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Dealing with randomness
Two weeks ago, I wrote an article regarding randomness on UNIX systems and libraries. In it, I dealt with some theoretical API issues, and real world issues of libraries being horribly misdesigned. Today I'd like to focus more on the current state of things, and further discuss real world problems.Randomness
To start, let us understand what randomness means. Any perceived randomness on your part is your inability to track all the variables. Meaning that so called randomness to us mere mortals is something that we do not know nor can predict, at least not with the tools we have at our disposal.The ideas behind randomness and determinism have long been battled over by philosophers and scientists, with the former debating whether it's possible for such things to exist, and the latter hunting for its existence. But thankfully, in terms of cryptography, even though randomness doesn't exist, we can make believe it does, and we can generate data that appears random to anyone from the outside. We can do so by using variables that are hard to track and influence, and extremely confusing algorithms.
Collecting unpredictable values is a challenge, and nothing is ever as unpredictable as one might think. Even if we were to look to quantum mechanics, which today is believed to be the closest thing to random that we know of, and measure some quantum behavior, the data might still be predicable, and therefore not random enough for cryptography needs. Cryptography Engineering pages 138-139 covers this in more detail, without even getting into possible advancements in the field. All the more so, being completely unpredictable is quite challenging using more conventional means.
Essentially, we're left with trying to do our best, without really being able to ensure we're doing things well. The less information we leak, and more unpredictable we behave, the better.
Expectations
We tend to make assumptions. We make assumptions that the people behind Linux know how to create a great operating system. We assumed the OpenSSL team knew how to create a secure library. We assume the OpenBSD developers know safe software engineering. However, assumptions may be wrong. Never assume, research and test it.
Like others, I tend to assume that different pieces of software were written correctly. A couple of years back, I was having trouble with a web server in conjunction with IPv6, which was built on top of a network server library. I assumed the library written by supposed experts with IPv6 knowledge knew what they were doing, and blamed the newness of IPv6, and assumed the issue was not with the library but with the network stacks in the OS.
For unrelated reasons I decided to improve my network development knowledge and purchased Unix Network Programming 3rd Edition, which is now over a decade old, yet includes instructions on properly working with IPv6. Turns out I was able to fix the bug in the networking library I was using by modifying a single line in it. After reading this book, I also realized why I was having some rare hard to reproduce bug in another application, and fixed the issue in two minutes.
The aforementioned book is considered the bible in terms of sockets programming. It explains common gotchas, and shows how to build robust networking software which works properly. We would assume the developers of a major networking library would have read it, but experience shows otherwise.
I hardly need to elaborate how this has applied elsewhere (hello OpenSSL).
False messiahs
Most software is written by people who aren't properly trained for their positions. They either get thrust into it, or dabble in something for fun, and it turns into something popular which people all over the globe use regularly. Suddenly popularity becomes the measurement for gauging expertise. Some kid with a spare weekend becomes the new recognized authority in some field with no real experience. People who end up in this position tend to start doing some more research to appear knowledgeable, and receive unsolicited advice, which to some extent propels them to be an expert in their field, at least at a superficial level. This can then further reinforce the idea that the previously written software was correct.Situations like this unfortunately even apply to security libraries and software which needs to be secure. We can read a book, which covers some important design nuance, and we'll think to ourselves that if we laymen read the popular book, surely those who need to be aware of its contents did, and assume the software we depend on every day is written properly. I've read a few books and papers over the years on proper random library gotchas, design, and more, yet used to simply use OpenSSL's functions in my code, as I assumed the OpenSSL developers read these books too and already took care of things for me.
Recently, the OpenBSD team actually reviewed OpenSSL's random functions, and realized that no, the OpenSSL developers did not know what they were doing. Since the Debian key generation fiasco a couple of years ago revealed they were using uninitialized variables as part of randomness generation, I suspected they didn't know what they were doing, but never bothered looking any deeper. It's scary to realize you may in fact know how to handle things better than the so-called experts.
I wrote an article the other day regarding how to protect private keys. A friend of mine after reading it asked me incredulously: You mean Apache isn't doing this?!
We have to stop blindly believing in certain pieces of software or developers. We must educate ourselves properly, and then ensure everything we use lives up to the best practices we know about.
