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How open source makes software more secure
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"Your secure software is open source; doesn't that make it less secure?"
This is a recurring question that we get at Benetech about Martus, our free, strongly encrypted tool for secure collection and management of sensitive information built and provided by the Benetech Human Rights Program. It's an important question for us and for all of our peers developing secure software in today's post-Snowden environment of fear and worry about surveillance. We strongly believe not only that open source is compatible with digital security, but that it's also essential for it.
Secure software is like a safe
Let me explain with the following analogy:
Think of encryption as a locked combination safe for your data. You may be the only one who has the combination, or you may entrust it to select few close associates. The goal of a safe is to keep unauthorized people from gaining access to its content. They might be burglars attempting to steal valuable business information, employees trying to learn confidential salary information about their peers, or a fraudster who wants to gain confidential information in order to perpetrate a scam. In all cases, you want the safe to keep your stuff secure and keep out unauthorized people.
Now let's say I'm choosing a safe for my valuables. Do I choose Safe Number One that's advertised to have half-inch steel walls, an inch-thick door, six locking bolts, and is tested by an independent agency to confirm that the contents will survive for two hours in a fire? Or, do I choose for Safe Number Two, a safe the vendor simple says to trust, because the design details of the safe are a trade secret? It could be Safe Number Two is made of plywood and thin sheet metal. Or, it could be that it is stronger than Safe Number One, but the point is I have no idea.
Imagine you have the detailed plans and specifications of Safe Number One, sufficient to build an exact copy of that safe if you had the right materials and tools. Does that make Safe Number One less safe? No, it does not. The security of Safe Number One rests on two protections: the strength of the design and the difficulty of guessing my combination. Having the detailed plans helps me, or safe experts, determine how good the design is. It helps establish that the safe has no design flaws or a second "back door" combination other than my own chosen combination that opens the safe. Bear in mind that a good safe design allows the user to choose their own combination at random. Knowing the design should not at all help an attacker in guessing the random combination of a specific safe using that design.
The real goal of security
There is no such thing as perfect security. Advertisements for "an uncrackable safe" are promising more than they can deliver. So, the goal of locking up your valuables is not to make them impossible to steal, but rather expensive to steal—whether in terms of money (better tools cost more), time, or the possibility of being sent to jail.
The more you raise the cost of cracking a safe, the more secure your valuables are.
Knowing the specifications of a safe, and hence what it would take to crack it, doesn't make it less secure. Knowing that the walls are half an inch thick might help a burglar know what tools are required to cut through a half inch of case hardened steel, but this knowledge doesn't make it less costly to do so. A well-designed safe with a hard-to-guess combination will discourage most attackers.
The analogy of the strong safe with an open design is directly applicable to secure software design.
Just as with the safe, the security of a strongly encrypted software tool is not compromised by by being open source code. In fact, a security software's source code being visible by others strengthens its security. And, by extension, the safety and privacy of its users.
Here's why: If the code is public and freely available for review, then the end users, their experts, and the open source community at large can verify that the software does exactly what it claims to do and that there are no "back doors." In a world where hyper-surveillance is the norm, it is only natural that users insist on commitment to transparency by software developers. This is especially critical for human rights defenders, activists, journalists, civil society groups, and other social justice actors whose digital security and physical safety are closely linked.
It may seem a paradox that opening up the source code of secure software actually makes it more trustworthy. As toolmakers, though, our goal is not to keep the software design secret, but rather protect the confidentiality of the information entrusted to the software. As the safe analogy shows, the strength of security of software depends on the quality of design and the difficulty of guessing the password. With a strong, openly accessible design, the other key security element is encouraging users to choose long, strong, non-obvious passwords. The combination of a secure design and a good confidential password makes it unlikely that all but the most dedicated and well-resourced attackers will be able to access the confidential information stored in open source security software.
Just as the most secure safe will eventually yield to a dedicated assault from an expert with plenty of time and resources, secure software will also eventually yield to a similar assault. The goal of secure software is to raise the cost of such attacks to the point where attackers rarely bother you—they'll attack your less secure neighbors!
Putting open source to work
At Benetech, we believe that collaboration and community best help deliver strong security. Here, the open source approach to software development makes it easier to collaborate and incorporate existing important innovations. In the case of Martus, we didn't have to re-implement cryptography libraries, as we used a strong open source one (Bouncy Castle). Likewise, we didn't need to reinvent anonymity tools, as we integrated Tor into Martus. In this way, our users benefit from an entire community that supports their work with better digital security tools.
The major funders of technology for human rights groups have concluded that open source software is more trustworthy for the activists they want to support. Some of them, like the Open Technology Fund, are actively encouraging their grantees to have their software audited by third party experts, and funding those audits.
With greater transparency, accountability, independent verifiability, and collaboration comes stronger security. The open source way moves us all towards that goal.