On Linux's /dev/random, /dev/urandom, getrandom()

[coderman]

Preface:
"""

This guy clearly thinks he's a lot brighter than everyone else... The people he's criticising aren't stupid, they just see just the job of the system RNG as to be extremely conservative, leading to decisions that are easy to criticise. You can alter this conservative behavior where appropriate.

For example, his ridicule of not trusting the Intel RNG "because NSA". The decision was actually made not to replace the random source with it because the whitening stage of the Intel RNG makes it near-impossible to tell whether the random source is working properly. IIRC they instead xor it in to all inputs to the random pool, which allows them to gain from it if it's working without losing if it doesn't.

"""
- https://np.reddit.com/r/netsec/comments/edaidy/on_linuxs_random_number_generation/fbhb55i/

The article:
https://research.nccgroup.com/2019/12/19/on-linuxs-random-number-generation/

I have been asked about the usefulness of security monitoring of entropy levels in the Linux kernel. This calls for some explanation of how random generation works in Linux systems.

So, randomness and the Linux kernel. This is an area where there is longstanding confusion, notably among some Linux kernel developers, including Linus Torvalds himself.

A long time ago (20+ years), a random generator was designed along the theory of “entropy depletion”. It goes thus: at any point, the generator has an internal state (the “entropy pool”) which has contents unknown to attackers. The amount of unknown-ness is called “entropy” (initially by analogy with thermodynamics, but that’s just an analogy; it’s not real thermodynamic entropy) expressed with a logarithmic scale, in bits; roughly speaking, if the pool contains “n bits of entropy” then this means that its unknown contents can be any of 2n potential values. (That’s a simplified definition that assumes that all potential values are equiprobable, but it will do for this explanation.)

Whenever some bits are obtained from the pool, they conceptually leak information about the pool: if you extract k random bits, then the remaining unknown-ness may be reduced down to 2n-k potential values, i.e. the entropy pool has been depleted by k bits.

Now, here, the important word is “conceptually”. The RNG will not give away its raw pool bits just like that; instead, it will use the pool as a seed in a cryptographically strong RNG, whose output is what is sent back to applications. The whole idea of a RNG being cryptographically strong is that it is computationally infeasible to actually obtain information about the seed by just observing the output. The RNG, effectively, plugs the leak, and no depletion occurs in any practical sense.

However, the person who designed that RNG was not very clear about that notion of “cryptographically strong”. In fact, out of some paranoid zeal, that person felt that some secret values required extra strong entropy, so that they may defeat adversaries that are powerful enough to break through the cryptographically strong RNG (it was in the 1990s, with the US rules on export of cryptography and the PGP craze, so these powerful adversaries were the usual fantasy of god-like NSA). If the CSRNG is assumed to be transparent, then depletion is back. Hence, the kernel was equipped with code that maintains a count of the current pool entropy contents, and /dev/random blocks when there is not enough to return the requested amount of pure NSA-proof randomness.

This is flawed reasoning, in several ways:

- The whole premise of entropy depletion is that cryptography does not work (the CSRNG does not prevent the leak), and yet the one and only example of values that require absolute randomness is “cryptographic keys”, i.e. the things that make sense only if cryptography, in fact, works. This is self-contradictory.
- The current amount of entropy in the pool is not known. It is estimated. Entropy is extracted from physical events (in particular exact timing of IRQs, as measured with the cycle counter), and this relies on that information being unpredictable by attackers. In other words, the god-like entities that can munch through cryptographic algorithms at breakfast are supposed not to be able to measure and accurately simulate physical systems. So much for divine abilities. In a sense, whether a given mechanism provides entropy is a matter of “this or that expert said that it does”; impossibility of accurate simulation comes from physics, specifically quantum mechanics, so the entropy pool estimator is based on a fair amount of trust in physicists such as Feynman or Bohr (but not Einstein, for that matter). But the entropy depletion is an assertion that cryptographers such as Shamir cannot be equally trusted.
- The measures from physical events are not uniform sequences of bits; they have biases, and successive measures can be correlated. Thus, the pool relies on some mixing which uses… a cryptographic hash function. So cryptography still has to be trusted to do something properly.