UNIX random interfaces
Linux invented two interfaces for dealing with random data which were then copied by the other UNIX-like operating systems. /dev/random which produces something closely related to what the OS believes is random data it collected, and /dev/urandom which produces an endless stream of supposedly cryptographically secure randomish data. The difference in the output between the two of them should not be discernible, so theoretically, using one in place of the other should not make a difference.There's a ton of websites online which tell developers to use only /dev/urandom, since the whole point of a cryptographically-secure pseudo-random number generator is to appear random. So who needs /dev/random anyway, and finite amounts of randomishness is problematic, so everyone should just use something which is unlimited. Then the Linux manual page will be blamed as a source of fostering confusion for suggesting there's situations that actually require /dev/random.
Now those who have an ounce of critical thinking within themselves should be questioning, if there's never a point with /dev/random, why did Linux invent it in the first place? Why does it continue to provide it with the same semantics that it always had? In fact, the manual page is doing nothing more than paraphrasing and explaining the comments in the source code:
* The two other interfaces are two character devices /dev/random and
* /dev/urandom. /dev/random is suitable for use when very high
* quality randomness is desired (for example, for key generation or
* one-time pads), as it will only return a maximum of the number of
* bits of randomness (as estimated by the random number generator)
* contained in the entropy pool.
*
* The /dev/urandom device does not have this limit, and will return
* as many bytes as are requested. As more and more random bytes are
* requested without giving time for the entropy pool to recharge,
* this will result in random numbers that are merely cryptographically
* strong. For many applications, however, this is acceptable.
Now if you believe the hype out there that for every situation, one should only use /dev/urandom, then you're implying you don't trust the developers of the Linux random interfaces to know what they're talking about. If they don't know what they're talking about, then clearly, they also don't know what they're doing. So why are you using any Linux supplied random interface, after all, Linux obviously handles randomness incorrectly!
FreeBSD actually makes the above argument, as they only supply the /dev/urandom interface (which is known as /dev/random on FreeBSD), which uses random algorithms created by leading cryptographers, and nothing remotely like what Linux is doing. Each of the BSDs in fact claim that their solution to randomness is superior to all the other solutions found among competing operating systems.
Linux on the other hand takes an approach where it doesn't trust the cryptographic algorithms out there. It has its own approach where it collects data, estimates the entropy of that data, and uses its own modified algorithms for producing the randomness that it does. Cryptographers are constantly writing papers about how non-standard Linux is in what it does, and numerous suggestions are made to improve it, in one case, even a significant patch.
Now I personally dislike Linux's approach to throw cryptographic practice out the window, but on the other hand, I like their approach in not putting too much faith into various algorithms. I like FreeBSD's approach to using well designed algorithms by cryptographers, but I dislike their inability to ensure the highest levels of security for when you really need it.
The cryptographers out there are actually divided on many of the issues. Some believe entropy estimation is utterly flawed, and cryptographically strong algorithms are all you need. Others believe that such algorithms are more easily attacked and prone to catastrophic results, and algorithms combined with pessimistic estimators and protective measures should be used.
In any case, while the various operating systems may be basing themselves on various algorithms, are they actually implementing them properly? Are they being mindful of all the issues discussed in the various books? I reviewed a couple of them, and I'm not so sure.
A strong point to consider for developers is that even though OS developers may improve their randomness in response to various papers those developers happen to stumble across or gets shoved in their faces, it doesn't necessarily mean you're free from worrying about the issues in your applications. It's common to see someone compiling and using some modern software package on say Red Hat Enterprise Linux 5, with its older version of Linux. I recently got a new router which allows one to SSH into it. Doing so, I saw it had some recentish version of BusyBox and some other tools on it, but was running on Linux 2.4!
Situations to be mindful of
There are many situations one must be mindful of when providing an interface to get random data. This list can be informative, but is not exhaustive.Your system may be placed onto a router or similar device which generates private keys upon first use. These devices are manufactured in bulk, and are all identical. These devices also generally lack a hardware clock. In typical operating conditions, with no special activity occurring, these devices will be generating identical keys, there's nothing random about them.
Similar to above, a full system may have an automated install process deploying on many machines. Or an initial virtual machine image is being used multiple times. (Do any hosting companies make any guarantees about the uniqueness of a VM you purchase from them? Are preinstalled private keys identical for all customers?)
A system may be designed to use a seed file, a file which contains some random data, which is stored on disk to ensure a random state for next boot-up. The system may be rebooted at some point after the seed file is used, but before it is updated, causing an identical boot state.
There can be multiple users on a system, which can influence or retrieve random data. Other users may very well know the sequences generated before and after some data was generated for another user. That can then be used in turn to determine what random data was generated for the other user.
Hardware to generate random data may contain a backdoor, and should not be fully trusted.
