There are few topics these days that can spark debate in software-development circles as quickly as the US National Security Agency (NSA). Recently, the NSA's director went on the record in public to advocate mandating government access to encrypted software systems. Such mandatory access is an idea that has been floated before, and although this time around the specifics are different, the tech industry has been just as receptive to the potential interference as one would expect. The plan outlined would make it virtually impossible to deploy certain free-software systems without running afoul of regulations and, while it does not seem particularly likely to be written into law any time soon, is has provoked quite a bit of discussion.

NSA Director Michael Rogers made the comments in question during a speech at Princeton University. The Washington Post published a write-up of the talk on April 10. The story quotes Rogers as saying "I don’t want a back door [...] I want a front door. And I want the front door to have multiple locks. Big locks." The distinction Rogers made was that "back doors" are hidden entry points; what he promoted are encryption systems with strong crypto—but where law enforcement has a way to access the keys.

The comments come while the White House is studying encryption policy, the Post story explains. In a February interview with Re/code, President Obama said "there’s no scenario in which we don’t want really strong encryption", but went on to say that law enforcement has a national-security interest in accessing encrypted communication. The difficulty, he said, is that encryption that is too strong makes it impossible for a software company to comply with a court order requesting specific documents.

All of this, of course, is familiar territory. A service provider (such as an email-hosting service) might be asked to turn over the emails and access logs associated with a particular user account. If all of the encryption keys needed to decrypt the account information are held by the user, the service provider cannot turn over any readable documents. What is different, however, is Rogers's suggestion.

For reference, in the mid-1990s the Clinton administration proposed a mandatory "key escrow" system, in which service providers would be required to create decryption keys independent of any keys held by the user or the service provider. Those keys would be turned over to a "trusted party" that would, in turn, release the appropriate key to the US government when required during criminal investigations. The proposal was an extension of the Clipper chip project, which was a hardware-based encryption system for digital phones. Each chip had a backdoor encryption key burned in during the manufacturing process; the backdoor key was held by the government. The Clipper chip failed in the marketplace, however, and was quickly abandoned.

The Wikipedia entry on key escrow links to a copy [PDF] of the 1996 CIA memo advocating a post-Clipper-chip escrow program. It makes for interesting historical reading, but the program was never implemented. The objections to it (apart from the risk driving software development away from the US) were straightforward: fear of abuse by government agencies or individuals, fear of abuse by the "trusted parties," and the general principle that individuals deserve to keep some of their communication private.

What Rogers proposed this time is a "split key" system. As with key escrow, an encryption key independent of the user's would be generated for each account—but in this system each of those keys would then be split into parts. The government would hold one half, and the service provider the other. Both pieces would have to be brought together to access a user account. That way, no single rogue actor could access a user's private data—regardless of whether the actor was from law enforcement or from the service provider.

The Post story cites critics of the proposal from Yahoo and from George Washington University’s Cyberspace Security Policy and Research Institute. There would be technological and logistical challenges to a mandatory form of such a system—imagine how many split keys would need to be generated and delivered to law enforcement on a daily basis for services as popular as Gmail and Facebook, for instance.

There is also the problem of keeping the split keys separate. Even if they are held by separate entities in the long term, they must be generated together and then distributed. That provides an opportunity for an attacker to copy both keys well before they reach the proper hands (including, of course, the service keeping its own copy of the government key from day one). Similarly, any time both key halves are used together, there would be another opportunity to steal or duplicate them.

In addition, critics of US government security policy may understandably have questions about how the government would exercise its right to meet with the service provider and access a suspicious account. Would such meetings be subject to gag orders or secret National Security Letters? Would the government be able to compel the service to turn over its half of the key, if it decided the stakes were particularly high?

Furthermore, under such a plan it might become illegal in the US to run non-compliant Internet services (possibly even for private use), which would put untold numbers of free-software projects in a bind. They would have to choose between implementing the mandatory split-key escrow service and losing US users. Free-software projects not based in the US would hardly be expected to merge in support for a US-government access program. No doubt some users in the US would continue to run their own services as they see fit, but they would do so at significant legal risk.

The Post story notes that, so far, there is no legislation proposed to implement what Rogers is asking for. It would seem to be a hard sell in the current climate; after the Edward Snowden leaks, cooperating with the NSA is a decidedly unpopular proposition in tech circles, and consumer interest in privacy issues is relatively high.

Nevertheless, privacy advocates are not taking anything for granted. The Electronic Frontier Foundation (EFF) criticized Rogers's comments, casting them in the same light as the Clipper chip and related proposals:

On the plus side, the EFF article is a welcome reminder that past attempts to mandate back (or front) doors in encryption products have failed. Historians will note that the Clipper chip fiasco contributed considerably to the growth of PGP and other software encryption projects, even though at that time encryption was considered "munitions" and was subject to export controls.

Whatever comes of the NSA's interest in split-key escrow technology, it will no doubt provoke considerable work from privacy-conscious software developers—perhaps leading to projects that will have just as much impact in the long term as PGP.