Notwithstanding its flaws, the entropy depletion theory got its followers early on, and was adamantly maintained by some big names in Linux kernel development, mostly because it is quite hard to admit to other people, and to oneself, that one might have got something wrong.

That theory is harmful in that /dev/random may block, possibly at inopportune moments. Typically, an OS or application installation may stall for long periods (possibly hours, I have seen it) because it tries to generate a cryptographic key (for SSH, PGP…) and the kernel does not believe it still has sufficient entropy. Note that, for all practical purposes, it really has more than enough entropy; the pool contents are not guessable by outsiders, since all previously extracted random elements were obtained through the CSRNG, and attackers do not have, in fact, the help of demoniac entities with unlimited computing abilities (or when they do, you usually have bigger problems).

To work around the blocking issues of /dev/random, an alternate API was added, called /dev/urandom. It’s the same as /dev/random, except that it does not block. Never. This is better, but not actually good: there are times when the entropy pool is really empty, namely during the early stages of the boot. At that point, the kernel did not obtain many physical events to work on, and it is conceivable that /dev/urandom output could be predicted. Note that this is not really about /dev/urandom, the special file: the early boot moments we are talking about are before there is any notion of a file; this is really about a single case, which is booting a diskless machine over the network, and mounting the root filesystem from a remote server. The relevant network protocol can need some randomness (e.g. TCP sequence numbers).

Since at least the early 2000s, Linux distributions have applied workarounds to ensure proper entropy at boot time, namely that a boot script injects the contents of a saved file upon boot, and immediately proceeds to regenerate the said file with /dev/urandom. In effect, this transports the entropy across reboots, so that even if the boot sequence was not enough, by itself, to generate enough entropy, the file contents would ensure that everything is all right.

Later on, a system call was added, to get randomness without having to open a file and use a file descriptor; it is named getrandom(). That system call finally implements the proper behavior, i.e. blocking until a sufficient amount of initial entropy has been gathered since last boot, but never blocking afterwards. Incidentally, this is what /dev/urandom does on sane systems (e.g. FreeBSD or macOS). Applications should simply use getrandom(), and be happy.

(Or not. Linux 5.3 will turn back getrandom() into /dev/urandom with its never-blocking behavior, because, quite frankly, Linus’s opinions on his own mastery of RNG theory exceed his actual abilities. The reasoning seems to be that if there is “not enough entropy”, then the application should make an interpretative dance of some kind to promote the appearance of new hardware events from which the entropy can be gathered. How the application does that, or why the kernel should not do it despite being much closer to the hardware, is not said.)

All of the above is the classical description, up to the early/mid-2010s. There are now a few extra relevant points to make:

- Virtual machines are a challenge to entropy gathering, in at least three ways:

- They provided access to virtual, emulated hardware only. The nice physical events from which entropy is supposed to come (thermal noise, mostly) are then just a simulation, and that which is simulated can, indeed, be simulated.
- The hypervisor can prevent access to the cycle counter (rdtsc opcode), which will further hinder attempts by the kernel to get entropy from the (not so) physical events.
- VM snapshots can be taken and replayed at will; each restart from the same snapshot will use the recorded pool contents.

- A contrario, sufficiently recent CPU have an embedded hardware generator which is totally available from VM (it’s the rdrand opcode on x86 CPU). The Linux kernel uses rdrand. It does not trust rdrand, because NSA (I’m not exaggerating! The kernel source code [explicitly calls out the NSA](https://github.com/torvalds/linux/blob/37d4e84f765bb3038ddfeebdc5d1cfd7e1ef688f/drivers/char/random.c#L1891)), so it will not count the rdrand output as worth any entropy. But it will still use it. In all edge cases described above (network boot, VM snapshots…), rdrand will by itself ensure that there is enough entropy for all practical purposes.

Given all the above mess, there are people who try to use extra hardware RNG, usually by injecting the random bytes obtained from these devices into the kernel (by writing into /dev/random). Surprise! It does not actually work; or, at least, not as well as is usually believed. It so happens that in 2017, a smart guy called Jason Donenfeld had a window of opportunity, to try to improve the RNG (he is the creator of Wireguard, and apparently Linus likes him). So, he changed the CSRNG for something decent and fast (the ChaCha20 stream cipher). As a byproduct of his changes, the CSRNG reseeds from the entropy pool only if the last reseeding occurred more than [five minutes](https://github.com/torvalds/linux/blob/37d4e84f765bb3038ddfeebdc5d1cfd7e1ef688f/drivers/char/random.c#L839) earlier (this is a part of the performance improvements). It still uses rdrand for each call, so it is very fine. But that means that any extra entropy, obtained from a dedicated hardware RNG (possibly a very costly one) and injected into the kernel, may be wholly ignored for up to five minutes. This incarnates a new high on the scale of uselessness.

