TPM Reference Code - TPM Crypto Library Introduction

• TPM Reference Code - TPM Crypto Library Introduction
  • See Also
  • Background
  • Components of a Cryptographic Protocol
  • TPM Crypto Support Library Design
    • Implications
  • Specifying the libraries

See Also

• Introduction
• Library Architecture
• Tpm BigNum Library
• Tpm Crypto Libraries
• Tpm Math Library
• Tpm Platform API

Background

As mentioned in Library Architecture, any TPM implementation using this Core library is expected to provide several additional libraries or components. This document describes the Crypto support libraries at a high level.

Components of a Cryptographic Protocol

Roughly speaking, any usable end-to-end crypto protocol breaks down into some combination of four groups of things:

Group	Algorithm Category	Examples
1	Symmetric primitives	Expand AES Key Schedule Encrypt a single 16-byte block
2	Hash primitives	Start Hash Hash Block End Hash
3	Big Number (BN) math primitives	Compute a^e mod N, where a,e,N are “big” integers
4	Cryptographic "Protocol"	Details building these primitives into end-to-end cryptographic operations.

Examples may help make this clearer. Depending on mode, encrypting the simplest message (a single block of data) with AES involves at least two operations from category 1:

• generating a key schedule, and
• performing the block encryption.

Similarly, hashing a message requires at least three operations from category 2:

• initializing the hash state, and
• extending the data block(s), and
• finalizing the hash.

Note: Correct Category 4 implementation is critical to security - many practical cryptographic weaknesses stem from errors in how the primitives are applied, even when the primitives are themselves secure.

TPM Crypto Support Library Design

The Core implementation strategy has been to link with (potentially separate) crypto libraries that provide the primitives from groups 1 & 2 and (a subset of) 3. The Core TPM reference library provides the remaining Group 3 and Group 4 code to create meaningful end-to-end cryptographic operations.

The interface to external libraries is implemented in the form of C language macros and function calls.

Implications

This design has pros and cons:

Pro	Con
Minimal requirements on crypto support libraries.	Difficult to take advantage of optimized higher-level crypto libraries
TPM specification able to evolve to support new commands and protocols without requiring crypto library changes.	Extra data conversions between internal and support library formats
When supported by the algorithm, allows blockwise/stream processing where the data would otherwise be too large.
Allows the Core library to do parent/child key derivation as described in the TPM spec.

This means the existing Core is not "plug-ready" to accept another cryptographic library that (for example) wants to provide all of groups 3 or 4 such as generating primary asymmetric keys, or an AES library that exposes only higher-level operations like EncryptMessageAesCbc(message, length).

This is true of symmetric block chaining implementations and key derivation functions, but has the greatest impact on asymmetric crypto implementation because the necessary API surface to support Symmetric and Hash operations is much smaller that the generalized large-number API surface required to implement arbitrary asymmetric crypto. Implementations could replace higher-level pieces of the Core library (e.g. the entire src/crypt folder) to directly provide desired behavior. This requires the crypto library (or a middle layer) to effectively rewrite TPM-specific code to understand TPM constructs rather than being an arbitrary cryptographic library.

Specifying the libraries

Any crypto library must provide a set of header files that are the interface between the Core Library and the particular crypto implementation. The Core code is informed of the name of these header files by three compile-time defines:

Build Flag	Interface File	Description
HASH_LIB	TpmTo[HASH_LIB]Hash.h	Hashing primitives to provide hashing and HMAC functionality
SYM_LIB	TpmTo[SYM_LIB]Sym.h	Symmetric (e.g. AES) primitives
MATH_LIB	TpmTo[MATH_LIB]Math.h	Math Interface used to provide asymmetric implementations

Each library must provide a header that matches one of the patterns above.

The TPMCmd folder provides the following implementations for these three interfaces:

Build Flag	Flag Value	Description
HASH_LIB	Ossl	OpenSSL Hash implementation
SYM_LIB	Ossl	OpenSSL AES implementation
MATH_LIB	TpmBigNum	TPM "bigNum" based asymmetric Math library. See TPM BigNum Library

At run-time, a simulator provides information about its crypto libraries via the GetCrypto{Sym,Hash,Math}Impl RPCs on the platform port.