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Comment by tptacek | Hacker News Reader

Here again I will posit:

Secure programs should rely on /dev/urandom, to the exclusion of other CSPRNGs, and should specifically eschew userland CSPRNG engines even when they're seeded from /dev/urandom.

This Android bug is another in a line of bugs in userland CSPRNGs, the most famous of which was Debian's OpenSSL CSPRNG bug which gave up everyone's SSH key. CSPRNGs have bugs. When you rely on a userland CSPRNG, you're relying on two single points of failure: the userland CSPRNG and the OS's CSPRNG. Failures of either aren't generally survivable.

There are people who are smarter than me (notably 'cperciva) who disagree with me on this, but this is an idea I got from DJB's NACL design paper; it's not my own crazy idea.

I got to eat dinner with Dan Bernstein the other day, by the way, and it turns out NACL is pronounced "lasagna".

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Except if /dev/urandom is using a hardware based random number generator, then you have to trust that the hardware hasn't received some NSA alterations at some point during the design.

The NSA did design a random number generator that likely had a backdoor in it: https://en.wikipedia.org/wiki/Dual_EC_DRBG#Controversy).

Here's Bruce Schneier talking about it: https://www.schneier.com/blog/archives/2007/12/dual_ec_drbg_...

Also it's in Windows (although it's not used by default but userspace programs could rely on it). https://www.schneier.com/blog/archives/2007/12/dual_ec_drbg_...

It would be possible for the NSA to go to Intel and get them to put in something in their random number generator that would let them to basically break the encryption by massively reducing the keyspace if they have the secret key.

No you don't. The only time you need to worry about the NSA having a back door into your random number generator is if you are worried either about the NSA attacking you (in which case you probably don't have a chance anyway), or are worried that some learned of the NSA's back door and is exploiting that.

That reminds me of a question I've been pondering on-and-off for a while: is it possible even in theory to use Sufficiently Advanced Mathematics (e.g. homomorphic encryption) to build a trusted computation environment on top of an untrustworthy foundation?

For example, I want to run a VM on EC2, knowing the NSA's after something I have. Is there some way I could, even theoretically, build my VM, that would protect it from the NSA compelling Amazon's cooperation in ripping the data right from my VM's memory at the hypervisor level?

Possibly. For instance, look up some papers on White Box Cryptography; these are implementations of things like AES designed to run AES computations on untrusted platforms without revealing their key (the key gets "baked in" to the code that runs the algorithm in such a way that it's theorized to be cryptographically hard to extract the key from the algorithm).

You are getting into the flip side of the DRM coin here; put differently: if there's a way to do what you want, there's also a way for the MPAA to do what it wants on PCs.

Personally, I think there is, at least in the dollar-cost model of security.

It's a good insight. There is distinction between I/O (rendering the video and audio to the host) and computations of a seemingly blackbox. Would also be an interesting avenue for malware to grow into if possible. The problem is there's always been decryption code that has to live somewhere that effectively unlocks the whole thing. Edit: PyPy but zk computing for malware.

Wouldn't this make nice foundations for hardware cryptographic tokens such as CryptoStick?

I'm out of my depth here, but since this conversation seems precise, and is discussing theoretical and practical implementations, does this paper[1] factor in? If so, is it fair to say that at some level, without auditing every aspect of hardware, and every aspect of all software used at build time and runtime, you can't know?

[1] http://cm.bell-labs.com/who/ken/trust.html

Funny you should mention this. I was asked to "suggest solutions" to this in 2008 as a client-facing AWS enterprise consultant in the mobile industry.

The naïve way relies on: "If there isn't a Rootkit or capture mechanism, it's secure if it's obscure." That will buy an iota of time.

The other way is to simply not trust the host and end-to-end encrypt everything important. This is a PITA, but there is no current viable alternative without a significantly rigorous and extensive project (would be in the form of a VM for servers &| something that runs on js like asm.js)

Mostly you need zero knowledge computation. Storage and network are easier probs.

