4 ms·

Seems like they store entropy in an on-disk file[1], and re-read that file on startup. This feature is available in Linux as well (see urandom(4) for an example

Seems like they store entropy in an on-disk file[1], and re-read that file on startup. This feature is available in Linux as well (see urandom(4) for an example of how to do it manually, though I think most distros bundle this in some manner); although apparently, according to the OP, it doesn't fully work? Seems like the man page sort of notes the problem in the article: > Writing to /dev/random or /dev/urandom will update the entropy pool with the data written, but this will not result in a higher entropy count. But, why not, man page, why not? I feel like if I, as root, write to /dev/random, I am saying that this is acceptable input for seeding the generator, and that it is on me to not, say, seed it with the exact same stuff every boot, or seed it with straight nuls, no? How does OpenBSD guarantee that this file exists, though? Perhaps making it a system default is wise, but what about brand new VMs/installs? Does the install just take the time to do the first generation of that file? The OpenBSD folks also appear to trust RDRANG, which would help not blocking. But I disagree with that trust decision. [1]: start reading here: https://www.openbsd.org/papers/hackfest2014-arc4random/mgp00019.html https://www.openbsd.org/papers/hackfest2014-arc4random/mgp00...

notaplumber 7y ago
> The OpenBSD folks also appear to trust RDRANG, which would help not blocking. But I disagree with that trust decision. It feeds the entropy pool, mixed in with many sources. OpenBSD also feeds in data from the AMD CCP as well. The point is it doesn't matter. https://man.openbsd.org/ccp https://man.openbsd.org/ccp And anyone running "echo badapples > /dev/random" in a loop, it still doesn't matter. https://man.openbsd.org/random.4 https://man.openbsd.org/random.4 https://man.openbsd.org/arc4random.9 https://man.openbsd.org/arc4random.9 > How does OpenBSD guarantee that this file exists, though? If you keep reading, OpenBSD's random subsystem always mixes old with new data from many sources. If the random.seed file is not available (for example, boot media), it is not fatal. There is a warning displayed. But the system rc(8) scripts handle creating the seed file at both boot and shutdown. https://www.openbsd.org/papers/hackfest2014-arc4random/mgp00019.html https://www.openbsd.org/papers/hackfest2014-arc4random/mgp00...
- deathanatos 7y ago
  > If you keep reading, OpenBSD's random subsystem always mixes old with new data from many sources. Sure. But say we don't trust RDRAND. It's the first boot, so you haven't got a seed file yet. Assuming you haven't got a HW generator at hand, what other source are there that don't take significant time to collect from? (I.e., I know you can get it from network timings, keyboard/mouse if they exist, etc., but that takes time, and would cause a call not wanting to return prior to there being sufficient data to block. If you're only mixing in unready or untrusted sources, you might get lucky and make it hard for an attacker, or worse, if your other sources are blocking b/c they're collecting data… you might not, but regardless it hardly seems principled?) It'd be willing to write off the first boot, except that I feel like a lot of things do potentially get initialized then; e.g., in a VM in the cloud, I think that's when host keys are initialized, for example, and certainly whatever application that VM might be hosting could have further requirements. Not blocking and issuing a warning would be to supply data prior to having sufficiently initialized a generator, no?
  - ori_b 7y ago
    > Sure. But say we don't trust RDRAND. It's the first boot, so you haven't got a seed file yet. Why not? The install media should be collecting entropy as it does the install, xoring it with a pool of random data built into it, and writing that to disk as part of the install. Take a look at https://github.com/openbsd/src/blob/master/distrib/miniroot/install.sub https://github.com/openbsd/src/blob/master/distrib/miniroot/..., specifically, feed_random and store_random
    - deathanatos 7y ago
      Okay, that's fair for install media installing to a physical machine such as a desktop. What about virtual machines / machines booting off an image? We can't put a seed in the image, or it'll get distributed to all downstream consumers. Also, there's the case posed in response to my first comment, about IOT devices which would ship with some factory-installed image. (I suppose there are some novel ways one could work around this such as somehow keeping a pool of one-shot images ready w/ just the seed added to them, but I don't feel like this is how real-world systems work. E.g., an AMI in AWS?)
earenndil 7y ago
I believe they xor the output of rdrand with that of other randomness sources; meaning, they don't have to trust that its output is actually random.
tytso 7y ago
/dev/random and /dev/urandom are world-writeable by design. This allows anyone to contribute to randomness via "cat ~/my_secrets > /dev/random". Of course, my_secrets might not be very secret, or controlled by the attacker. So there is no credit given to that. There is a way for a root process, like systemd, to contribute trusted entropy to the entropy pool, using an ioctl. Of course, then it is systemd which will be taking on the responsibility if it turns out that /var/lib/random_seed turns out to be replicated onto a zillion static images on some IOT device, so it turns out /var/lib/random_seed wasn't actually all that random to begin with
justincormack 7y ago
There is an ioctl to write with an ascribed entropy from Linux.
ori_b 7y ago
> The OpenBSD folks also appear to trust RDRANG, which would help not blocking. But I disagree with that trust decision. Mixing more data into a random source cannot make it worse. OpenBSD does not use rdrand alone.
- nitrogen 7y ago
  This is unlikely, but wouldn't it be conceivable that a malicious hardware "RNG" that lives in the CPU and has access to the internal state of the OS RNG (and knows common OS algorithms) could always supply the worst possible value to counteract whatever is in the pool?
  - ori_b 7y ago
    Yeah, but you don't need to use rdrand in that scenario -- the cpu could also just tamper with the pool at will, regardless of the RDRAND instruction.
    - lifthrasiir 7y ago
      djb has noted that RDRAND is a particularly attractable means to exfiltration back in 2014 [1]. So it is not simply irrelevant. [1] https://blog.cr.yp.to/20140205-entropy.html https://blog.cr.yp.to/20140205-entropy.html

