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Rocks to Avoid When Developing TPM Applications

When using the TPM in an application, there are two major pitfalls to avoid. First, the TPM (or another component on the motherboard) may die, or users may upgrade their equipment. If the motherboard is replaced, any keys that are locked to the TPM go away. Second, if data is locked to PCRs (a process called sealing), and the things measured into the PCRs are updated, that data is no longer unsealable.

Both of these problems amount to the same thing: management of the keys and data locked to a TPM needs to be carefully considered. An example of how do this well is found in Microsoft's BitLocker application, which first came out with Windows Vista Enterprise.

Microsoft BitLocker

Microsoft gave careful consideration to both of the previously described problems when it created the BitLocker application, originally embedded in the Enterprise edition of Vista. This program was used to do full-disk encryption of the hard disk on which Windows resided. To do this, early in the boot sequence BitLocker obtained a key from the TPM. This key was sealed to PCRs that represented the boot sequence of the computer up to the point where the kernel was loaded into memory. BitLocker could also require the user to enter a password. To enable management of the encryption key used for full-disk encryption, the sealed key was used as a key encrypting key (KEK) and used to encrypt the full-disk encryption key. The actual key used for the full-disk encryption key could be then backed up by also encrypting it using a very long random password. This password could be kept secure elsewhere (for example, on a USB key locked in a safe). This way, if the motherboard was replaced, the TPM died, or the hard disk was moved into a new system, the data stored on it was still accessible.

Additionally, Microsoft gave thought to the problem caused by people upgrading their BIOS. Such an upgrade prevented the TPM from being able to unseal the KEK. Although the random-number backup sufficed for recovery in this case, Microsoft decided it would make more sense for an administrator doing the BIOS upgrade, who already had access to the decrypted data, to have a means to temporarily leave the fulldisk encryption key in the clear while the BIOS upgrade was performed and then reseal it to the TPM's new PCR values after the BIOS upgrade. It is important to realize that making things easy for the user at a small cost to security (leaving the drive open for the brief time while a BIOS upgrade was taking place) is usually a good tradeoff. Security that is hard to use is seldom used.

When IBM came out with its first TPM solutions, several years before BitLocker saw the light of day, it also had to keep manageability problems in mind.

IBM File and Folder Encryption

IBM had a similar problem when it allowed storage keys to be used for file and folder encryption to the TPM, and it solved the issue in a similar way. Instead of generating a random number, IBM wanted to let users type the answer to questions in order to recover the disk encryption key; this key was normally encrypted with the KEK, which in turn was protected by the TPM. This can be dangerous, because it may allow an attacker to simply try many answers to these questions in the hope of generating the correct answer and unlocking the drive. IBM's solution to this problem was clever. The company realized that although in normal use the key needed to be available almost immediately, in the case of recovery, it was fine if it took several minutes to recover the data. Therefore IBM performed a hash operation on the answers to the questions over and over again until a few minutes had passed, noted the number of operations, and then used the resulting value as a key to encrypt the file and folder encryption key. It then stored the number of operations and the encrypted blob on the hard disk. In order to decrypt this blob, someone had to spend several minutes for every attempt to answer the questions. This quickly becomes impractical for an attacker, but it costs a user only a few minutes in the case of recovery.

When TPM 2.0 was being designed, the architects had experience with the multitude of problems caused by managing TPMs, so new features were built into 2.0 to help solve these issues. One specific problem that is encountered repeatedly in security software is the need to manage authorizations (passwords). For example, someone changes a password while on a plane or late at night at a hotel, when they aren't connected to the network; then, the next day, they can't remember their password. Or someone working for a corporation quits or (worse yet) dies and leaves important corporate data encrypted on their hard disk without telling anyone their password. IT organizations are assumed to be able to fix problems like this—but it's hard to see how they can. TPM 2.0 enhanced authorization was designed to help fix the issue of managing passwords.

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