The intractable problem is eventually an answer needs to be presented somewhere. For now, you should minimize exposure as much as possible through all means available.

We're working on a similar problem at PrivateCore: Protecting VM data in-use on outsourced infrastructure.

We're running a high-assurance, remotely attestable hypervisor inside the CPU cache and encrypting all access to main memory. This protects against threats from the physical layer, like cold boot, DMA attacks, NVDIMMs, bus analyzers, etc.

It's not quite what you're talking about in your Amazon and NSA scenario. Amazon doesn't let you bring your own hypervisor to run on bare metal and the NSA can compromise the CPU itself.

However, our approach does give you assurance that someone with physical access can't easily snapshot your VM memory.

> remotely attestable hypervisor

How does this bit work, by the way? What's stopping an altered hypervisor from lying to say it's unaltered? (This is the classic "how do you verify a player on your FPS isn't running a bot instead of a game-client" problem in a nutshell.)

Today we rely on the TPM to measure the state of the system using Intel TXT. These measurements are stored in platform configuration registers (PCRs) on the TPM device.

There are known TPM and LPC bus vulnerabilities. That is why long-term we will move away from that dependency by utilizing upcoming CPU features.

> our approach does give you assurance that someone with physical access can't easily snapshot your VM memory.

But how do you know the VM (or rather the hypervisor for the vm) is running on physical hardware, and not in a hypervisor?

I can't think of a way you could be certain of this remotely? Perhaps you could be on-site for the boot-up, and then rely on the fact that snapshotting is very hard -- but it sounds rather fragile...

Still very interesting project! I've been thinking a bit on "running inside the L/1/2/3 chache"-lately - but I hadn't thought about the particular idea that you could treat RAM as "external" -- assuming you could guarantee that you're always in cache.

You must first remotely attest the hypervisor using TXT before deploying a VM to run on it.

Today, that attestation process relies on a TPM and a signed certificate chain baked in by the TPM manufacturer. This is standard stuff out of the Trusted Computing Group.

One more thing to add, this isn't just a personal side project. We're a company and have a beta product deployed to early adopters.

I realized that you probably did have a product -- unfortunately that doesn't mean that the product works (not trying to imply anything about your product/company here; it's just (as you probably know) there many companies selling security solutions; and very few that seem to be selling security solutions that work...).

Are you aware of:

  "Overcoming TPM by exploiting EFI overflow"
  http://www.youtube.com/watch?v=4bM3Gut1hIk&feature=player_detailpage&t=1655

and:

  "Coreboot/bios malware":
  http://www.youtube.com/watch?v=umBruM-wFUw&feature=player_detailpage&t=2297

It's an interesting use of TPM -- and sounds like a sound approach, assuming there aren't any bugs in the TPM software... which might be too big an assumption.

I don't suppose any of your software is available as open source? Where can I/we learn more?

I don't know anything about his product but let me just chime in here to say that Steve Weis is the real deal.

Hi. We're aware of TPM vulnerabilities. The one you link to is not relevant. However, there are attacks to extract EK private keys, which we know the cost of conducting. It's significantly higher than other low-cost attacks.

We're also aware of vulnerabilities on the the LPC bus. The latter can be addressed with existing TPM 1.2 features -- although they aren't enabled by default.

There are CPU features in the pipeline which may make the TPM unnecessary. We're also working on some new attestation techniques which may help.

We measure and attest the state of the system with TXT. If that works as advertised, you would measure changes to the BIOS, SINIT, opt ROMs, ACM modules, etc.

If you don't trust the hardware, then you've already lost, no matter what algorithmic construction you are using. How are you going to trust your random number generator if 1 + 1 = 2 except when it equals NSA?

(This is one of those times where I regret HN that has twin interests in software security as an engineering science and software security as a political statement.)

> If you don't trust the hardware, then you've already lost, no matter what algorithmic construction you are using. How are you going to trust your random number generator if 1 + 1 = 2 except when it equals NSA?