The entropy depletion cult is not very happy about that. Over time, it has spun off some extra sects, including [haveged](https://www.issihosts.com/haveged/). This particular piece of software is not only operating on the flawed notion of entropy depletion, but also on the belief that where the kernel, which has access to the hardware, fails, a userland software, without access to the hardware, can succeed. In practice, it’s another moving part in a complicated but ultimately meaningless ritual.

Some words on statistical tests. The NIST has come up with a bunch of statistical tests meant to measure the quality of a random source. This is a semi-flawed idea. Any good CSRNG will pass these tests successfully. Many atrocious CSRNG will pass them, too. A bias that can be detected through statistics is the sign of an extremely bad design. The whole point of cryptography is to defeat intelligent attackers who have computers and know exactly what kind of software we run; this gives them a lot more power than statistics. Consider for instance the following RNG: from a seed s, it generates output in 32-byte chunks by doing the following:

- Replace s with SHA-256(s)
- Output s

- Goto 1 (until all requested output as been produced)

This will pass the FIPS 140-2 statistical tests with flying colours; none of the statistical tests will see anything wrong with that. But, of course, the first 32 bytes that this RNG outputs are its current internal seed; any attacker observing a 32-byte output chunk can compute all subsequent output with 100% reliability.

To be brief, a CSRNG should be unbreakable by smart attackers with lots of resources. If a CSRNG does not pass statistical tests, then this means that it can be broken by a chimpanzee. But if it passes the tests, this does in no way prove that it is cryptographically strong; it just means that the chimpanzee will be stumped.

Now, statistical tests are randomized in nature. Each such test is really a measure of implausibility: the test gets an output, then tries to compute how improbable such an outcome can be, assuming that the source is perfectly random. For instance, if there was less than 1 chance in 1000 to have such a bias, then the test reports a “significant bias” (this is the way all experimental science works, the probability being called “p-value” and the significance threshold being traditionally 0.05, i.e. that which could happen with probability 1/20 or less is considered significant).

1 in 1000 is a low probability. But if you run 10000 tests, each with a 1 in 1000 threshold, then there will be some “failures”: things that happen with probability 1 in 1000, do happen fairly reliably if you try 10000 times. Thus, the fact that some FIPS tests occasionally report a “failure” is here meaningless.

Summary. Monitoring entropy levels on Linux systems is not very useful. From a security point of view, the entropy estimates by the kernel are quite off. It may happen that some applications insist on reading from /dev/random instead of /dev/urandom (or from getrandom()), and block unduly for the sake of that specific brand of fetishism. This should not happen often. OpenSSL uses /dev/urandom, for instance. So does Java by default. The occasional blocking is so irksome that there has been great pressure to make all apps use the non-blocking /dev/urandom; only the very few apps that nobody really uses (e.g. PGP) still insist on /dev/random.

To fully ensure that no blocking ever happens, a simple solution might be to simply make /dev/random a link to /dev/urandom. (In recent Linux distributions, /dev is a virtual filesystem, so such renaming must be done at each boot, which may or may not be easy.)

---

A better lay of the land:
https://www.2uo.de/myths-about-urandom

Myths about /dev/urandom

There are a few things about /dev/urandom and /dev/random that are repeated again and again. Still they are false.

I'm mostly talking about reasonably recent Linux systems, not other UNIX-like systems.

/dev/urandom is insecure. Always use /dev/random for cryptographic purposes.

Fact: /dev/urandom is the preferred source of cryptographic randomness on UNIX-like systems.

/dev/urandom is a pseudo random number generator, a PRNG, while /dev/random is a “true” random number generator.

[Fact:](https://www.2uo.de/myths-about-urandom#structure-of-linux-s-random-number-generator) Both /dev/urandom and /dev/random are using the exact same CSPRNG (a cryptographically secure pseudorandom number generator). They only differ in very few ways that have nothing to do with “true” randomness.

