Here's my ldpreload hack: rerouting /dev/rand to dev/urand -- because I disagree with gpg's fears on entropy. Now it's as fast as generating a private key with ssh-keygen or openssl:
It is incorrect. Both /dev/urandom and /dev/random are connected to a CSPRNG. Once a CSPRNG is initialized by SUFFICIENT unpredictable inputs, it's forever unpredictable for (practically) unlimited output (something 2^128). If the CSPRNG algorithm is cryptographically-secure, and the implementation doesn't leak its internal state, it would be safe to use it for almost all cryptographic purposes.
However, the original design in the Linux kernel was paranoid enough, it blocks /dev/random (even if a CSPRNG can output unlimited random bytes) if the kernel thinks the the output has exceeded the estimated uncertainty from all the random events. Most cryptographers believe if a broken CSPRNG is something you need to protect yourself from, you already have a bigger trouble, and it's unnecessary from a cryptographic point-of-view to be paranoid about a properly-initialized CSPRNG. /dev/random found on other BSDs is (almost) equivalent to Linux's /dev/urandom.
However, /dev/urandom has its own issues on Linux. Unlike BSD's implementation, it doesn't block even if the CSPRNG is NOT initialized during early boot. If you automatically generate a key for, e.g. SSH, at this point, you'll have serious troubles - predictable keys, so reading from /dev/random still has a point, although not for 90% of the programs. I think it's a prefect example of being overly-paranoid about unlikely dangers, while overlooking straightforward problems that are likely to occur.
The current recommended practice is to call getrandom() system call (and arc4random()* on BSDs) when it's available, instead of reading from raw /dev/random or /dev/urandom. It blocks, until the CSPRNG is initialized, otherwise it always outputs something.
*and no, it's not RC4-based, but ChaCha20-based on new systems.
This isn't quite true. The BSDs random (and urandom) block until initially seeded, unlike Linux's urandom. Then they don't block. (Like the getrandom/getentropy behavior.)
> The current recommended practice is to call getrandom() system call (and arc4random() on BSDs) when it's available, instead of reading from raw /dev/random or /dev/urandom. It blocks when the CSPRNG is initialized, but otherwise it always outputs something.
+1 (I'd phrase that as "blocks until the CSPRNG is initialized," which for non-embedded systems will always be before userland programs can even run, and for embedded should not take long after system start either).
Not just that, but if you have a threat model where you actually need information theoretic security (e.g. you're conjecturing a computationally unbounded attacker or at least a quantum computer)-- the /dev/random output is _still_ just a CSPRNG and simply rate limiting it doesn't actually make a strong guarantee about the information theoretic randomness of the output. To provide information theoretic security the function design would need to guarantee that at least some known fraction of the entropy going in actually made it to the output. Common CSPRNGs don't do this.
So you could debate if information theoretic security is something someone actually ever actually needs-- but if you do need it, /dev/random doesn't give it to you regardless.
[And as you note, urandom doesn't block when not adequately seeded ... so the decision to make /dev/random block probably actually exposed a lot of parties to exploit and probably doesn't provide strong protection even against fantasy land attacks :(]
This is an interesting point I hadn't thought about before, so thanks for that. I suppose if you're generating a OTP or something like that, there might be some small advantage to using /dev/random, but the probability of it making a difference is pretty remote.
The one thing I haven't been able to figure out is why Linux hasn't "fixed" both /dev/random and /dev/urandom to block until they have sufficient entropy at boot and then never block again. That seems like obviously the optimal behavior.
The Real Solution™ is to make /dev/random and /dev/urandom the same thing, and make them both block until properly seeded. And replace the current ad-hoc CSPRNG with a decent one, e.g. Fortuna. There were patches almost 15 years ago implementing this (https://lwn.net/Articles/103653/), but they were rejected.
There's simply no good reason not to fix Linux's CSPRNG.
Unless all of your cryptography is information-theoretically secure, there is no problem using a PRNG.
If you happen to be using a an information-theoretically secure algorithm than you are theoretically weaker using a limited entropy PRNG; but there is no practical implications of this.
https://github.com/dsaul/UDELibXprop-Legacy
I was writing a dock program over a decade ago, and java programs didn't put the PID on the window, whereas everything else did.
Had to fix it somehow...
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.
And only Linux has /dev/urandom.
(Note, that's from 2014; today I would recommend getrandom() instead.)
The one and only danger is during the machine's boot process, because while /dev/random and /dev/urandom use the same data:
* on linux /dev/random has a silly and unfounded entropy estimator and will block at arbitrary points (used to be a fad at some point, but cryptographers have sworn off it e.g. Yarrow had an entropy estimator but Fortuna dropped it)
* also on linux, /dev/urandom never blocks at all, which includes a cold start, which can be problematic as that's the one point where the device might not be seeded and return extremely poor data
In fact the second point is the sole difference between getrandom(2) and /dev/urandom.
If you're in a steady state scenario (not at the machine boot where the cold start entropy problem exists) "just use urandom" is the recommendation of pretty much everyone: tptacek, djb, etc…
https://www.2uo.de/myths-about-urandom/
https://sockpuppet.org/blog/2014/02/25/safely-generate-rando...
http://blog.cr.yp.to/20140205-entropy.html (see bottom of page)
AFAIK, there's another important difference: getrandom(2) doesn't use a file descriptor (so it'll work even if you're out of file descriptors, or in other situations where having an open fd is inconvenient), and it doesn't need access to a /dev directory with the urandom device.
So, recompile every program and every subsequent update to use this functionality or...bind mount. One of these sounds easier than the other.