You can verify the random number generator. If you know the algorithm and the seed values you can run it on multiple different platforms, or with a pen and paper and verify that the output is as expected and repeatable. If you have large amounts of entropy you are feeding into it, you can log it for testing purposes.

There are also apparently some EC based algorithms that can be used to fix or at lease reduce the impact of a compromised random number generator.

That might not protect against a active attack on your specific system by the NSA (they could send/embedded a magic packet that gives them total control over the CPU for example), might even be possible for it to happen on the NIC controller rather than the CPU if it has access to the system bus. At the least they could flip it into some kind of backdoor random number mode by embedding some extra data in a TLS handshake or whatever. But it should protect against widespread passive surveillance.

Reducing the impact of a CSPRNG state compromise is a basic design goal for CSPRNGs, and you don't need ECC to do it; the "NSA backdoored" RNG you're referring to is one that no system anyone knows about actually uses, for that reason.

Again: you're making a point that has nothing to do with my comment. If you don't trust RDRAND, don't use it. But you should still be using the OS's CSPRNG. Just make sure your OS isn't using RDRAND. Done and done.

It's about verifiability, you can verify if a CPU is adding incorrectly, there's no way of verifying if the random source is generating randomness correctly.

Integers form a closed group (in the group theory sense) under multiplication and addition, so if you were concerned about specific calculations being compromised you could verify the answer via alternative calculations and testing for consistency.

Faking consistency is likely impossible without causing a huge amount other calculations (which the CPU will do as part of day-to-day operations) to fail. Making the CPU alter calculations only under very specific circumstances and in an undetectable way would require a huge amount of complexity.

On the other hand we know several fairly trivial ways of faking reversable randomness using standard crypto algos that would be statistically undetectable without taking the hardware apart.

Tell you what: You write executable code which tests whether, e.g., Ruby's Fixnum addition is actually adding or not, without using introspection on the implementation. (i.e. You're allowed to test any combination of inputs and outputs, but cannot look within the black box.) Feel free to use number theory, statistics, or a web service which wraps Cthulu in JSON.

I will provide a script which monkeypatches Fixnum such that your test script still reports "Yep, it's still adding correctly" and which will predictably cause 1 + 1 to equal the string "LOL I AM THE NSA" under attacker-chosen conditions.

There are many, many ways I could do this as the crafty adversary, but I anticipate my implementation would be roughly seven lines long and trivially implementable in hardware. (At complexity somewhere between a half-adder and the circuits which we had to hand-draw for our finals in hardware design, which -- obviously -- are a drop in the ocean of complexity on a modern CPU.)

Say you wanted your backdoor to kick-in when I was calculating the fibonacci sequence, will you also detect when I was calculating it via the golden ratio, or via continued fractions, etc. If I can write a test that can show that there's a backdoor because it's not behaving consistently then your attack is significantly weaker then a RNG attack.

You can't just break one calculation under specific circumstances, you have to break any way of verifying it which means compromising pretty much every calculation.

(not to mention you'd have to figure out that it's the fibonacci sequence being calculated and not some other calculation that happens to start with 1+1)

Remember you don't have an interactive attack, you have one chance to build something and ship it out in hardware, and it then has to compromise software that's going to be written in the future.

The beauty of the RNG attack is that it's undetectable, introduces a backdoor into a huge number of systems and it only makes the system vulnerable to the attacker and not to anyone else.

Super double-secret squirrel sources have said there exists a file "nsa_does_fixnum.rb" which is 1006 bytes long and the SHA512 hash of 896e12642f5720d830572a8b11e4c729da3a20c57402758d22a25c46c3c2e6 bb4b09de9030ad591ecb51b8a9a4d4c119cefbf4d51a72db219d94150fb63a 3623. (Concat them -- I broke it into a few lines to avoid breaking the comment page on low-resolution devices.) Our intrepid squirrel didn't get a good look at what it is doing but claims "It doesn't use any external library and doesn't do anything even remotely tricky." They claim that it secretly modifies any Ruby1.8 interpreter to cause 1 + 1 to equal "LOL THIS IS THE NSA" under some circumstances.

