The Future of TLS

There exist many implementations of TLS. Unfortunately, alongside the proliferation of implementations, there is a proliferation of serious defects. These defects aren't just limited to the design of the protocol, they are endemic. The cipher choices, the ways in which the primitives are combined, everything.

I want to lay out a roadmap to a situation where we can have an implementation of TLS that we can be confident in. Given the amount commerce that is currently undertaken using TLS, and the fact that other, life-and-death (for some) projects pull TLS libraries (don't roll your own!) and entrust them, I'd say that this is a noble goal.

Let's imagine what would make for a "strong" TLS 1.3 & appropriate implementation.

We start with primitive cipher and mac selection. We must ensure that well-studied ciphers are chosen. No export ciphers, no ciphers from The Past (3DES), and the ciphers must not show any major weaknesses (RC4). Key sizes must be at least 128-bit or more. If we're looking to defend against quantum computers, the key-size must be at least 256-bit. A similar line of reasoning should be applied to the asymmetric and authentication primitives.

Further, the specification should aim to be as small, simple, self-contained and obvious as possible to facilitate review. It should also shy away from recommendations that are known to be difficult for programmers to use without a high probability of mistakes, for instance, CBC mode.

Basic protocol selections should ensure that a known-good combination of primitives is used. For example, encrypt-then-MAC should be chosen. We head off any chosen ciphertext attacks by rejecting any forged or modified ciphertexts before even attempting to decrypt the cipher text. This should be a guiding principle. Avoid doing anything with data that is not authenticated.

The mandatory cipher suite in TLS 1.2 is TLS_RSA_WITH_AES_128_CBC_SHA. While this can be a strong cipher, it's not guaranteed. It's quite easy to balls-up AES CBC, to introduce padding oracles, and the like. I would argue that the default should be based around AES GCM, as this provides authenticated encryption without even so much as lifting a finger. Even the choice of MAC in the current default is looking wobbly. It's by no means gone, but people are migrating away from HMAC-SHA1 to better MAC constructions. I would recommend exploiting the parallelism on current-generation technologies by allowing a PMAC.

There should also be allowances, and maybe even a preference towards well-studied ciphers that are designed to avoid side-channels, such as Salsa20 and ChaCha20.

There should be no equivalent of the current "null" ciphers. What a terrible idea.

I like that in the current RFC, the maximum datagram size is specified. That means that the memory usage per-connection can be bounded, and the potential impact of any denial of service attack better understood before deployment.

For the implementation, I am not fussy. Upgrading an existing implementation is completely fine by me. However, it should be formally verified. SPARK Ada may be a good choice here, but I have nothing against ACSL and C. The latter "could" be applied to many existing projects, and so may be more applicable. There are also more C programmers than there are Ada programmers.

Personally, I think SPARK Ada would be a fantastic choice, but the license scares me. Unfortunately, ACSL has it's own host of problems. Primarily that the proof annotations for ACSL are not "as good" as for SPARK, due to the much more relaxed language semantics in C.

Any implementation which is formally verified should be kept as short and readable as reasonably possible, to facilitate formal review. The task of any reviewers would be to determine if any side-channels existed, and if so, what remediation actions could be taken. Further, the reviewers should generally be checking that the right proofs have been proven. That is, the specification of the program is in-line with the "new" TLS specification.

Responsibilities should be shirked where it makes good sense. For instance, randomness shouldn't be "hoped for" by simply using the PID, or the server boot time. Use the OS provided randomness, and if performance is paramount, use a CSPRNG periodically reseeded from true random (on Linux, this is supposed to be provided by /dev/random).

The implementation should, as far as possible, aim to meet the specification, as well as providing hard bounds for the time and memory usage of each operation. This should help with avoiding denial of service attacks.

