Equivalent of /dev/urandom on Windows?

Question

My application would like to get a random number, preferably with entropy if available, but does not need cryptographic quality, and would like to do ensure that the call does not block if the system entropy pool is depleted (e.g. on a server in a farm). I am aware of CryptGenRandom (http://msdn.microsoft.com/en-us/library/windows/desktop/aa379942(v=vs.85).aspx), but its behaviour with respect to blocking under adverse entropy conditions is not specified. On Unix, /dev/urandom supports this use case. Is there equivalent functionality available on Windows? I would prefer to avoid using a non-system RNG simply to get non-blocking semantics.

Answer 1

You could wait for one good seed full of entropy and follow GMasucci advice to pre-generate a long list of random numbers.

Unless your system is already compromised it seems that a good seed it's good enough to generate a series of non-related numbers as discussed in http://www.2uo.de/myths-about-urandom/

From the discussion I get that a continuous feed of ("true"/"fresh") random numbers it's only needed if your system state (your sources of entropy are known and the attacker knows their current state) it is compromised at some point. After feeding your block cypher more randomness, the predictability of its output will get lower.

Source of seeds? Two or more pieces of trusted software that are less likely to be already compromised. I try to blur out the predictability of the functions that use time functions as seed: local rand_function() + some variable delay + mysql's rand(). From there, a list of pseudo-random numbers generated by some good library.

Answer 2

For a toy application, you could use rand(), but the implementation on Windows is of notoriously poor quality.

A better bet is simply to include a suitable pseudo-random number generator in your program. The Mersenne Twister is a good choice IMO, particularly as there are plenty of available implementations (including in the C++11 standard library and in Boost).

Answer 3

If I need non-blocking behaviour on random numbers, I generally pre-generate n numbers and store them in an in memory variable: ie if I know I will need 30 random numbers per second, takes 3 seconds to compute them (including blocks), then I will pre-generate 300 while the main code is loading, store them in an array or vector and use them at need; whilst using them I generate another one on a separate thread every time I use one up, replacing the utilised random number with the newly generated one and moving on to the next one in the list, that way when I hit the limit (in this case 300) I know when I can simply start again at the start of my array/vector/list and all the random numbers are fresh and will be non-blocking (as they are pre-generated).

This means you can use any random number generator you like and not worry about blocking behaviour, however it has the expense of utilising more ram, negligible however for the sort of coding I need random numbers for.

Hope this helps, as I couldn't fit this all into a comment:)