Desktop version

Home arrow Computer Science arrow A Practical Guide to TPM 2.0

History of Development of the TPM Specification from 1.1b to 1.2

The first widely deployed TPM was TPM 1.1b, which was released in 2003. Even at this early date, the basic functions of a TPM were available. These included key generation (limited to RSA keys), storage, secure authorization, and device-health attestation. Basic functionality to help guarantee privacy was available through the use of anonymous identity keys, based on certificates that could be provided with the TPM; owner authorization was required to create those identity keys. A new network entity called a privacy certificate authority (CA) was invented to provide a means to prove that a key generated in the TPM came from a real TPM without identifying which TPM it came from.

Areas of dynamic memory inside the TPM, called Platform Configuration Registers (PCRs), were reserved to maintain the integrity of a system's boot-sequence measurements. PCRs, together with identity keys, could be used to attest to the health of the system's boot sequence. This began the building of a secure architecture based on the TPM anchor, one of the key aims of the TCG.

One specific non-goal of the TCG was making the TPM design immune to physical attacks. Although such capabilities are possible, it was decided to leave physical protections to the manufacturers as an area where they could differentiate. Any software attacks are within scope for TPM-based security, however.

In a manner similar to smart-card chips attached to the motherboard, IBM PCs were the first to use TPMs (similar security coprocessors had been used in mainframe computers for decades). HP and Dell soon followed suit in their PCs, and by 2005 it was difficult to find a commercial PC that did not have a TPM.

One drawback of TPM 1.1b was incompatibilities at the hardware level. TPM vendors had slightly different interfaces, requiring different drivers, and package pinouts were not standardized. TPM 1.2 was developed from 2005–2009 and went through several releases. Its initial improvements over 1.1b included a standard software interface and a mostly standard package pinout.

The TCG realized that although TPM 1.1b protected keys against attackers who did not know a key's authorization password, there was no protection against an attacker trying one password after another in an attempt to guess a correct password. Attackers who do this usually try passwords from a dictionary of common passwords; this is known as a dictionary attack.

The TPM 1.2 specification required that TPMs have protection against dictionary attacks.

Privacy groups complained about the lack of implementations of privacy CAs. This led to the inclusion in TPM 1.2 of new commands for a second method of anonymizing keys to help address this concerns—direct anonymous attestation (DAA)—and a method of delegating key authorization and administrative (owner-authorized) functions.

It turned out that shipping a machine with a certificate for its TPM's endorsement keys on the hard disk was in many case impractical, because IT organizations often erased the hard disk when they received it and installed their own software load. When they did so, the certificate was deleted. In order to provide a solution, a small amount of nonvolatile RAM (usually about 2KB) was added in TPM 1.2; it had specialized access controls along with a small number of monotonic counters.

The 1.1b specification had a means of copying migratable keys from one TPM to another TPM in case a machine died or needed to be upgraded. This process required the approval of the key owner and the TPM owner. It was designed with the assumption that an IT administrator would be the TPM owner and the user would be the key owner. But in 1.2, the user needed to be able to use the TPM owner authorization to mitigate dictionary attacks and create NVRAM, which made this design impractical. Therefore, in 1.2, another technique was designed to let users create keys that could only be migrated by a designated third party. Such keys could be certified to this effect and hence were called Certified Migratable Keys (CMKs).

Signing keys are often used to sign contracts, and having a timestamp of when the signing takes place is useful. The TCG considered putting a clock into the TPM, but the problem was that the TPM loses power whenever the PC is turned off. Although putting a battery in the TPM is possible, it is unlikely that the increased function would be worth the higher cost. Therefore the TPM was given the ability to synchronize an internal timer with an external clock and then to sign with the value of the internal timer. As an example, this combination could be used to determine when a contract was signed. This functionality also lets the TPM be used to distinguish how much time elapsed between two signature operations performed by the TPM.

No changes were made to existing application programming interfaces in 1.2, which preserved binary compatibility of software written to the 1.1b specification. A side effect was that the TPM 1.2 specification became even more complex, because special cases had to be used to maintain compatibility.

Before TPMs became ubiquitous, security coprocessors such as smart cards were used by some applications to store keys to identify users and keys to encrypt data at rest. TPMs are well equipped to take over this task. But they can do much more: because the security coprocessor is integrated onto the system's motherboard in the form of a TPM, it has additional uses (such as device identification) that are detailed in Chapter 3.

TPM 1.2 was deployed on most x86-based client PCs from 2005 on, began to appear on servers around 2008, and eventually appeared on most servers. Just having hardware does nothing, however—software needs to use it. In order to make use of the TPM hardware, Microsoft supplied a Windows driver, and IBM open sourced a Linux driver. Software began to be deployed, as described in Chapter 4.

< Prev   CONTENTS   Next >

Related topics