5 ms·

> "to be blocked until enough entropy is available" which is fine. The idea that entropy is a consumeable resource is a crypto myth that needs to die.

tlb 8y ago
It's not consumable, but it can be in short supply right after boot. So /dev/random should block until there's sufficient entropy, but never after that.
- cperciva 8y ago
  The problem with "block until you have enough entropy" is that the amount of entropy you have is literally unknowable.
  - tux1968 8y ago
    How can that be true? The kernel at least, knows how much you have and knows to block consumption until it crosses the desired threshold.
    - cperciva 8y ago
      The kernel knows how many bytes you've fed into it. But it has no fundamentally sound way of computing a lower bound on how much entropy those bytes contained; the best approaches involve calculating an upper bound and then say "well hopefully it's at least X% of this".
- cbhl 8y ago
  If I recall correctly, at one point, the network interface was used as a source of entropy. Then someone demonstrated that sending the right sequence of network packets to a machine would let you control the key that got generated. So they removed it. Then folks discovered -- in production -- that some cloud computing environments just don't get any other new entropy after boot, and so instances would hang on generating SSH host keys. Some folks went to /dev/urandom. Other folks decided to seed instances with entropy from another computer (with fancy names like "cloud entropy service"). And then someone had to decide how that machine gets entropy (like plugging in an FM radio into the mic jack).
  - justinclift 8y ago
    > some cloud computing environments just don't get any other new entropy after boot For environments like that, I think Haveged is the general approach these days. Latest dev code (revived project) is now here: https://github.com/jirka-h/haveged https://github.com/jirka-h/haveged It's the (officially blessed, I think) continuation from the original Haveged: http://www.issihosts.com/haveged/ http://www.issihosts.com/haveged/
- wbl 8y ago
  RDRAND is a thing
  - clarry 8y ago
    What's the alternative on ARM?
- deleted 8y ago
  [deleted]

tlb 8y ago

It's not consumable, but it can be in short supply right after boot. So /dev/random should block until there's sufficient entropy, but never after that.

cperciva 8y ago
The problem with "block until you have enough entropy" is that the amount of entropy you have is literally unknowable.
- tux1968 8y ago
  How can that be true? The kernel at least, knows how much you have and knows to block consumption until it crosses the desired threshold.
  - cperciva 8y ago
    The kernel knows how many bytes you've fed into it. But it has no fundamentally sound way of computing a lower bound on how much entropy those bytes contained; the best approaches involve calculating an upper bound and then say "well hopefully it's at least X% of this".
cbhl 8y ago
If I recall correctly, at one point, the network interface was used as a source of entropy. Then someone demonstrated that sending the right sequence of network packets to a machine would let you control the key that got generated. So they removed it. Then folks discovered -- in production -- that some cloud computing environments just don't get any other new entropy after boot, and so instances would hang on generating SSH host keys. Some folks went to /dev/urandom. Other folks decided to seed instances with entropy from another computer (with fancy names like "cloud entropy service"). And then someone had to decide how that machine gets entropy (like plugging in an FM radio into the mic jack).
- justinclift 8y ago
  > some cloud computing environments just don't get any other new entropy after boot For environments like that, I think Haveged is the general approach these days. Latest dev code (revived project) is now here: https://github.com/jirka-h/haveged https://github.com/jirka-h/haveged It's the (officially blessed, I think) continuation from the original Haveged: http://www.issihosts.com/haveged/ http://www.issihosts.com/haveged/
wbl 8y ago
RDRAND is a thing
- clarry 8y ago
  What's the alternative on ARM?
deleted 8y ago
[deleted]