/dev/random is unambiguously the better choice for cryptography. Even if /dev/urandom were comparably secure, there's no reason to choose the latter.

[Fact:](https://www.2uo.de/myths-about-urandom#what-s-wrong-with-blocking) /dev/random has a very nasty problem: it blocks.

But that's good! /dev/random gives out exactly as much randomness as it has entropy in its pool. /dev/urandom will give you insecure random numbers, even though it has long run out of entropy.

[Fact:](https://www.2uo.de/myths-about-urandom#what-about-entropy-running-low) No. Even disregarding issues like availability and subsequent manipulation by users, the issue of entropy “running low” is a straw man. About 256 bits of entropy are enough to get computationally secure numbers for a long, long time.

And the fun only starts here: how does /dev/random know [how much entropy](https://www.2uo.de/myths-about-urandom#estimate) there is available to give out? Stay tuned!

But cryptographers always talk about constant re-seeding. Doesn't that contradict your last point?

[Fact:](https://www.2uo.de/myths-about-urandom#re-seeding) You got me! Kind of. It is true, the random number generator is constantly re-seeded using whatever entropy the system can lay its hands on. But that has (partly) other reasons.

Look, I don't claim that injecting entropy is bad. It's good. I just claim that it's bad to block when the entropy estimate is low.

That's all good and nice, but even the man page for /dev/(u)random contradicts you! Does anyone who knows about this stuff actually agree with you?

[Fact:](https://www.2uo.de/myths-about-urandom#the-random-and-urandom-man-page) No, it really doesn't. It seems to imply that /dev/urandom is insecure for cryptographic use, unless you really understand all that cryptographic jargon.

The man page does recommend the use of /dev/random in some cases (it doesn't hurt, in my opinion, but is not strictly necessary), but it also recommends /dev/urandom as the device to use for “normal” cryptographic use.

And while appeal to authority is usually nothing to be proud of, in cryptographic issues you're generally right to be careful and try to get the opinion of a domain expert.

And yes, quite a few [experts](https://www.2uo.de/myths-about-urandom#orthodoxy) share my view that /dev/urandom is the go-to solution for your random number needs in a cryptography context on UNIX-like systems. Obviously, their opinions influenced mine, not the other way around.

---------------------------------------------------------------

Hard to believe, right? I must certainly be wrong! Well, read on and let me try to convince you.

I tried to keep it out, but I fear there are two preliminaries to be taken care of, before we can really tackle all those points.

Namely, [what is randomness](https://www.2uo.de/myths-about-urandom#true-randomness), or better: what kind of randomness am I talking about here?

And, even more important, I'm really [not being condescending.](https://www.2uo.de/myths-about-urandom#you-re-saying-i-m-stupid) I have written this document to have a thing to point to, when this discussion comes up again. More than 140 characters. Without repeating myself again and again. Being able to hone the writing and the arguments itself, benefitting many discussions in many venues.

And I'm certainly willing to hear differing opinions. I'm just saying that it won't be enough to state that /dev/urandom is bad. You need to identify the points you're disagreeing with and engage them.

You're saying I'm stupid!

Emphatically no!

Actually, I used to believe that /dev/urandom was insecure myself, a few years ago. And it's something you and I almost had to believe, because all those highly respected people on Usenet, in web forums and today on Twitter told us. Even the [the random and urandom man page](https://www.2uo.de/myths-about-urandom#the-random-and-urandom-man-page) seems to say so. Who were we to dismiss their convincing argument about “entropy running low”?

This misconception isn't so rampant because people are stupid, it is because with a little knowledge about cryptography (namely some vague idea what entropy is) it's very easy to be convinced of it. Intuition almost forces us there. Unfortunately intuition is often wrong in cryptography. So it is here.

True randomness

What does it mean for random numbers to be “truly random”?

I don't want to dive into that issue too deep, because it quickly gets philosophical. Discussions have been known to unravel quickly, because everyone can wax about their favorite model of randomness, without paying attention to anyone else. Or even making himself understood.

I believe that the “gold standard” for “true randomness” are quantum effects. Observe a photon pass through a semi-transparent mirror. Or not. Observe some radioactive material emit alpha particles. It's the best idea we have when it comes to randomness in the world. Other people might reasonably believe that those effects aren't truly random. Or even that there is no randomness in the world at all. Let a million flowers bloom.