Quick ig1, you are freedom's last, best hope. Write actual computer code which can, by adding numbers together and inspecting their output, determine whether your Ruby interpreter has been compromised by the NSA. You're lucky, since the NSA has already shipped their exploit (or did they?), they can't modify it in response to your detection code. Bad news, though: if your detection code fails and an interpreter which includes the backdoor can, after passing your detection code, still get the wrong answer for 1 + 1, an innocent user fails to find the backdoor and suffers a FatalHitByCruiseMissileError. You don't get to say "OK, so in hindsight, now that I see the backdoor addressing it was pretty darn easy. Mop up the mess and I'm sure to win round two."

Sorry, I don't see what you're trying to argument here, since CPUs aren't Ruby interpreters with libraries. CPUs have much stronger limitations under which they have to operate. Remember the Pentium DIV bug? It was discovered.

I'm writing assembly code, I've spent time also to measure the time of the machine operations etc, so I simply can't imagine the valid argument which ignores exact limitations present in actual hardware.

People in this thread seem to be arguing past each other. Patrick is specifically disputing the claim (a few posts up) that you can always tell if the hardware is adding incorrectly. His argument is that if it's deliberately manufactured to add almost correctly, except in certain carefully tailored cicrumstances, the chance of that being detectable by a third party are pretty slim. The Ruby stuff is just a metaphor.

(Google for "Illinois Malicious Processors" for what an attacker might actually do. A few extra transistors here and there can lay you wide open.)

I have referred to

http://en.wikipedia.org/wiki/Pentium_FDIV_bug

I still don't see how adding "almost incorrectly" would have not be detected in practice. In practice you also have to implement "almost incorrectly" in a way that doesn't affect the performance of your CPU. We are talking about processors, not Ruby interpreters.

The Pentium FDIV bug was inadvertent and blatant, and was found because people were bound to notice the faulty output.

You can't compare that kind of error to a deliberately hidden flaw.

The possible flaws with Lenovo computers haven't, as far as I know, been found anyone other than the spooks. (https://news.ycombinator.com/item?id=6108980)

Those flaws aren't "adding" bugs but remote access "features." Intel has also such technology in the new BIOSes and we ordinary users unfortunately can do less and less about it every time some new technology comes.

Can you please break that hash on multiple lines or quote it? It breaks the layout of the comment page.

Your attack relies upon security by obfuscation, hoping that no-one will notice that the behaviour is incorrect (we obviously disagree about the chance that such an attack will be spotted but I think to some degrees that's irrelevant).

The RNG attack has correct behaviour under every statistical and blackbox analysis. The only way you can break it externally is if you have the key.

If you were designing a cryptosystem you'd pick the one where the security lies in the key and not in obfuscation. The same applies when designing an attack you want to keep secret.

None of this comment has anything to do with my comment. If you don't want to use RDRAND, compile a kernel that doesn't.

To avoid hobgoblins wearing Faraday cages without denying the reality of the privacy debate, open source hw can make them quieter. I think it's going to happen because of PC market forces (IBM/Lenovo, Dell) and prior examples (OpenSparc). The startup costs (eda, test equipment) are doable because there are enough people with specialized expertise that could contribute.

If you don't trust the hw or vhost, you're stuffed. Political upwards and technical downwards are the ways to make sure this doesn't happen.

Linux's /dev/urandom does not trust hardware RNGs any more than other entropy sources. All entropy sources are stirred into the same entropy pool.

Does it use multiple pools a-la Fortuna?

There's an input pool and 2 output pools but it's not like Fortuna. This is the most up to date analysis of the Linux entropy system I'm aware of http://eprint.iacr.org/2012/251.pdf

> Except if /dev/urandom is using a hardware based random number generator, then you have to trust that the hardware hasn't received some NSA alterations at some point during the design.

That is a valid concern that needs to be part of your risk assessment. "Do I want to protect my secrets from a well funded government agency?"[1]

But there are other risks that need to be thought about too. Some people seem to think that hardware RNGs are better than software. Often they're not, they're lousy.

