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TPM Software Stack

This book is primarily about TPM 2.0 devices. However, a TPM without software is like a car with a full tank of gas but no driver; it has great potential but isn't going anywhere.

This chapter, in preparation for the rest of the book, introduces you to the TPM's “driver” [1], the TPM Software Stack (TSS). A good understanding of this topic will enable you to understand subsequent code examples in this book.

The TSS is a TCG software standard that allows applications to intercept the stack, that is, be written to APIs in the stack at various levels in a portable manner. Applications written to the TSS should work on any system that implements a compliant TSS. This chapter describes the layers of the TSS with a particular focus on the System API and Feature API layers. The other layers are described at a high level.

The Stack: a High-Level View

The TSS consists of the following layers from the highest level of abstraction to the lowest: Feature API (FAPI), Enhanced System API (ESAPI), System API (SAPI), TPM Command Transmission Interface (TCTI), TPM Access Broker (TAB), Resource Manager (RM), and Device Driver. [2]

Most user applications should be written to the FAPI, because it's designed to capture 80% of the common use cases. Writing to this layer is the TPM equivalent of writing in Java, C#, or some other higher-level language.

The next layer down is the ESAPI, which requires a lot of TPM knowledge but provides some session management and support for cryptographic capabilities. This is like writing in C++. At the time of this writing, the ESAPI specification is still a work in progress, so it isn't described in this chapter.

Applications can also be written to the SAPI layer, but this requires much more TPM 2.0 expertise. This is analogous to programming in C instead of a higher-level language. It provides you with access to all the functionality of the TPM but requires a high level of expertise to use.

TCTI is the layer used to transmit TPM commands and receive responses.

Applications can be written to send binary streams of command data to the TCTI and receive binary data responses from it. This is like programming in assembly.

The TAB controls multiprocess synchronization to the TPM. Basically it allows multiple processes to access the TPM without stomping on each other.

The TPM has very limited on-board storage, so the Resource Manager is used in a manner similar to a PC's virtual memory manager to swap TPM objects and sessions in and out of TPM memory. Both the TAB and the RM are optional components. In highly embedded environments that don't have multiprocessing, these components are neither needed nor, in some cases, desired.

The last component, the device driver, handles the physical transmission of data to and from the TPM. Writing applications to this interface is possible as well and would be like programming in binary.

Figure 7-1 illustrates the TSS software stack. Some points to note:

• Although typically there is only one TPM available to applications, multiple TPMs could be available. Some of these could be software TPMs, such as the Microsoft simulator; others may be accessed remotely over the network—for instance, in the case of remote administration.

• Generally, components from the SAPI on up the stack are per-process components.

• Components below the SAPI are typically per-TPM components.

• Although Figure 7-1 doesn't show it, TCTI may be the interface between the RM and the device driver. In this case, the TCTI appears at multiple layers in the stack.

• At this time, we think the most common implementation will combine the TAB and the RM into a single module.

Figure 7-1. TSS diagram

The following sections describe each of the TSS layers.

  • [1] This is not to be confused with an OS device driver.
  • [2] The device driver isn't officially part of the TCG-defined TSS, but it makes sense to discuss it in this chapter because it's one of the layers in a TPM software stack.
 
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