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The C Standard rand() function makes no guarantees as to the quality of the random sequence produced. The numbers generated by some implementations of rand() have a comparatively short cycle , and the numbers can be predictable. Applications that have strong pseudorandom number requirements must use a generator that is known to be sufficient for their needs.

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To achieve the best random numbers possible, an implementation-specific function must be used. When unpredictability is crucial and speed is not an issue, as in the creation of strong cryptographic keys, use a true entropy source, such as /dev/random, or a hardware device capable of generating random numbers. Note that the The /dev/random device can block for a long time if there are not enough events going on to generate sufficient entropy.

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On Windows platforms, the CryptGenRandom() function can be used to generate cryptographically strong random numbers. Note that the The exact details of the implementation are unknown, including, for example, what source of entropy CryptGenRandom() uses. From the The Microsoft Developer Network CryptGenRandom() reference [MSDN] :states

If an application has access to a good random source, it can fill the pbBuffer buffer with some random data before calling CryptGenRandom(). The CSP [cryptographic service provider] then uses this data to further randomize its internal seed. It is acceptable to omit the step of initializing the pbBuffer buffer before calling CryptGenRandom().

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