Entities

• Permanent

• Non-volatile indexes

• Objects

• Non-persistent entities

• Persistent entities

• Entity names

Permanent

• It is defined by the specification.

  • Cannot be created nor deleted.

• Persistent hierarchies

• Ephemeral hierarchies

• Dictionary attack lockout reset

• PCRs

• Password authorization session

• Platform NV enable

Permanent Entities

Persistent hierarchies

Their elements persist across power-off.

Each one has an authorization and a policy.

• Platform

• Storage

• Endorsement

Ephemeral hierarchies

Their elements disappear across power-offs. But the hierarchies do not.

Have an authorization and a policy.

NULL hierarchy.

Dictionary attack lockout reset

A mechanism designed to overrule lockout states entered upon an excessive number of authentication failures.

Has an authorization and a policy.

PCRs

Arrays of registers

• Accessed by index.

• Organized in banks by algorithm.

Each has an authorization and a policy

• For updating only.

• Reading is always allowed.

Password authorization session

Sessions help to organize several commands under a common authorization policy.

Password authorization sessions are special.

• They only last for a command.

• Thus, they are implicit (thus, permanent).

Platform NV enable

The access to non-volatile (NV) platform data is controlled by this control.

Platform NV data belongs to the platform hierarchy.

TPM non-volatile (NV) indexes

A kind of pointer (or handle) to NV storage. With a structure unknown to the TPM.

Have more attributes than objects. Which have a structure known by the TPM.

Each one has an authorization value and a policy.

• Authorizations can be changed by the index owner.

• Policies cannot be changed.

They belong to a hierarchy.

• And are cleared when the hierarchy is cleared.

TPM NV index types

Ordinary

• Can contain any value, of arbitrary length.

Counter

• Can contain an increment-only 64-bit value.

• Initialized to the largest value that any counter has ever had in the TPM.

Bit field

• 64-bit array.

• Initialized at zero, bits can only be set (never reset).

Extend

• Similar to a PCR.

Objects

Key or data.

Each one has authorization data and a policy.

• Authorizations can be changed by the object owner.

• Policies cannot be changed.

They belong to a hierarchy.

• And are cleared when the hierarchy is cleared.

Their use requires a TPM non-persistent entity: the session.

Non-persistent entities

They disappear on power-on.

• Reset (upon reboot).

• They can be saved out of the TPM.

  • But not reloaded after a power cycle.

Are preserved across restarts and resumes.

• These operations send a different order to the TPM.

Authorization sessions.

Persistent entities

An object that the owner of a hierarchy sets to persist across power cycles.

Usually, keys, and just a few, since the TPM has a small persistent memory.

Names

Names prevent the reuse of handles

• They are given to entities that can reuse handles.

• They are unique entity identifiers.

Permanent entities have no name

• Their handle is constant.

Other entities have a name that is a hash of their public data.

Hierarchies

A hierarchy is a collection of related entities.

• Managed as a group.

• Can be enabled or disabled.

They include:

• Permanent entities

  • Hierarchy handles

• A private, random cryptographic root

  • Seed

• Key trees

  • Starting on a primary key

• NV indexes

• A private proof value

  • To check whether a value was produced by the TPM

Platform hierarchy

Intended to be in control of the platform manufacturer.

It is always configured by the manufacturer's firmware upon a boot.

• Which can also enable or disable it.

Storage hierarchy

Intended to be in control of the platform owner.

• Company

• End user

Intended for non-privacy-sensitive operations

All its contents can be cleared by the owner

• Useful when changing owners.

Endorsement hierarchy

Intended to be in control of the platform user.

The TPM chip generates its seed when first powered on.

• Endorsement primary seed.

Several primary keys are derived from the seed.

• And certified by the TPM manufacturer.

  • Certification attests they belong to a genuine TPM.

• Usually, these are encryption/decryption keys.

  • And not signing keys.