Dual-EC, BitLocker disk encryption and conspiracy theories

[Full disclosure: this blogger worked at MSFT but has not been involved in BitLocker development]

Infosec community is still looking for a replacement since the cancellation of TrueCrypt. Last year the mysterious group behind the long standing disk-encryption system announced they were discontinuing work. In a final insult to users, they suggested current users migrate to BitLocker, the competing feature built into Windows. It could not have been worse timing, just when NCC Group announced to great fanfare their completion of an unsolicited security audit on the project. (Not to worry; there are plenty of audit opportunities left in OS/2 and DEC Ultrix for PDP11, to take other equally relevant systems as TrueCrypt.) What to do when your favorite disk encryption system has reached end-of-life? Look around for competing alternatives and weigh their strengths/weaknesses for starters. However a recent article on Intercept looking at Windows BitLocker spends more time spinning conspiracy theories than helping users migrate to BitLocker. There are four “claims” advanced:

  • Windows supports the dual-EC random number generator (RNG) which is widely believed to have been deliberately crafted by the NSA to be breakable
  • BitLocker is a proprietary implementation, and its source code is not available for review
  • MSFT will comply with law-enforcement requests to provide content
  • MSFT has removed the diffuser from BitLocker without a good explanation, demonstrably weakening the implementation

Let’s take these one by one.

“Windows has dual-EC random number generator”

It is true that Windows “next-generation” crypto API introduced in Vista supports dual-EC RNG, widely believed to have been designed by the NSA with a backdoor to allow predicting its output. In fact it was a pair of MSFT employees who first pointed out in a very restrained rump-session talk at 2007 Crypto conference that dual-EC design permits a backdoor without speculating on whether NSA itself had availed itself of the opportunity. Fast forward to Snowden revelations, and RSA Security finds itself mired in a PR debacle when it emerged that the company accepted $10M payment from the NSA for incorporating dual-EC.

Overlooked in the brouhaha is that while dual-EC has been available as an option in Windows crypto API, it was never set as the default random number generator. Unless some application went out of its way to request a different RNG— and none of the built-in Windows features including BitLocker ever did that— the backdoor would have sat idle. (That said it creates interesting opportunities for post-exploit payloads: imagine state-sponsored malware whose only effect on target is switching default system RNG, with no other persistence.)

From a product perspective, the addition of dual-EC RNG to Vista can be considered as a mere “checkbox” feature aimed at a vocal market segment. There was a published standard from NIST called SP800-90 laying down a list of officially-sanctioned RNG. Such specifications may not matter to end-users but carry a lot of weight in government/defense sector where deployments are typically required to operate in some NIST-approved configuration. That is why the phrase “FIPS-certified” makes frequent appearances in sales materials. From MSFT perspective, a lot of customers required those boxes to be checked as a prerequisite for buying Windows. Responding to market pressure, MSFT added the feature and did so in exactly the right way such “niche-appeal” features should be introduced: away from the mainline scenario, with zero impact on majority of users who do not care about it. That is the main difference between RSA and Windows: RSA made dual-EC the default RNG in their crypto library. Windows offered it as an option, but never set as the default the system RNG. (It would have made no sense; in addition to security concerns, it was plagued by dog-slow performance compared to alternatives based on symmetric ciphers such as AES counter-mode.)

Bottom line: Existence of a weak RNG as an additional option to satisfy some market niche— an option never used by default— has no bearing on the security of BitLocker.

“BitLocker is not open-source”

Windows itself is not open-source either but that has never stopped people from discovering hundreds of significant vulnerabilities by reverse engineering the binaries. Anyone is free to disassemble any particular component of interest or single-step through it in a debugger. Painstaking as that effort may be compared to reading original source, thousands of people have made a career out of this within the infosec community. In fact Microsoft even provides debug symbols drawn from source-code to make that task easier. As far as closed-source binaries go, Windows is probably the most carefully examined piece of commercial software with an entire cottage industry of researchers working to make that process in crafty ways. From comparing patched binaries against their earlier version to reveal silently fixed vulnerabilities to basic research on how security features such as EMET operate, being closed-source has never been a hurdle to understanding what is going on under the hood. The idea that security research community can collectively uncover hundreds of very subtle flaws in the Windows kernel, Internet Explorer or the graphics subsystem— massively complex code-bases compared to BitLocker— while being utterly helpless to notice a deliberate backdoor in disk encryption is laughable.

Second, many people past and present did get to look at Windows source code at their leisure. Employees, for starters. Thousands of current and past MSFT employees had the opportunity to freely browse Windows code, including this blogger during his tenure at MSFT. (That included the separate crypto codebase “Enigma” which involved signing additional paperwork related to export-controls.) To allege that all of these people, many of whom have since left the company and spoken out in scathing terms about their time, are complicit in hiding the existence of a backdoor or too oblivious/incompetent to notice its presence is preposterous.

And it is not only company insiders who had many chances to discover this hypothetical backdoor. Some government customers were historically given access to Windows code to perform their own audit. More recently the company has opened transparency centers in Europe inviting greater scrutiny. The idea that MSFT would deliberately include a backdoor with full knowledge that highly sophisticated and cautious customers— including China, not the most trusting of US companies— would get to pore over every line, or for that matter provide doctored source-code to hide the backdoor, is equally preposterous.

Bottom-line: Being open-source may well improve the odds for security community at large to identify vulnerabilities in a particular system. (But even that naive theory of “given enough eyeballs, all bugs are shallow” has been seriously questioned in the aftermath of Shellshock and never-ending saga of OpenSSL) But being closed-source in and of itself can not be a priori reason to disqualify a system on security grounds, much less serve as “evidence” that a hidden backdoor exists after having survived years of reverse-engineering in arguably the most closely scrutinized proprietary OS in the world. Linking source-code availability to security that way is a non-sequitur.