HWrngs can have subtle failure rates which are hard to detect.

Once you've done all the de-skewing and other checks they can be quite low bandwidth.

I have a bunch of links to reading about HWRNGs here - (https://news.ycombinator.com/item?id=6060636)

And here's a really nice thread (https://news.ycombinator.com/item?id=1453299)

[1] Although if you want to defend yourself against a well funded secret government agency you need to worry about more than a weak RNG.

(notably 'cperciva)

I'm confused, what's the apostrophe eliding? ;-)

I quote HN usernames. It's a weird me thing that I've done ever since I saw a user named "the". If I did @cperciva that would make intuitive sense to everyone, but then it wouldn't work for people not on Twitter.

Huh, I never noticed that before. It's a useful notation which I might well adopt. Thanks!

I'm assuming Bobby Tables can't get a nick on HN, then?

(That is, nicks like 'nick, and "' or '1'='1' -- '" aren't allowed?).

> but then it wouldn't work for people not on Twitter.

I think the @user idiom is by now understood by anyone not relying exclusively on pigeons to send their missives.

In places outside Twitter, the @foo idiom addresses a comment to that user, but that's not always what I intend when I reference a user. I'm simply trying to prevent evaluation of a token as English text.

I understand where you are coming from, but I think that at this point the interpretation of @foo as an idiom for not addressing but merely mentioning a user has permeated most online cultures.

Those who really do not know (yet) will quickly understand from context.

All in all, I think @foo is less confusing than 'foo, and will spare you questions like in this thread.

The English language is evolving. We've created a vocative tense for English using the "@" symbol. "Et tu, @Brutus?"

I believe he speaks with a lisp.

(Please do not throw anything at me.)

It's a Lisp way of quoting (i.e. preventing from being evaluated) symbols :).

I think this is fundamentally wrong, in the way 'secure programs should do their own crypto' is wrong. Secure programs should be able to rely on the platform provider's API to provide a CSPRNG. If the platform doesn't, it's the platform's fault and let's all go yell at them. It seems odd to put this at the doorstep of app developers. Android app developers should reasonably expect that SecureRandom does what it says on the tin as should consumers of similar apis in iOS, .net, OS X, Windows, you name it. "I'm a Java app, but really, in fact, I happen to know underneath it all it's Linux thus I will read some random file descriptor". It's a Chewbacca defense, it makes no sense!

Would that every platform had done what Golang did and simply wrap /dev/random. In the meantime, you have to make a choice: do you trust your language designer's CSPRNG AND the OS CSPRNG on which it (and the rest of the system) doubtless depends, or instead trust only the OS CSPRNG?

I think iOS does just that see the function SecRandomCopyBytes in http://opensource.apple.com/source/Security/Security-55179.1... it just wraps /dev/random

Sure but down that path lies 'did Intel and the NSA backdoor my CPU' tinfoilhattery. If you're an app developer, rather than implementor of original crypto from dry sticks and a bit of flint, you have to trust something and the platform API is a sane place to start. Google, who have specialists on staff, seems to have got this wrong at least twice. It seems unreasonable to expect a person who's simply looking for a nonce to do better or to keep careful track of how to do better because the APIs at hand just don't do what they claim they do.

This comment appears to be animated by a belief that the NSA backdoors the Linux source code.

Wait, wat? I thought we were talking about 'What is the reasonable line that cryptography API users should assume is secure'. Which is something sensible people can disagree on.

I just happen to think you're wrong (and the NaCl paper you mention actually provides advice to crypto library writers, not users, for the question at hand). I'm not animated by anything any more than you're in the secret employ of the cartesian product of all possible three letters.

I'm just not following why you think the NSA makes /dev/random risky. /dev/random is not an interface to hardware random numbers; it's the logical interface to the OS's own CSPRNG, which uses multiple sources of entropy.