The deployment would be toughest. One could suppose that a few extra cipher suites and a TLS 1.2 mode of operation could be added in to ease deployment, but this would seriously burden any reviewers and implementors. The existing libraries could implement the new TLS specification, without too much hassle, or even go through their existing code-bases and try to move to a formally-verified model, then add in the new version. Once a sufficient number of servers started accepting the new TLS, a movement in earnest to a formally verified implementation could begin (unless this route was taken from the start by and existing library).

Any suitable implementation that wants major uptake would ensure that it has a suitable license and gets itself out there. Current projects could advertise their use of formal methods to ensure the safety of their library over others to try to win market share.

We had the opportunity to break backwards compatibility once and fix many of these issues. We did not, and we've been severely bitten for it. We really go for it now, before the next heartbleed. Then we can go back to the fun stuff, rather than forever looking over our shoulders for the next OpenSSL CVE.

My Heart Bleeds for OpenSSL

Overview

Today has been eventful, to say that least. My colleague and I patched several services, re-issued keys for them and started the ball rolling on revocation. It took all sodding day.

The progenitor of this has caused a lot of hubris and internet opinions.

Internet Opinions

Basically, a lot of people have said a lot of things, and I'd like to address one or two of them in something more than a pithy tweet.

Dump OpenSSL

I have seen this almost everywhere I looked. The call to arms to fling out OpenSSL, or plain re-write it. I get it, I understand that OpenSSL is not the healthiest code base. I often use it as an example of an extremely unhealthy code base.

However, just because your friend is ill, doesn't mean you take them out behind the chemical sheds and shoot them.

OpenSSL has a long and "rich" history. In fact, it's got so much history, that some of it has become useful! Rewriting is almost always a mistake.

Depending on how deep we go, we could end up throwing everything away, the CA infrastructure, the cipher suite flexibility, the vast quantities of knowledge that surround the OpenSSL ecosystem and allow thousands of developers to work in a more secure manner than would otherwise be possible.

I want to cover the cipher suite flexibility quickly, just to point out how much of a fantastic idea it is. The server and client tell each other what their most-prefered ciphers are, then they agree on the strongest one that they both support.

This has massive benefits, for instance, when BEAST et. al. came along and effectively broke block cipher based OpenSSL for a while, we could jump to RC4 for a little while. When that was broke, we all hauled ass back to AES. This gives incredibly flexibility when dealing with many attacks, by simply being able to circumvent the affected parts.

Secondly, last I checked, OpenSSL still has support for DES and RC4. This allows for graceful handling of legacy clients. This is very important when you have a very large, heterogeneous installed base.

If we lost cipher suite flexibility, we'd open ourselves up to so much more Bad Stuff.

The CA Infrastructure I'm less happy with, but that's a post for another day, and, frankly, I'd be surprised if anyone seriously improved on it for your average Joe in the next 2 - 3 years. I've not looked into it much, but NameCoin, or a NameCoin-like protocol looks promising from afar.

Suddenly retraining literally tens of thousands of developers, system administrators, etc. I'm sure that will be a cheap, quick and widely accepted move. It could happen, but it's quite entrenched. I would see the move happening over several years, with what ever "comes next" supporting developers as they move from one platform to the other. In fact, it could be TLS 1.3 (I think 1.2 is the latest at the time of writing, correct me in the comments below if I'm wrong) as implemented in *drum roll* OpenSSL.

Throw Money at the Problem

I do not think OpenSSL's developers are monkeys. If you look at the project, it's effectively dying in a lot of respects. It needs more than money, it needs a huge injection of resource, people, contributions, and more than anything else, enthusiasm.

It's kinda sad that one of the most critical pieces of software to most people's lives these days has the appeal to developers of a punch right in the face.

This is mostly because the OpenSSL project is much more organic than most other projects out there -- it's grown.

I have looked at the OpenSSL code, I've tried to ... do ... things to it. It's a big and intimidating code base, and I really did not want to break anything. So much so that I abandoned my idea. I looked into the abyss, and the abyss looked into me.