Cryptographers often circumvent this philosophical debate by disregarding what it means for randomness to be “true”. They care about unpredictability. As long as nobody can get any information about the next random number, we're fine. And when you're talking about random numbers as a prerequisite in using cryptography, that's what you should aim for, in my opinion.

Anyway, I don't care much about those “philosophically secure” random numbers, as I like to think of your “true” random numbers.

Two kinds of security, one that matters

But let's assume you've obtained those “true” random numbers. What are you going to do with them?

You print them out, frame them and hang them on your living-room wall, to revel in the beauty of a quantum universe? That's great, and I certainly understand.

Wait, what? You're using them? For cryptographic purposes? Well, that spoils everything, because now things get a bit ugly.

You see, your truly-random, quantum effect blessed random numbers are put into some less respectable, real-world tarnished algorithms.

Because almost all of the cryptographic algorithms we use do not hold up to information-theoretic security. They can “only” offer computational security. The two exceptions that come to my mind are Shamir's Secret Sharing and the One-time pad. And while the first one may be a valid counterpoint (if you actually intend to use it), the latter is utterly impractical.

But all those algorithms you know about, AES, RSA, Diffie-Hellman, Elliptic curves, and all those crypto packages you're using, OpenSSL, GnuTLS, Keyczar, your operating system's crypto API, these are only computationally secure.

What's the difference? While information-theoretically secure algorithms are secure, period, those other algorithms cannot guarantee security against an adversary with unlimited computational power who's trying all possibilities for keys. We still use them because it would take all the computers in the world taken together longer than the universe has existed, so far. That's the level of “insecurity” we're talking about here.

Unless some clever guy breaks the algorithm itself, using much less computational power. Even computational power achievable today. That's the big prize every cryptanalyst dreams about: breaking AES itself, breaking RSA itself and so on.

So now we're at the point where you don't trust the inner building blocks of the random number generator, insisting on “true randomness” instead of “pseudo randomness”. But then you're using those “true” random numbers in algorithms that you so despise that you didn't want them near your random number generator in the first place!

Truth is, when state-of-the-art hash algorithms are broken, or when state-of-the-art block ciphers are broken, it doesn't matter that you get “philosophically insecure” random numbers because of them. You've got nothing left to securely use them for anyway.

So just use those computationally-secure random numbers for your computationally-secure algorithms. In other words: use /dev/urandom.

Structure of Linux's random number generator

An incorrect view

Chances are, your idea of the kernel's random number generator is something similar to this:

[Wrong view]

“True randomness”, albeit possibly skewed and biased, enters the system and its entropy is precisely counted and immediately added to an internal entropy counter. After de-biasing and whitening it's entering the kernel's entropy pool, where both /dev/random and /dev/urandom get their random numbers from.

The “true” random number generator, /dev/random, takes those random numbers straight out of the pool, if the entropy count is sufficient for the number of requested numbers, decreasing the entropy counter, of course. If not, it blocks until new entropy has entered the system.

The important thing in this narrative is that /dev/random basically yields the numbers that have been input by those randomness sources outside, after only the necessary whitening. Nothing more, just pure randomness.

/dev/urandom, so the story goes, is doing the same thing. Except when there isn't sufficient entropy in the system. In contrast to /dev/random, it does not block, but gets “low quality random” numbers from a pseudorandom number generator (conceded, a cryptographically secure one) that is running alongside the rest of the random number machinery. This CSPRNG is just seeded once (or maybe every now and then, it doesn't matter) with “true randomness” from the randomness pool, but you can't really trust it.

In this view, that seems to be in a lot of people's minds when they're talking about random numbers on Linux, avoiding /dev/urandom is plausible.

Because either there is enough entropy left, then you get the same you'd have gotten from /dev/random. Or there isn't, then you get those low-quality random numbers from a CSPRNG that almost never saw high-entropy input.

Devilish, right? Unfortunately, also utterly wrong. In reality, the internal structure of the random number generator looks like this.