I don't think that at all, I'm sorry a dumb NSA joke confused the issue. I think app writers should be calling SecureRandom (or platform equivalents) and expect secure results. Just like app writers should use TLS, despite its faults. My argument is that reading '/dev/urandom' is akin to opening a raw socket and hand-crafting packet headers because you don't trust TLS.

I'll stop harping on the backdoor point. What kinds of things do you think developers can do wrong reading from urandom? There are a lot of things you can get wrong using raw sockets instead of TLS; a universe of them.

My real issue isn't that devs use SecureRandom. Use it by default; it's not like we'd doc a bug on you for doing that.

My real issue is that the library developers who expose SecureRandom don't themselves simply pull the random bytes from urandom.

I'll stop harping on the backdoor point.

I never even made such a point! At all! If anything, I'd like to know how I gave you that impression so I can avoid it in the future.

What kinds of things do you think developers can do wrong reading from urandom?

Well for one thing it's entirely unportable. I think cperciva pointed out some difference on how it behaves on FreeBSD vs Linux. How does it behave on OS X? On Windows? What are the failure modes?

My real issue is that the library developers who expose SecureRandom don't themselves simply pull the random bytes from urandom.

I'm in 74211004% agreement with you there, as is the NaCl paper. Centralize randomness, ensure that it's reviewable and done right. In fact, the NaCl paper suggests:

"A structural deficiency in the /dev/urandom API provided by Linux, BSD, etc. is that using it can fail, for example because the system has no available file descriptors. In this case NaCl waits and tries again. We recommend that operating systems add a reliable urandom(x,xlen) system call."

Again, my stipulation is that this is very much not the job of app developers. The Android bitcoin wallet developers seem to be a case in point - they don't seem to have done anything particularly wrong at all. The platform failed them. So should we be berating platform providers or should we insist app-writers keep close track of various security developments? I think it's very much the former.

Sorry, I was trying to concede that you weren't making that point, but did it clumsily.

So here is the interesting question as I see it:

What's safer for developers, reading from /dev/urandom, or calling the platform CSPRNG when the platform CSPRNG does something other than reading from /dev/urandom?

I think the answer might be /dev/urandom. A Debian maintainer is not going to accidentally comment the randomness out of the Linux kernel CSPRNG. Nobody is going to miss the logic bug that causes the Linux kernel random number generator to not even try to seed itself.

What's safer for developers, reading from /dev/urandom, or calling the platform CSPRNG when the platform CSPRNG does something other than reading from /dev/urandom?

I think you're right about this and the (rather fiddly and nerdy) point where we disagree is on where the line of abstraction lies. I don't think you have any business reading /dev/urandom any more than you have using, say, AES-NI

Nobody is going to miss the logic bug that causes the Linux kernel random number generator to not even try to seed itself.

Well, NetBSD more or less did that twice in a row earlier this year...

I'd be curious to know how he pronounces Google's NaCl. ;)

edit: for those unaware, the NACL 'tptacek is referring to is [0] - which long predates [1].

[0] http://nacl.cr.yp.to/

[1] https://developers.google.com/native-client/

I always thought it was weird hearing Google devs pronounce it like "na-cull" (like na in "na na na na na na" and "cull" as in killing something) instead of "salt".

What's the CS part of CSPRNG?

Cryptographically secure.

Thanks

cryptographically secure.

http://en.wikipedia.org/wiki/Cryptographically_secure_pseudo...

This comment and the fact that it is so highly rated shows just how easy it is to fall into the trap of "I'll just create my own better crypto!" /dev/urandom is insecure by design. It is a non-blocking version of /dev/random and works by first depleting /dev/random, and then falling back to a very low quality RNG once /dev/random is depleted.

/dev/random is the right way to provide hardware RNG in a UNIX environment. However, programs using it can be blocked for extremely long times if there is no dedicated hardware available for providing RNG. I have regularly seen blocking times in the multiple hour range. This is not practical for most programs.