“MSFT will comply with law-enforcement requests”

This is a very real concern for content hosted in the cloud. For data stored on servers operated by MSFT such as email messages at Hotmail/Outlook.com, files shared via One Drive or Office365 documents saved to the cloud, the company can turn over content in response to an appropriate request from law enforcement.  MSFT is not alone in that boat either; same rules apply to Google, Facebook, Twitter, Yahoo, DropBox and Box. Different cloud providers compete along the privacy dimension based on product design, business model, transparency and willingness to lobby for change. But they can not hope to compete in the long run on their willingness to comply with existing laws on the books or creative interpretations of these laws.

All that aside, BitLocker is disk encryption for local content. It applies to data stored on disk inside end-user machine and removable media such as USB thumb-drives. It is not used to protect content uploaded to the cloud. (Strictly speaking one could use it to encrypt cloud storage, by applying BitLocker-To-Go on virtual disk images. But that is at best a curiosity, far from mainstream usage.)

On the surface then it seems there is not much MSFT can do if asked to decrypt a seized laptop with BitLocker enabled. If disk encryption is implemented properly, only the authorized user possesses the necessary secret to unlock. And if there is some yet-to-be-publicized vulnerability affecting all BitLocker usage such as cold-boot attacks, weak randomness or hardware defects in  TPMs, there is no need to enlist MSFT assistance in decryption. Law enforcement might just as well exploit that vulnerability on their own, using their own offensive capabilities. Such a weaknesses would have existed all along, before the laptop is seized pursuant to an investigation. There is nothing MSFT can do to introduce a new vulnerability after the seizure, any more than they can go back in time to back-door BitLocker before it was seized.

But there is a catch. Windows 8 made a highly questionable design decision to escrow BitLocker keys to the cloud by default. These keys are stored associated with the Microsoft Live account, presumably as a usability improvement against forgotten passphrases. If a user were to forget their disk encryption passphrase or the TPM used to protect keys malfunctions, they can still recover as long as they can  log into their online account. That capability provides a trivial way for MSFT to assist in the decryption of BitLocker protected volumes: tap into the cloud system to dig up escrowed keys. Good news is that default behavior can be disabled; in fact, it is disabled by default in enterprise systems presumably because MSFT realized IT departments would not tolerate such a cavalier attitude around key management.

Bottom-line: There is a legitimate concern here, but not in the way the original article envisioned. Intercept made no mention of the disturbing key-escrow feature in Windows 8. Instead the piece ventures into purely speculative territory around Government Security Program from 2003 and other red-herrings around voluntary public/private-sector cooperation involving MSFT.

“MSFT removed the diffuser”

For a change, this is a valid argument. As earlier posts mentioned, full-disk encryption suffers from a fundamental limitation: there is no room for an integrity check. The encryption of one sector on disk must fit exactly on that one sector. This would not be a problem if our only concern was confidentiality, or preventing other people from reading the contents of data. But it is a problem for integrity, detecting whether unauthorized changes were made. In cryptography this is achieved by adding an integrity check to data. That process is frequently combined with encryption because both confidentiality and integrity are highly desirable properties.

But in FDE schemes without any extra room to stash an integrity check, designers are forced to take a different approach. They give up on preventing bad guys from making changes, but try to make sure those changes can not be controlled with any degree of precision. In other words you can flip bits in the encrypted ciphertext stored on disk, and it will decrypt to something (without an integrity check, there is no such thing as “decryption error”) but that something will be meaningless junk; or so the designers hope. The original BitLocker diffusers attempted to achieve that effect, by “mixing” the contents within a sector such that modifying even a single bit of encrypted data would result in randomly introducing errors all over the sector after decryption. That notion was later formalized in cryptographic literature, standardized into modes such as XTS that are now supported by self-encrypting disk products on the market.

Fast forward to Windows 8 and the diffuser mysteriously goes away, leaving behind vanilla AES encryption in CBC mode. With CBC mode it is possible to introduce partially controlled changes at the level of AES blocks. (“Partial” in the sense that one block can be modified freely but then the previous block is garbled.) How problematic is that? It is easy to imagine hypothetical scenarios based on what the contents of that specific location represent. What if it is a flag that controls whether firewall is on and you could disable it? Or registry setting that shuts off ASLR? Or enables kernel-debugging, which then allows controlling the target with physical access? It turns out a more generic attack is possible in practice involving executables. The vulnerability was already demonstrated with LUKS disk-encryption for Linux. Suppose that sector on disk happens to hold an executable file that will be run by the user. Controlled changes mean the attacker can modify the executable itself, controlling what instructions will be executed when that sector is decrypted to run the binary. In other words, you get arbitrary code execution. More recently, the same attack was demonstrated against the diffuser-less BitLocker.

So there is a very clear problem with this MSFT decision. It weakens BitLocker against active attacks where the adversary gets the system to decrypt the disk after having tampered with its contents. That could happen without user involvement if decryption is done by TPM alone. Or it may be an evil-maid attack where the laptop is surreptitiously modified but the legitimate owner, being oblivious, proceeds to unlock the disk by entering their PIN.

Bottom-line: Windows 8 did weaken BitLocker, either because the designers underestimated the possibility of active attacks or made a deliberate decision that performance was more important. It remains to be seen whether Windows 10 will repair this.


Leave a Reply

Please log in using one of these methods to post your comment:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google+ photo

You are commenting using your Google+ account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )


Connecting to %s