Key management

Can generate keys internally, and export them.

• Public keys can be exported unprotected.

• Private keys can be exported protected. Wrapped.

Can import keys, previously exported by itself, produced by others.

TPM as a key cache

Many times keys must be used within the TPM-confined environment, but the TPM resources should not limit their number.

Solution:

• Off-load keys to external repositories (wrapped)

• Import keys of interest

• Use cached keys

• Clear cached keys from TPM

Key trees (aka key hierarchies)

Sequence of keys.

• Starting from a top-most, primary key

• Hierarchies can have several key trees

Primary keys are generated from hierarchy seeds and a public template:

• Algorithm, key size, policy, usage.

• Arbitrary data (kind of label).

The key derivation is idempotent.

• Same seed, same template -> same key.

Storage keys

Keys that encrypt keys.

• A storage key is a parent of a child's key.

• It can be used to wrap the child key.

  • For exporting it.

They cannot be used for arbitrary decryption.

• It could compromise wrapped child keys.

Changing seeds

Different seed -> different key tree, the previous one is naturally flushed.

This should not happen to the endorsement seed.

• Because endorsement keys are pre-certified by the TPM manufacturer.

• Getting the certification again is complex.

• Can only be changed by the platform builder (OEM).

  • This is not an issue for the platform owner.

  • New keys can always be created with different templates.

Endorsement primary keys

Need to be certified by the TPM manufacturer.

• The TPM manufacturer uses it to generate candidate primary keys.

  • Using well-defined templates.

• The public keys are exported and certified.

  • The manufacturer keeps the certificates.

• The keys are removed from the TPM.

  • The TPM keeps the seed that allows the recreation of keys with the same templates used by the manufacturer.

Key duplication

Copy of a key into a different hierarchy. It continues to exist in its original place.

Primary keys cannot be duplicated

• Because they cannot be wrapped.

• They are generated from a seed.

  • And a template.

Duplication of parent keys can duplicate their children.

• Duplication group.

Key attributes

Type

• Symmetric

• Asymmetric

Usage

• Signing

• Encryption

• Decryption

User restrictions

• Restrict signing

  • Attestation

• Restrict decryption

  • Storage

Duplication restrictions

• Fixed TPM

  • Cannot be duplicated

• Fixed parent

  • Can only be duplicated under a duplicated parent