Even relying on /dev/random isn't necessarily safe. I have seen setups where people connect /dev/random to /dev/urandom in an attempt to eliminate these long block times on /dev/random. A quick google search can bring up multiple examples of how to do this. I'll leave it to you to think about out why this is a bad idea, but the result is that you cannot even trust /dev/random.

Your comment more or less expands on an urban legend. The legend started because of the way the Linux man page for random(4) was written.

Thankfully, Thomas Pornin wrote a definitive debunking of the urban legend here:

http://security.stackexchange.com/questions/3936/is-a-rand-f...

In short: that's not how urandom works. It's not a "fallback to a crappy RNG". Random and urandom are two different interfaces to the same CSPRNG engine. One of them happens to block based on entropy estimates, but that's a design weakness of the Linux CSPRNG, not a great feature of a CSPRNG in general.

While considering how highly rated my comment was, and weighing your own expertise on cryptographic randomness, you might also consider reading the NaCL design paper (Bernstein, Lange, Schwabe) for more details on why you want to use the OS CSPRNG instead of your own. Or, for that matter, consult the NaCL source code to see which they use.

Finally, to your main point: the reason you should use the OS CSPRNG (typically: urandom) is precisely so that you don't end up using some random bit of aspirational app crypto. The OS has to get its CSPRNG right no matter what, so just let it do the work for you.

Host-based security is a whole dead horse discussion, but I still believe Def-in-Dpht means "use everything you can get your hands on; reveal the least as possible."

Correct me if I'm wrong: which OSes have desirable CSPRNG properties like Fortuna (entropy pools esp.). I've looked at OSX, Win8, Free/Net&Open BSD and still haven't found anything satisfying other than class of algo. (Ignore whether closed/open source and IP issues.) I would like to see all mainstream OSes do these well enough.

OS X does. The kernel CSPRNG in Linux is comparatively primitive.

I don't think you understand the problem.

The problem is that the PRNG has a weak default entropy source. The same problem existed in the kernel for ages. See http://www.factorable.net

The real advice here ought to be that if you are building an application that generates keys, you should make sure that the system has appropriate entropy before generating the keys. Don't assume the PRNG (whether it's /dev/urandom or OpenSSL) does it for you.

The problem with this comment is that /dev/random wasn't broken, and the OpenSSL CSPRNG used by Android Java was.

Further, it is harder to imagine a circumstance in which /dev/random could be broken and an app-layer CSPRNG like OpenSSL's could not be broken.

It's a little fuzzy in this case because part of the patch feeds entropy back to /dev/random, but observe (a) that the bulletin doesn't tell developers to re-key if they were depending on /dev/random (as some apps do), despite naming 5 other interfaces that do demand rekeying, and (b) that this particular patch would be an inadequate fix for a broken /dev/random anyways.

It's true that you care about entropy at cold start either way. But your choice of CSPRNGs doesn't resolve that problem for you, except that the OS needs to have secure random regardless and so you're boned whether or not you rely on /dev/random, and hence are better off just relying on /dev/random.

Do you mean /dev/random or /dev/urandom? They are different, AFAIK.

Random and urandom are interfaces to the same kernel CSPRNG. The difference is that reads from random block until an internal counter that estimates the entropy in the CSPRNG crosses a threshold, and reads from urandom don't do that.

In practice, this is a distinction without a meaningful difference. In more modern implementations (like OS X) the CSPRNG works the same way with both interfaces.

You should generally prefer urandom.

There is a wellspring of urban legands about the security difference between these two interfaces because the Linux man page is misleading.

And why /dev/urandom instead of /dev/random?

The entropy needed to generate random numbers is a finite resource. If you don't have enough entropy stored up, /dev/random may block until such a time as it has sufficient entropy.

/dev/urandom reuses existing entropy to produce more pseudo-random numbers. It may be less "random", but will never block.

more at http://en.wikipedia.org/wiki//dev/random

And that should be a very good reason for using random instead of urandom

I don't want less entropy thank you. If I don't have enough then let's wait until some more become available

I would use urandom if I need random but I don't care about their quality (even though urandom is good most of the time)

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