[tor-bugs] #17697 [Tor]: Add crypto_rand unit tests to check for predictable values

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#17697: Add crypto_rand unit tests to check for predictable values
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     Reporter:  teor         |      Owner:
         Type:  enhancement  |     Status:  new
     Priority:  Medium       |  Milestone:  Tor: 0.2.8.x-final
    Component:  Tor          |    Version:
     Severity:  Normal       |   Keywords:
Actual Points:               |  Parent ID:
       Points:               |    Sponsor:
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 Add unit tests that check for common RNG failure modes, such as
 returning all zeroes, identical values, or incrementing values
 (OpenSSL's rand_predictable feature).

 See my branch rand-failure-modes, it's based on bug17686_v2_027 in #17686.
 https://github.com/teor2345/tor.git

 As well as doing these checks during unit tests, we could also do these
 checks every time we reseed OpenSSL's PRNG. (With an appropriately large
 buffer so that the chance of failure during normal operation is
 astronomically improbable, or at least less probable than a bit-flip in
 the return value.)

 In particular, OpenSSL's rand_predictable feature concerns me - we
 currently don't check if Tor is run against an OpenSSL with it on.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/17697>
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