[tor-bugs] #13696 [Tor]: Use syscall-based entropy reading where possible.

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#13696: Use syscall-based entropy reading where possible.
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 Reporter:  nickm        |          Owner:  yawning
     Type:  enhancement  |         Status:  needs_revision
 Priority:  Medium       |      Milestone:  Tor: 0.2.???
Component:  Tor          |        Version:
 Severity:  Normal       |     Resolution:
 Keywords:  tor-relay    |  Actual Points:
Parent ID:               |         Points:
  Sponsor:               |
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Comment (by yawning):

 Replying to [comment:11 teor]:
 > I can't see the point of calling a system call that wraps /dev/random.
 We'd just be reducing performance, and exposing ourselves to any security
 issues in the wrappers.

 Disagree, at least for Linux's getrandom(), which does happen to pull it's
 entropy from /dev/urandom, in that behavior is a lot more clear and well
 defined (Eg: The syscall will block if the device isn't sufficiently
 seeded, requests under a certain size will not be interrupted by signals
 or return a short read).

 OpenBSD's getentropy() is a better getrandom() in that there's less
 complexity and error handling required, though I'm sure the OpenBSD people
 will raise eyebrows at us for not using arc4random().

 If the OSX call happens to provide similar advantages over reading from
 /dev/urandom or similar, it should be used for those reasons.

 Ultimately this is just fairly arbitrary tinfoil hattery since OpenSSL
 will seed itself from the character device on all the platforms just
 mentioned, and our explicit strong random calls fall back on failure
 (#17687).

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/13696#comment:12>
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