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Key Generator

Arguably, the TPM's greatest strength is its ability to generate a cryptographic key and protect its secret within a hardware boundary. The key generator is based on the TPM's own random number generator and doesn't rely on external sources of randomness.

It thus eliminates weaknesses based on weak software random number generators or software with an insufficient source of entropy.

Primary Keys and Seeds

TPM keys can form a hierarchy, with parent keys wrapping their children. Primary keys are the root keys in the hierarchy. They have no parent. Chapter 9 discussed the general concept of hierarchies and their use cases. Their specific application to keys is discussed under “Key Hierarchy.”

This section describes, in a linear flow, the creation and destruction of primary keys. In the narrative, the caller is some software that is provisioning the TPM, sending commands and receiving responses, whereas the TPM is the device that processes

the commands. Provisioning software (see Chapter 19) typically performs these steps. Although end users may use primary keys, they would not typically be creating them.

Primary keys are created with the aptly named command TPM2_CreatePrimary.

If you're familiar with TPM 1.2, you know that it has one key equivalent to the TPM 2.0 primary key: the storage root key (SRK), which is persistently stored in the TPM.

TPM 2.0 permits an unlimited number of primary keys, which don't need to be persistent. Although you might think the number would be limited by the TPM persistent storage, it's not. Primary seeds, described shortly, permit the expansion.

There were two reasons TPM 1.2 could function with one SRK. First, it had only one algorithm and key size for wrapping keys, RSA-2048. The design of TPM 2.0, of course, permits multiple algorithms and key sizes. Second, TPM 1.2 has only one key hierarchy: the storage hierarchy. TPM 2.0 has three hierarchies, each with at least one root. Chapter 9 discussed the general concept of hierarchies and their use cases.

How can a TPM with limited persistent storage have an unlimited number of root keys? A root can't exist outside the TPM because it has no parent to wrap its secret parts. The answer is the primary seeds.

Each of the three persistent hierarchies has an associated primary seed: the endorsement primary seed, the platform primary seed, and the storage primary seed. These seeds never leave the TPM. They're the secret inputs to key-derivation functions. When the TPM creates a primary key, it uses a primary seed plus a public template. The template includes all the items you would normally expect when specifying a key: the algorithms and key size, its policy, and the type of key (signing, encryption, and so on). The caller can also provide unique data in the template. The unique data is input in the public key area of the template.

The key-derivation function is fixed and repeatable. For the same seed, the same template always produces the same key. By varying the unique data in the template, the caller can create an unlimited number of primary keys.

When the TPM creates a primary key, it remains on the TPM in volatile memory.

The caller now has two choices. A limited number of primary keys can be moved to persistent memory using the TPM2_EvictControl command. Other keys can remain in

volatile memory.

If more primary keys are needed than can fit in persistent storage or volatile memory, some can be flushed (from volatile storage) or moved from persistent storage and then flushed. Because the seed is persistent, the key isn't lost forever. If the caller knows the template, which may be completely public, the TPM can re-create the identical key on demand. If the key being regenerated is an RSA key, this process may take a lot of time. If the key is an elliptic curve cryptography (ECC), AES, or HMAC key, the process of creating a primary key is very fast. In most use cases, at least one storage primary key is made persistent in the TPM for the storage hierarchy, to act in a manner similar to the SRK.

How would this work in practice? In TPM 1.2, there was one endorsement key and an associated certificate signed by the TPM vendor. They resided in persistent storage, so that when the final user got a system with a TPM on it, the user also had a certificate stored in the TPM's NVRAM that matched the endorsement key stored in the TPM. In TPM 2.0, there can be many key/certificate pairs—at least one for each algorithm the TPM implements. However, the end user may not want to consume valuable persistent storage for keys and certificates that aren't being used, even if they could fit.

A possible solution, which TPM vendors are expected to implement, is to have the manufacturer use the endorsement seed to generate several endorsement primary keys and certificates using a standard set of algorithms, each with a well-known template. One popular key, say RSA-2048, and its certificate can be moved to persistent storage. The vendor flushes the other keys but retains the certificates.

The TCG Infrastructure work group has defined several such templates for endorsement primary keys. The RSA template uses RSA 2048, SHA-256, and AES-128. The ECC template uses ECC with the NIST P256 curve, SHA-256, and AES-128. Both use the same authorization policy, which requires knowledge of the endorsement hierarchy password. This delegates the key authorization to the endorsement hierarchy administrator. The unique data is empty, a trivial well-known value. The attributes (see “Key Types and Attributes”) are fixedTPM and fixedParent true, as expected for an endorsement key that should never be duplicated. User With Auth and adminWithPolicy are specified so that a policy must always be used, not a password, which is appropriate because the TPM vendor has no way of passing a password to the end user. The key is a restricted decrypt key: that is, a storage key.

Suppose the end user desires a different primary key. That user can flush the one that was provisioned with the TPM and generate a new one with their algorithm of choice.

Magic happens now! Because the seed is unchanged and the user creates the primary key using the same template, they get the exact same key that the TPM vendor created. The user can treat the public part as an index into a TPM vendor certificate list. That list could even be on a public server. The user retrieves the certificate and is ready to go. This key-generation repeatability (the same seed and the same template always yield the same key) permits the TPM vendor to generate many keys and certificates during manufacturing, but not have to store them in the limited TPM nonvolatile storage.

The end user can regenerate them as needed.

Note that the vendor must generate all needed primary keys and vendor certificates in advance. Because the seed is secret, the vendor would otherwise not be able to determine that a public key value came from the vendor's TPM.

Once a seed is changed, the primary keys can no longer be re-created, and any keys residing in the TPM based on the old seed are flushed. This means any certificates the vendor created also become worthless. Creating a new certificate for a TPM endorsement key (EK) signed by the vendor would be very difficult. Because of this, changing the seed to the endorsement hierarchy is controlled by the platform hierarchy, which in practice means the OEM. This makes it difficult for an end user to change this seed. On the other hand, by simply choosing a random input in the template, the end user can create their own set of endorsement keys that are totally independent of the EKs the vendor produced.

the user has several primary storage keys that serve as the root for a key hierarchy. They can't all fit in persistent storage. If the user creates the keys using well-known templates, they can be re-created as needed.

the tpM commands are as follows:

• TPM2_NV_Read: reads the well-known template from tpM NV space. the tpM vendor may provision several templates (for example, one for rsa and one for eCC) on the tpM, and these templates match the provisioned key certificates. the user may also have enterprise-wide templates.

• TPM2_CreatePrimary: specifying the template.

• TPM2_EvictControl: Can optionally be used to make several keys persistent. especially for rsa keys, this saves the time required to regenerate them. Keys can also remain in volatile memory and be re-created after each power cycle.

the user wishes to create a primary key using a user secret in the template rather than using a well-known template. again, there are more primary keys than can fit in persistent storage. the user stores the secret in a tpM NV index, with suitable read access control, and retrieves it when needed to re-create the primary key.

the tpM commands are as follows:

• TPM2_NV_Write: Writes and protects a user secret.

• TPM2_NV_Read: reads the secret using appropriate authorization. the secret is inserted into the key template.

• TPM2_CreatePrimary: specify the template, which includes a user secret, to generate a custom primary key.

 
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