Hardware to generate random data may break, or be influenced in some manner, causing the generated data to not actually be random.
Two machines next to each other may be measuring the same environmental conditions, leading one machine to know the random data being generated for the other.
A process may be inheriting its state from its parent, where both of them will be generating the same sequences, or multiple children will be generating the same sequences.
Wall-clock time can be adjusted, allowing the same time to occur multiple times, which in turn will lead to identical random states where wall-clock time is the only differentiating factor.
Possible Solutions
A proper solution for identical manufacturing or deployments would be to write a unique initial state to each one. However, this cannot be relied upon, as it requires compliance by the manufacturer or deployer, which can increase costs and complexity, and good tools to do so are not available.Devices generally have components which contain serial numbers. These serial numbers should be mixed into initial states, minimally ensuring that identical devices do not have identical states. As an example, routers will have different MAC addresses. They may even have multiple MAC addresses for different sides of the router, or for wireless. Be aware however that it is possible for an outsider to collect all the MAC addresses, and thus reveal the initial state.
If a hardware clock is available, it should be mixed into the boot-up state to differentiate the boot-up state from previous boots and other identical machines that were booted at different times.
Random devices should not emit data until some threshold of assurances are reached. Linux and NetBSD provide an API for checking certain thresholds, although race conditions make the API a bit useless for anything other than ensuring the devices are past an initial boot state. FreeBSD now ensures its random device does not emit anything till some assurances are met, but older versions did not carry this guarantee. Also be wary of relying on /dev/random for assurances here, as my prior random article demonstrated that the device under that path may be swapped for /dev/urandom, a practice done by many sysadmins or device manufacturers.
Seed-files should not solely be relied upon, as embedded devices may not have them, in addition to the reboot issue. Above techniques should help with this.
The initial random state provided to applications should be different for each user and application, so they are not all sharing data in the same sequence.
Hardware generators should not be used without some pattern matching and statistical analysis to ensure they are functioning properly. Further, what they generate should only be used as a minor component towards random data generation, so they cannot solely influence the output, and prevent machines in the same environment producing the same results.
Due to application states being cloned by fork() (or similar), a system wide random interface can be more secure than an application state (thus OpenSSL when directly using /dev/urandom can be more secure than various flavors of LibreSSL). For application level random interfaces, they should have their states reset upon fork(). Mixing in time, process ID, and parent process ID can help.
Always use the most accurate time possible. Also, mix in the amount of time running (which cannot be adjusted), not just wall-clock time.
Many of the above solutions alone do not help much, as such data is readily known by others. This is a common library mistake where libraries tend to try system random devices, and if that fails, fall back on some of these other sources of uniquish data. Rather, system random devices should be used, which mix in all these other sources of uniquish data, and upon failure, go into failure mode, not we can correct this extremely poorly mode.
Usage scenarios
Not only does random data have to be generated correctly, it has to be used correctly too.A common scenario is attempting to get a specific amount of random values. The typical approach is to divide the random value returned by the amount of possible values needed, and take the remainder. Let me demonstrate why this is wrong. Say your random number generator can generate 32 possible values, meaning any number from 0 to and including 31, and we only need to choose between five possible values.
As can be seen from this chart, modulo reducing the random number will result in 0 and 1 being more likely than 2, 3, 4. Now if we were to suppose lower numbers are inherently better for some reason, we would consider such usage fine. But in that scenario, our random number generator should be:
function GetInherentlyGoodRandomNumber()Being that the above is obviously ridiculous, we want to ensure equal distribution. So in the above scenario, if 30 or 31 occurs, a new random number should be chosen.
{
return 0 //Lower numbers are inherently better!
}
Similar issues exist with floating point random numbers. One generally uses floating point random numbers when one wants to use random values with certain probabilities. However, typical algorithms do not provide good distributions. There is an entire paper on this topic, which includes some solutions.
The C library's fork() call (when it exists) should be resetting any random state present in C library random functions, but unfortunately that's not done in the implementations I've checked (hello arc4random). The issue extends to external libraries, which most of them never even considered supplying secure_fork() or similar. Be wary to never use fork() directly. You almost always want a wrapper which handles various issues, not just random states.
Further reading
Pretty much everything I stated here is a summary or rehash of information presented in important books and papers. Now based on my past experiences that I described above, you're probably not going to read anything further. However, if the topics here apply to you, you really should read them.- Secure Programming Cookbook
- Practical Random Number Generation
- Generating Pseudo-random Floating-Point Values
- Cryptography Engineering
- Weak Keys
- Weak Keys Extended
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