A better simplification

Before Linux 4.8

[Before Linux 4.8]

See the big difference? The CSPRNG is not running alongside the random number generator, filling in for those times when /dev/urandom wants to output something, but has nothing good to output. The CSPRNG is an integral part of the random number generation process. There is no /dev/random handing out “good and pure” random numbers straight from the whitener. Every randomness source's input is thoroughly mixed and hashed inside the CSPRNG, before it emerges as random numbers, either via /dev/urandom or /dev/random.

This is a pretty rough simplification. In fact, there isn't just one, but three pools filled with entropy. One primary pool, and one for /dev/random and /dev/urandom each, feeding off the primary pool. Those three pools all have their own entropy counts, but the counts of the secondary pools (for /dev/random and /dev/urandom) are mostly close to zero, and “fresh” entropy flows from the primary pool when needed, decreasing its entropy count. Also there is a lot of mixing and re-injecting outputs back into the system going on. All of this is far more detail than is necessary for this document.

Another important difference is that there is no entropy counting going on here, but estimation. The amount of entropy some source is giving you isn't something obvious that you just get, along with the data. It has to be estimated. Please note that when your estimate is too optimistic, the dearly held property of /dev/random, that it's only giving out as many random numbers as available entropy allows, is gone. Unfortunately, it's hard to estimate the amount of entropy.

The Linux kernel uses only the arrival times of events to estimate their entropy. It does that by interpolating polynomials of those arrival times, to calculate “how surprising” the actual arrival time was, according to the model. Whether this polynomial interpolation model is the best way to estimate entropy is an interesting question. There is also the problem that internal hardware restrictions might influence those arrival times. The sampling rates of all kinds of hardware components may also play a role, because it directly influences the values and the granularity of those event arrival times.

In the end, to the best of our knowledge, the kernel's entropy estimate is pretty good. Which means it's conservative. People argue about how good it really is, but that issue is far above my head. Still, if you insist on never handing out random numbers that are not “backed” by sufficient entropy, you might be nervous here. I'm sleeping sound because I don't care about the entropy estimate.

So to make one thing crystal clear: both /dev/random and /dev/urandom are fed by the same CSPRNG. Only the behavior when their respective pool runs out of entropy, according to some estimate, differs: /dev/random blocks, while /dev/urandom does not.

From Linux 4.8 onward

[From Linux 4.8]

In Linux 4.8 the equivalency between /dev/urandom and /dev/random was given up. Now /dev/urandom output does not come from an entropy pool, but directly from a CSPRNG.

[We will see shortly](https://www.2uo.de/myths-about-urandom#the-csprngs-are-alright) why that is not a security problem.

What's wrong with blocking?

Have you ever waited for /dev/random to give you more random numbers? Generating a PGP key inside a virtual machine maybe? Connecting to a web server that's waiting for more random numbers to create an ephemeral session key?

That's the problem. It inherently runs counter to availability. So your system is not working. It's not doing what you built it to do. Obviously, that's bad. You wouldn't have built it if you didn't need it. I'm working on safety-related systems in factory automation. Can you guess what the main reason for failures of safety systems is? Manipulation. Simple as that. Something about the safety measure bugged the worker. It took too much time, was too inconvenient, whatever. People are very resourceful when it comes to finding “inofficial solutions”.

But the problem runs even deeper: people don't like to be stopped in their ways. They will devise workarounds, concoct bizarre machinations to just get it running. People who don't know anything about cryptography. Normal people.

I'm working on safety-related systems in factory automation. Can you guess what the main reason for failures of safety systems is? Manipulation. Simple as that. Something about the safety measure bugged the worker. It took too much time, was too inconvenient, whatever. People are very resourceful when it comes to finding “inofficial solutions”.

Why not patching out the call to random()? Why not having some guy in a web forum tell you how to use some strange ioctl to increase the entropy counter? Why not switch off SSL altogether?

In the end you just educate your users to do foolish things that compromise your system's security without you ever knowing about it.

It's easy to disregard availability, usability or other nice properties. Security trumps everything, right? So better be inconvenient, unavailable or unusable than feign security.

But that's a false dichotomy. Blocking is not necessary for security. As we [saw](https://www.2uo.de/myths-about-urandom#a-better-simplification), /dev/urandom gives you the same kind of random numbers as /dev/random, straight out of a CSPRNG. Use it!

