Random fork-safety

One of the most important issues in the proper cryptographic use of random numbers is that random numbers must not be reused. Since the UNIX fork() system call duplicates the entire process state, a random number generator which does not take this issue into account will produce the same sequence of random numbers in both the parent and the child (or in multiple children), leading to cryptographic disaster. (i. e. people being able to read your communications)

OpenSSL's default random number generator mixes in the PID, which provides a certain degree of fork safety. However, once the PIDs wrap, new children will start to produce the same random sequence as previous children which had the same PID. This is unlikely to happen in most common cases, but it is not impossible, which makes the issue even more insidious.

The most comprehensive explanation of this problem is probably this blog post:

• OpenSSL PRNG Is Not (Really) Fork-safe - martinboßlet - Aug 21, 2013

However, since this issue has been "rediscovered" and discussed multiple times, here are some additional links (some are also linked from the above article)

• openssl-dev: recycled pids causes PRNG to repeat - Eric Wong - Apr 15, 2011
• Ruby bug 4579: SecureRandom + OpenSSL may repeat with fork - Apr 15, 2011
• pgsql: Reset OpenSSL randomness state in each postmaster childprocess - Tom Lane - Mar 27, 2013
• Google+ discussion of Android SecureRandom issue, caused by OpenSSL PID wraparound - Aug 14, 2013
• openssl-users: DLL hell - Nico Williams - Aug 15, 2013
  • openssl-dev: not fork-safe if pids wrap (continuation of above openssl-users thread) - Aug 22, 2013
• openssl-dev: Patch to increase rng entropy after forking - Ernst-Udo Wallenborn - Sept 18, 2013
• openssl-dev: Mixing time into the pool - Ben Laurie - Sept 21, 2013
  • commit mentioned in above message