An injection of cash and some good C developers on the project as their day job would make a huge dent in this. Their first job could be making the code amenable to testing and testing the result. Their second job could be adding newer, safer cipher suites, and so on. But beyond that, OpenSSL is not Java or C#. There's no benevolent banker running the show, there is only the dedication and hard work of the volunteers.

Once the project has had a good cleanup, (and probably a few bug fixes!), an audit of some sort would definitely be nice, but I think that this is of limited value. OpenSSL is a constantly evolving project. It is not the code to run a jet engine -- written once and run on well-known hardware. It's changing almost daily.

The only way to get that level of assurance would be to build an automated verification suite, or similar. I'm looking towards Frama-C, though something with more power may be required. That way, the assurances would be quite hard to break, in much the same way as introducing regression bugs is made harder by decent unit testing.

Conclusion

I feel for the OpenSSL developers, all two of them, I really do. They have come under a huge amount of fire recently. I would not be surprised if they just downed tools to go live in a cabin in the woods tomorrow, out of sheer frustration and upset. When their code works, they get no thanks, when it breaks, they never hear the end of it.

Unfortunately for some projects, it just becomes time to stop adding features, and to go and address some platform problems, to run a comb through it and straighten things out. This is often made hard by a lack of a large test suite.

We did a similar thing with our product at my day job recently. We stopped work on features for nearly 2 years to drag the system out of the dark ages, add in a real security system, upgrade the underlying technology, really improve our test suite, etc. It's painful, slow and incredibly frustrating at the time. But when you're finished, continuing to work with your product is so much easier. It stops the "Let's rewrite!" urge quite well.

Edit, 2014-04-09

There is some discussion over on Hacker News (Hello!). I'd like to address a couple of the points.

SSL/TLS /= OpenSSL

I completely agree. The reason this comes across as a point that needs to be made is that, in writing the post in the middle of the night, immediately before going to bed without hashing the ideas out right, I've managed to put issues that were solely OpenSSL's fault right next to the spec's fault.

The reason that I talk about the potential for losing things like cipher suite flexibility is that a new library would likely implement something AES & RSA based with SHA-2 or SHA-3, and leave the others for a rainy day. We need more cipher suites at our fingertips than that (and good, safe, defaults).

Beyond that, unfortunately, attacks like Lucky Thirteen were basically other attacks, but using timing information to extra data. This sort of attack is only possible with the Mac-then-Encrypt construction that SSL/TLS uses, thus being the problem with the spec. The fact that it is a timing exploit is an issue with OpenSSL. I know the bug is effectively intractable, but I'm also aware that OpenSSL has fixed it to a satisfactory level.

You can't spell

I ager.

Seriously though, while I do make an effort, it was the middle of the night and for some reason my spell checker was not red-lining. Opening up the post this morning gave me an opportunity to go through and fix these issues ;-)

OpenSSL isn't formally verified!?

No, neither is any part of your browser, your kernel, your hardware, the image rendering libraries that your browser uses, the web servers you talk to, or basically any other part of the system you use.

The closest to formally verified in your day-to-day life that you're going to get may well be the brakes on your car, or the control systems on a jet engine. I shit you not.

We live on fragile hopes and dreams.

Monoculture Leads to Mass Vulnerability

I think that this is a real problem. However, this is crypto code. I know, I know, it's still a systems problem in lots of ways, but I can't avoid the feeling of "don't roll your own."

Asking normal developers to choose between several competing libraries with different interfaces, vulnerabilities, performance characteristics, etc. would likely result in carnage. We know that developers, when asked to choose cobble together cipher primitives, are basically pants-on-heads retarded, why would they be in a position to make those kinds of decisions on a larger scale?

I that out best bet is to to ensure that OpenSSL is a rock-solid piece of software with things like automated verification, extensive testing and independent audits. Unfortunately, that's extremely expensive (don't forget, re-writing is likely more expensive).

Blogger is Awful

Yes. I am always surprised that my blog doesn't render in a browser with no JS.