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How Trustworthy Is Microsoft BitLocker Disk Encryption?

Journalists at The Intercept, the site founded by Glenn Greenwald and others as a home for Edward Snowden-style disclosures and national security coverage, published a deep dive on Thursday into the security of Microsoft's BitLocker full-disk encryption technology.

In "Microsoft Gives Details About Its Controversial Disk Encryption," The Intercept's Micah Lee followed up on an earlier how-to he'd written about using BitLocker, among other full-disk encryption technologies for various platforms. Lee serves as a combination journalist and resident technologist who helps the site handle the operational security, including source protection and cryptography, for The Intercept. Based on security concerns raised in the feedback to the how-to article, Lee approached Microsoft about specific issues and got some interesting replies from an unnamed Microsoft spokesperson.

According to Lee's piece, the main concerns of the security community about Microsoft's BitLocker technology, which first shipped in versions of Windows Vista, include:

  1. That it's closed-source Microsoft code that no one but Microsoft and those it invites, be they technology partners or government agencies, may inspect -- a common security community concern about nearly all of Microsoft's proprietary code.

  2. That BitLocker may rely on a pseudorandom number generator (PRNG) called Dual_EC_DRBG, short for Dual Elliptic Curve Deterministic Random Bit Generator, that many experts believe has been compromised by the U.S. National Security Agency.

  3. That an important component of BitLocker security, called the "Elephant diffuser," was removed from Windows 8 to potentially weaken its security.

  4. That Microsoft's real, reported and rumored track record of cooperating with U.S. law enforcement and intelligence agencies makes any security solutions produced by the company automatically suspect, a concern that is closely related to the closed-source-versus-open-source question.

On the PRNG question, Microsoft told Lee that Dual_EC_DRBG is not used by BitLocker or by Windows itself by default. "It has never been the default, and it requires an administrator action to turn it on," Lee quoted the spokesperson as saying. Instead, BitLocker uses the default Windows algorithm, CTR_DRBG.

While the Elephant diffuser was removed, Microsoft cited performance problems caused by the diffuser and a lack of compliance with the U.S. Federal Information Processing Standards (FIPS). "[The Elephant diffuser is] not FIPS compliant, so certain companies and government clients can't use it," Lee quoted Microsoft as saying. Lee also pointed out that LUKS, the respected disk encryption technology for Linux, also lacks a diffuser and is vulnerable to the same types of attacks that not having Elephant diffuser exposes in Windows.

An even less reassuring response, from a technical security and privacy standpoint, came on a question about whether Microsoft can provide access to BitLocker disks to comply with a government order. "The spokesperson told me they could not answer that question," Lee wrote.

In the earlier how-to piece, Lee presented BitLocker as "the best of several bad options for Windows users." In the deep dive, Lee also looks at alternatives, including TrueCrypt, VeraCrypt, CipherShed, BestCrypt, Symantec Endpoint Encryption and DiskCryptor, and arrives at roughly the same conclusion.

"Balancing trust, ease of use, transparency, apparent robustness, compatibility and resources for squashing bugs, BitLocker comes out ahead for the average user," Lee concludes. "Whatever you choose, if trusting a proprietary operating system not to be malicious doesn't fit your threat model, maybe it's time to switch to Linux."

Full-disk encryption is a major pillar of any complete security solution these days. The Intercept has done the Microsoft community a service by turning up some hard facts about the security of Microsoft's full-disk encryption technology. Lee's deep dive gives anyone considering Windows disk encryption the information that allows them to approach the myriad challenges and risks with open eyes.

Posted by Scott Bekker on June 04, 2015 at 1:07 PM