Problem - Kevin Tafuro

11.4 Using the Standard Windows Randomness
Infrastructure
Problem
You want to use random numbers on a Windows system.
Solution
Use CryptGenRandom( ) unless you absolutely need entropy, in which case see Recipe
11.8 and Recipes 11.20 through 11.23.
Discussion
Microsoft allows you to get cryptographically strong pseudo-random numbers using
the CryptoAPI function CryptGenRandom( ). Unfortunately, there is no provision for
any way to get entropy. The system does collect entropy behind the scenes, which it
uses to improve the quality of the cryptographically strong pseudo-random numbers
it gets.
Therefore, if this interface is being used to bind to the API we describe in Recipe
11.2, we can only implement spc_rand( ) and spc_keygen( ), both of which will be
exactly the same. If you want to try to get actual entropy on Windows, the only
solution as of this writing is to use EGADS, which we discuss in Recipe 11.8. Alternatively,
you can collect it yourself, as discussed in Recipes 11.20 through 11.23.
To use CryptGenRand( ), you must first acquire an HCRYPTPROV context. To do this, use
the function CryptAcquireContext( ), which we discuss in some detail in Recipe 5.25.
With an HCRYPTPROV context in hand, you can call CryptGenRandom( ), which will
return TRUE if it is successful; otherwise, it will return FALSE, but it should never fail.
CryptGenRandom( ) has the following signature:
BOOL CryptGenRandom(HCRYPTPROV *hProv, DWORD dwLen, BYTE *pbBuffer);
This function has the following arguments:
hProv
Handle to a cryptographic service provider obtained via CryptAcquireContext( ).
dwLen
Number of bytes of random data required. The output buffer must be at least
this large.
pbBuffer
Buffer into which the random data will be written.
580 | Chapter 11: Random Numbers
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