The CSPRNGs are alright

But now everything sounds really bleak. If even the high-quality random numbers from /dev/random are coming out of a CSPRNG, how can we use them for high-security purposes?

It turns out, that “looking random” is the basic requirement for a lot of our cryptographic building blocks. If you take the output of a cryptographic hash, it has to be indistinguishable from a random string so that cryptographers will accept it. If you take a block cipher, its output (without knowing the key) must also be indistinguishable from random data.

If anyone could gain an advantage over brute force breaking of cryptographic building blocks, using some perceived weakness of those CSPRNGs over “true” randomness, then it's the same old story: you don't have anything left. Block ciphers, hashes, everything is based on the same mathematical fundament as CSPRNGs. So don't be afraid.

What about entropy running low?

It doesn't matter.

The underlying cryptographic building blocks are designed such that an attacker cannot predict the outcome, as long as there was enough randomness (a.k.a. entropy) in the beginning. A usual lower limit for “enough” may be 256 bits. No more.

Considering that we were pretty hand-wavey about the term “entropy” in the first place, it feels right. As we saw, the kernel's random number generator cannot even precisely know the amount of entropy entering the system. Only an estimate. And whether the model that's the basis for the estimate is good enough is pretty unclear, too.

Re-seeding

But if entropy is so unimportant, why is fresh entropy constantly being injected into the random number generator?

djb [remarked](http://blog.cr.yp.to/20140205-entropy.html) that more entropy actually can hurt.

First, it cannot hurt. If you've got more randomness just lying around, by all means use it!

There is another reason why re-seeding the random number generator every now and then is important:

Imagine an attacker knows everything about your random number generator's internal state. That's the most severe security compromise you can imagine, the attacker has full access to the system.

You've totally lost now, because the attacker can compute all future outputs from this point on.

But over time, with more and more fresh entropy being mixed into it, the internal state gets more and more random again. So that such a random number generator's design is kind of self-healing.

But this is injecting entropy into the generator's internal state, it has nothing to do with blocking its output.

The random and urandom man page

There has actually been an updated version of the Linux kernel man page for /dev/random and /dev/urandom. Unfortunately, a simple web search still turns up the old, deficient version I'm describing here in the top results. Furthermore, many Linux distributions still ship the old man pages. So unfortunately this section needs to stay a bit longer in the essay. I'm so looking forward to deleting it!

The man page for /dev/random and /dev/urandom is pretty effective when it comes to instilling fear into the gullible programmer's mind:

> A read from the /dev/urandom device will not block waiting for more entropy. As a result, if there is not sufficient entropy in the entropy pool, the returned values are theoretically vulnerable to a cryptographic attack on the algorithms used by the driver. Knowledge of how to do this is not available in the current unclassified literature, but it is theoretically possible that such an attack may exist. If this is a concern in your application, use /dev/random instead.

Such an attack is not known in “unclassified literature”, but the NSA certainly has one in store, right? And if you're really concerned about this (you should!), please use /dev/random, and all your problems are solved.

The truth is, while there may be such an attack available to secret services, evil hackers or the Bogeyman, it's just not rational to just take it as a given.

And even if you need that peace of mind, let me tell you a secret: no practical attacks on AES, SHA-3 or other solid ciphers and hashes are known in the “unclassified” literature, either. Are you going to stop using those, as well? Of course not!

Now the fun part: “use /dev/random instead”. While /dev/urandom does not block, its random number output comes from the very same CSPRNG as /dev/random's.

If you really need information-theoretically secure random numbers (you don't!), and that's about the only reason why the entropy of the CSPRNGs input matters, you can't use /dev/random, either!

The [current, updated version](http://man7.org/linux/man-pages/man4/random.4.html) of the man page says in no uncertain terms:

> The /dev/random interface is considered a legacy interface, and /dev/urandom is preferred and sufficient in all use cases, with the exception of applications which require randomness during early boot time; for these applications, getrandom(2) must be used instead, because it will block until the entropy pool is initialized.

The man page is silly, that's all. At least it tries to redeem itself with this:

> If you are unsure about whether you should use /dev/random or /dev/urandom, then probably you want to use the latter. As a general rule, /dev/urandom should be used for everything except long-lived GPG/SSL/SSH keys.

