2uo.de

> this slightly time-saving but potentially dangerous choice?

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-random-numbers/

http://blog.cr.yp.to/20140205-entropy.html (see bottom of page)

How does a blocking CSPRNG mitigate this attack?

Again, it makes no sense to say that a CSPRNG can start "running low" on entropy.

Here's what djb has to say about this:

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.

(Unless you're talking about the early boot seeding problem that /dev/urandom has on linux, which is a very real problem).

For reference, here's the classic source I think you're referring to: https://www.2uo.de/myths-about-urandom

> Here's another article you can argue with:
> http://www.2uo.de/myths-about-urandom/

Which backs up my point that using /dev/urandom blindly without knowing about some of the edge cases isn't a good idea, and lists those edge cases and what to do about them (which is what I suggested you do too).

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

Which also lists the edge cases I'm talking about.

Hey look if you just want to say "I'm aware of the edge cases and don't want to put them in my gist for others to see" that's fine with me, but dodging the issue by claiming there are no edge cases (and then listing 3 articles which all mention the edge cases) isn't the right reply I think.

Don't be surprised if someone suggests that a gist listing security best practices list some edge cases that go along with the blind advice too. Feel free to disagree, but at least disagree honestly.

Random numbers are used to generate your TLS certificate key or your SSH key. If one can predict how a TLS or SSH key was generated they could impersonate the key holder.

Here is an explanation of what was possible when a Debian packager mistakenly introduced a patch which reduced the SSL certificate keyspace in 2008: https://jblevins.org/log/ssh-vulnkey

The possibilities of keys which were generated by this random number generator was so small, that a brute-force attack on keys was feasible.

That being said, for years, random number generators have been using random signals coming to your computer (key strokes, network packets, ...) and feeding them into a sponge function. You don't need lava lamps or pendulums to generate random numbers, it's just for the press.

https://www.2uo.de/myths-about-urandom/

From https://github.com/smuellerDD/jitterentropy-rngd

> Using the Jitter RNG core, the rngd provides an entropy source that feeds into the Linux /dev/random device if its entropy runs low. It updates the /dev/random entropy estimator such that the newly provided entropy unblocks /dev/random.

This is a red flag. Entropy doesn't run low. https://www.2uo.de/myths-about-urandom

I'm calling it now: Don't use VeraCrypt.

They made a very questionable decision based on the sort of ignorance that leads people to use /dev/random and haveged rather than RtlGenRandom (Windows), getrandom(2) (new Linux), or /dev/urandom (old Linux).

Cryptography engineering requires care and this sort of ignorance tends to undermine secure implementations.

/dev/random is a PRNG, and predictable. You shouldn't use it for security applications but only for specific state/algorithm randomness.
/dev/urandom requires hardware noise/key events, etc. to generate its entropy. These become hard to find when your dealing with purely virtualized installations.

This is not true, at least on Linux. /dev/random is actually closer to what you describe as /dev/urandom. It is a cryptographically strong randomness source that blocks depending on the state of its internal "entropy pool". /dev/urandom is also a cryptographically strong randomness source (seeded from /dev/random), but it does not block.

Here's a great blog post with more info: http://www.2uo.de/myths-about-urandom/