notaplumber 7y ago

> The OpenBSD folks also appear to trust RDRANG, which would help not blocking. But I disagree with that trust decision. It feeds the entropy pool, mixed in with many sources. OpenBSD also feeds in data from the AMD CCP as well. The point is it doesn't matter. https://man.openbsd.org/ccp https://man.openbsd.org/ccp And anyone running "echo badapples > /dev/random" in a loop, it still doesn't matter. https://man.openbsd.org/random.4 https://man.openbsd.org/random.4 https://man.openbsd.org/arc4random.9 https://man.openbsd.org/arc4random.9 > How does OpenBSD guarantee that this file exists, though? If you keep reading, OpenBSD's random subsystem always mixes old with new data from many sources. If the random.seed file is not available (for example, boot media), it is not fatal. There is a warning displayed. But the system rc(8) scripts handle creating the seed file at both boot and shutdown. https://www.openbsd.org/papers/hackfest2014-arc4random/mgp00019.html https://www.openbsd.org/papers/hackfest2014-arc4random/mgp00...

deathanatos 7y ago
> If you keep reading, OpenBSD's random subsystem always mixes old with new data from many sources. Sure. But say we don't trust RDRAND. It's the first boot, so you haven't got a seed file yet. Assuming you haven't got a HW generator at hand, what other source are there that don't take significant time to collect from? (I.e., I know you can get it from network timings, keyboard/mouse if they exist, etc., but that takes time, and would cause a call not wanting to return prior to there being sufficient data to block. If you're only mixing in unready or untrusted sources, you might get lucky and make it hard for an attacker, or worse, if your other sources are blocking b/c they're collecting data… you might not, but regardless it hardly seems principled?) It'd be willing to write off the first boot, except that I feel like a lot of things do potentially get initialized then; e.g., in a VM in the cloud, I think that's when host keys are initialized, for example, and certainly whatever application that VM might be hosting could have further requirements. Not blocking and issuing a warning would be to supply data prior to having sufficiently initialized a generator, no?
- ori_b 7y ago
  > Sure. But say we don't trust RDRAND. It's the first boot, so you haven't got a seed file yet. Why not? The install media should be collecting entropy as it does the install, xoring it with a pool of random data built into it, and writing that to disk as part of the install. Take a look at https://github.com/openbsd/src/blob/master/distrib/miniroot/install.sub https://github.com/openbsd/src/blob/master/distrib/miniroot/..., specifically, feed_random and store_random
  - deathanatos 7y ago
    Okay, that's fair for install media installing to a physical machine such as a desktop. What about virtual machines / machines booting off an image? We can't put a seed in the image, or it'll get distributed to all downstream consumers. Also, there's the case posed in response to my first comment, about IOT devices which would ship with some factory-installed image. (I suppose there are some novel ways one could work around this such as somehow keeping a pool of one-shot images ready w/ just the seed added to them, but I don't feel like this is how real-world systems work. E.g., an AMI in AWS?)

earenndil 7y ago

I believe they xor the output of rdrand with that of other randomness sources; meaning, they don't have to trust that its output is actually random.

tytso 7y ago

/dev/random and /dev/urandom are world-writeable by design. This allows anyone to contribute to randomness via "cat ~/my_secrets > /dev/random". Of course, my_secrets might not be very secret, or controlled by the attacker. So there is no credit given to that. There is a way for a root process, like systemd, to contribute trusted entropy to the entropy pool, using an ioctl. Of course, then it is systemd which will be taking on the responsibility if it turns out that /var/lib/random_seed turns out to be replicated onto a zillion static images on some IOT device, so it turns out /var/lib/random_seed wasn't actually all that random to begin with

justincormack 7y ago

There is an ioctl to write with an ascribed entropy from Linux.

ori_b 7y ago

> The OpenBSD folks also appear to trust RDRANG, which would help not blocking. But I disagree with that trust decision. Mixing more data into a random source cannot make it worse. OpenBSD does not use rdrand alone.

nitrogen 7y ago
This is unlikely, but wouldn't it be conceivable that a malicious hardware "RNG" that lives in the CPU and has access to the internal state of the OS RNG (and knows common OS algorithms) could always supply the worst possible value to counteract whatever is in the pool?
- ori_b 7y ago
  Yeah, but you don't need to use rdrand in that scenario -- the cpu could also just tamper with the pool at will, regardless of the RDRAND instruction.
  - lifthrasiir 7y ago
    djb has noted that RDRAND is a particularly attractable means to exfiltration back in 2014 [1]. So it is not simply irrelevant. [1] https://blog.cr.yp.to/20140205-entropy.html https://blog.cr.yp.to/20140205-entropy.html