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A 'TPM chip' is:

a secure cryptoprocessor that can store cryptographic keys that protect information

FIPS 140-2 requires a cryptographic module, which can be hardware, software, or both that have been certified.

If I was using Bitlocker as a cryptographic module, would I need a TPM as another piece of that cryptographic module?

Is a TPM required for FIPS 140-2 security level 1 compliance?

What about security level 2?

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up vote 10 down vote accepted

No, absolutely not. FIPS 140-2 level 1 is essentially a test that your cryptographic module:

  • uses approved algorithms;
  • implements those algorithms correctly;
  • does not leak like a sieve.

Software can be validated at FIPS 140 level 1, there is no need to involve special hardware. FIPS 140 level 1 does not indicate resistance against an attacker with hardware access, unless the hardware is part of the target of evaluation. If you have a requirement for FIPS 140 level 1 compliance, then using Bitlocker (an approved version thereof, under an approved version of Windows operating in FIPS mode) is sufficient, whether or not the secrets are stored in a TPM.

At FIPS 140 level 2, the hardware must be part of the target of evaluation, but the security requirement is limited to tamper-evidence. A TPM is overkill for that (TPMs offer tamper resistance, so in principle a suitably designed TPM-based system could reach FIPS 140 level 3, though there are sticky points, and going beyond level 2 is not likely to be practical). You can browse the list of (non-secret) FIPS-140 validated modules. Note that the list does not include Bitlocker+TPM combinations, and in fact no TPM has been certified at FIPS 140 level 2 so far.

One of the hurdles in certifying a TPM is that it includes a large number of algorithms, many of them non-approved. The TPM does not provide a good way to disable these algorithms or to isolate all FIPS mode data from non-FIPS mode data. Yet this would have to be done, and tracked not only during operation but through the lifetime of the TPM, since the TPM includes non-volatile storage. (How do you know a secret key stored in FIPS mode is not leaked to non-FIPS-mode where it's used to encrypt some data with a broken algorithm?) Another hurdle is that all certified algorithms undergo a self-test when the device boots, and this takes a while (TPMs have pretty slow chips).

By the way, if you do not have a legal or contractual requirement for FIPS conformance, and you don't need to quote FIPS conformance as a commercial argument, then don't worry about FIPS conformance. Especially at level 1, especially when running on a complex, open OS with a poor track record on security like Windows. FIPS conformance is not an indication of security, and AviD's law of regulatory compliance very much applies here: FIPS conformance reduces the risk of the penalties of non-conformance. If you're running under Windows, I recommend using Bitlocker, not because of any conformance, but because it was written by specialists, has been heavily tested, and is being actively maintained. That way, you can concentrate on the real weakness in your overall security, like the fact that 50% of your users are going to use 1234 as their PIN and the other 50% are going to use their birthday.

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Thanks Gilles, great response. – Mark Rogers Jan 9 '12 at 18:36
Do you have any pointers for your claim on TPMs? TPMs do NOT offer tamper resistance or even tamper evidence. In fact physical attacks are completely out of scope of the TCG specification. Some vendor may of course chose to make his TPM tamper resistant, and some people claim the physical security will be improved when TPM moves into the CPU, but currently nobody does that. And since there is currently no real demand for TPMs yet, Intel surely won't extend their CPUs with such things. – pepe Jan 9 '12 at 19:20
@Gilles - You clearly are not a fan of Windows, its perfectly secure, if it wasn't then it wouldn't be used by the very people that required FIPS 140 compliance. – Ramhound Jan 9 '12 at 19:59
@pepe I'm going by my understanding of current TPM manufacturing processes, which I believe leverage some smartcard manufacturing technologies (and in principle a TPM could be implemented in a smartcard, though I don't think any manufacturer goes near that). You're right that the TPM specs don't mandate any such security (a TPM could be implemented in pure software). I see my wording was ambiguous: I meant to write that it would be possible to reach higher levels with a TPM designed for this purpose, not that any TPM would meet the requirements. – Gilles Jan 9 '12 at 20:21
@Gilles: Maybe they re-used some components, but note that the LPC interface to the TPM is susceptible to manipulation. In the past, this allowed a trivial attack where one could reset the TPM at runtime, resetting PCRs, completely breaking its advanced security features (seal/attest). Many vendors even chip the TPM as optional, pluggable component, and don't issue endorsement keys. So they often don't even fulfill TCG specs. – pepe Jan 9 '12 at 22:21

If you're asking to extend Bitlocker security, the TPM is certainly a good and cheap start. But it will not give you regulatory compliance, and it will also not give you complete security.

Basically, what remains as attack on Bitlocker (apart from passphrase brute-forcing and physically tapping the TPM's LPC bus) are user interface spoofing attacks. E.g. I replace your bootloader to record the password that you type into the bitlocker dialog, or I fake the bitlocker dialog itself. Using a TPM can make such attacks much harder, since the keys are bound to a "correct" bootloader. However, one could still replace the bootloader, store the password for later retrieval, then restore the original bootloader and reboot. As a user, you only see an unexpected reboot and maybe won't wonder much about what just happened.

This "evil maid" attack is well-known, was mentioned in the InvisibleThings blog some time ago and the German Fraunhofer SIT also has a nice video demonstration:

Theoretically, the TPM can be used as a drop-in replacement for a smartcard. The Trousers software stack for Linux actually exports a PKCS#11 interface for that purpose, and AFAIK Windows 8 will enable access to the TPM via their Card Service Provider API. However, in that respect the TPM will only provide you with a basic protection against PIN brute-forcing. Existing TPMs have no guarantees regarding tamper evidence. Moreover, you don't have a secure PIN entry method and no secure display. Anyone can intercept the PIN on its way to the TPM, just like with class 1 smartcard readers. In the future, trusted execution technology (TXT,SVM,TrustZone) might give you secure user I/O, but this is far from usable today.

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So your saying that a TPM does not qualify as part of the 'cryptographic module' as defined in FIPS 140-2? But bitlocker alone could still be used to comply with FIPS 140-2 security level 1 compliance, right? Are you saying that Bitlocker could not be used alone for compliance? – Mark Rogers Jan 9 '12 at 20:19
@MarkRogers No, pepe is going beyond the issue of FIPS compliance into actual security concerns. For my part, I'll refrain without knowing your security requirements; pepe's advice may be spot on or irrelevant to your particular situation (but it's a good read anyway). – Gilles Jan 9 '12 at 22:43
I extended my answer with some more text. I'm not into regulatory compliance. If your hardware token does not need a secure PIN entry method, a TPM might be suitable. As pointed out by Gilles, no TPMs received such certification yet. But TPM test suites exist since a few years and current TPMs probably mostly fulfill at least the TCG spec. Especially for HP/Lenovo Laptops, maybe also Dell. – pepe Jan 9 '12 at 22:44
@pepe & Gilles - thanks for the clarifications and the detailed information, it will be of great assistance. – Mark Rogers Jan 9 '12 at 22:49
Sorry I didn't choose yours, it was still a good, correct response. – Mark Rogers Jan 9 '12 at 23:55

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