Trusted Computing Group's (TCG) TPM2 Software Stack (TSS). 

This stack consists of the following layers from top to bottom:

 - Feature API (FAPI) as described in the TCG Feature API (FAPI) 
Specification along with TCG TSS 2.0 JSON Data Types and Policy 
Language Specification This API is designed to be very high-level API,
intended to make programming with the TPM as simple as possible. The API
functions are exposed through a single library: libtss2-fapi.

 - Enhanced System API (ESAPI) as described in the TCG TSS 2.0 Enhanced 
System API (ESAPI) Specification This API is a 1-to-1 mapping of the 
TPM2 commands documented in Part 3 of the TPM2 specification. 
Additionally there are asynchronous versions of each command. In 
addition to SAPI, the ESAPI performs tracking of meta data for TPM 
object and automatic calculation of session based authorization and 
encryption values. Both the synchronous and asynchronous API are exposed
through a single library: libtss2-esys.

 - System API (SAPI) as described in the TCG TSS 2.0 System Level API 
(SAPI) Specification This API is a 1-to-1 mapping of the TPM2 commands 
documented in Part 3 of the TPM2 specification. Additionally there are 
asynchronous versions of each command. These asynchronous variants may 
be useful for integration into event-driven programming environments. 
Both the synchronous and asynchronous API are exposed through a single 
library: libtss2-sys.

 - Marshaling/Unmarshaling (MU) as described in the TCG TSS 2.0 
Marshaling/Unmarshaling API Specification This API provides a set of 
marshaling and unmarshaling functions for all data types define by the 
TPM library specification. The Marshaling/Unmarshaling API is exposed 
through a library called libtss2-mu.

 - TPM Command Transmission Interface (TCTI) as described in the TCG 
TSS 2.0 TPM Command Transmission Interface (TCTI) API Specification. 
This API provides a standard interface to transmit / receive TPM command
 / response buffers. It is expected that any number of libraries 
implementing the TCTI API will be implemented as a way to abstract 
various platform specific IPC mechanisms. Currently this repository 
provides several TCTI implementations: libtss2-tcti-device, 
libtss2-tcti-tbs (for Windows), libtss2-tcti-swtpm and 
libtss2-tcti-mssim. The former should be used for direct access to the 
TPM through the Linux kernel driver. The latter implements the protocol 
exposed by the Microsoft software TPM2 simulator.

 - The TCG TSS 2.0 Overview and Common Structures Specification forms 
the basis for all implementations in this project. NOTE: We deviate 
from this specification by increasing the value of TPM2_NUM_PCR_BANKS 
from 3 to 16 to ensure compatibility with TPM2 implementations that 
have enabled a larger than typical number of PCR banks. This larger 
value for TPM2_NUM_PCR_BANKS is expected to be included in a future 
revision of the specification.