that it will later sign. It produces a ticket that says, “I calculated this hash and I assert that it is a hash that I will sign.” It signs the ticket by producing an HMAC using a secret only the TPM knows. Later, the HMAC is presented along with the ticket. The ticket is verified using the same secret HMAC key. Because only the TPM knows the HMAC key, it knows the ticket was produced by the TPM (authenticity) and that it has not been altered (integrity).

The TPM can do something similar when storing data outside the TPM. It can encrypt that data with a secret only it knows and then decrypt it again when loading it back inside the TPM. In this case, a symmetric key is used for encryption.

 
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