Fine. I think it's unnecessary, but if you want to use /dev/random for your “long-lived keys”, by all means, do so! You'll be waiting a few seconds typing stuff on your keyboard, that's no problem.

But please don't make connections to a mail server hang forever, just because you “wanted to be safe”.

Orthodoxy

The view espoused here is certainly a tiny minority's opinions on the Internet. But ask a real cryptographer, you'll be hard pressed to find someone who sympathizes much with that blocking /dev/random.

Let's take [Daniel Bernstein](http://www.mail-archive.com/cryptography@randombit.net/msg04763.html), better known as djb:

> Cryptographers are certainly not responsible for this superstitious nonsense. Think about this for a moment: whoever wrote the /dev/random manual page seems to simultaneously believe that
>
> (1) we can't figure out how to deterministically expand one 256-bit /dev/random output into an endless stream of unpredictable keys (this is what we need from urandom), but
>
> (2) we _can_ figure out how to use a single key to safely encrypt many messages (this is what we need from SSL, PGP, etc.).
>
> For a cryptographer this doesn't even pass the laugh test.

Or [Thomas Pornin](http://security.stackexchange.com/questions/3936/is-a-rand-from-dev-urandom-secure-for-a-login-key/3939#3939), who is probably one of the most helpful persons I've ever encountered on the Stackexchange sites:

> The short answer is yes. The long answer is also yes. /dev/urandom yields data which is indistinguishable from true randomness, given existing technology. Getting “better” randomness than what /dev/urandom provides is meaningless, unless you are using one of the few “information theoretic” cryptographic algorithm, which is not your case (you would know it).
>
> The man page for urandom is somewhat misleading, arguably downright wrong, when it suggests that /dev/urandom may “run out of entropy” and /dev/random should be preferred;

Or maybe [Thomas Ptacek](http://sockpuppet.org/blog/2014/02/25/safely-generate-random-numbers), who is not a real cryptographer in the sense of designing cryptographic algorithms or building cryptographic systems, but still the founder of a well-reputed security consultancy that's doing a lot of penetration testing and breaking bad cryptography:

> Use urandom. Use urandom. Use urandom. Use urandom. Use urandom. Use urandom.

Not everything is perfect

/dev/urandom isn't perfect. The problems are twofold:

On Linux, unlike FreeBSD, /dev/urandom never blocks. Remember that the whole security rested on some starting randomness, a seed?

Linux's /dev/urandom happily gives you not-so-random numbers before the kernel even had the chance to gather entropy. When is that? At system start, booting the computer.

FreeBSD does the right thing: they don't have the distinction between /dev/random and /dev/urandom, both are the same device. At startup /dev/random blocks once until enough starting entropy has been gathered. Then it won't block ever again. In the meantime, Linux has implemented a new syscall, originally introduced by OpenBSD as getentropy(2): getrandom(2). This syscall does the right thing: blocking until it has gathered enough initial entropy, and never blocking after that point. Of course, it is a syscall, not a character device, so it isn't as easily accessible from shell or script languages. It is available from Linux 3.17 onward.

On Linux it isn't too bad, because Linux distributions save some random numbers when booting up the system (but after they have gathered some entropy, since the startup script doesn't run immediately after switching on the machine) into a seed file that is read next time the machine is booting. So you carry over the randomness from the last running of the machine.

In the meantime, Linux has implemented a new syscall, originally introduced by OpenBSD as getentropy(2): getrandom(2). This syscall does the right thing: blocking until it has gathered enough initial entropy, and never blocking after that point. Of course, it is a syscall, not a character device, so it isn't as easily accessible from shell or script languages. It is available from Linux 3.17 onward.

Obviously that isn't as good as if you let the shutdown scripts write out the seed, because in that case there would have been much more time to gather entropy. The advantage is obviously that this does not depend on a proper shutdown with execution of the shutdown scripts (in case the computer crashes, for example).

And it doesn't help you the very first time a machine is running, but the Linux distributions usually do the same saving into a seed file when running the installer. So that's mostly okay.

Virtual machines are the other problem. Because people like to clone them, or rewind them to a previously saved check point, this seed file doesn't help you.

But the solution still isn't using /dev/random everywhere, but properly seeding each and every virtual machine after cloning, restoring a checkpoint, whatever.

tldr;

Just use /dev/urandom!
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