A simple solution is to use Window's Cryptography API.
include \masm32\include\Advapi32.inc
includelib \masm32\lib\Advapi32.lib
...
RandomBytes proc dwLength:DWORD,pBuffer:PVOID
LOCAL hProvider:HANDLE
invoke CryptAcquireContext,ADDR hProvider,0,0,PROV_RSA_FULL,CRYPT_VERIFYCONTEXT or CRYPT_SILENT
invoke CryptGenRandom,hProvider,dwLength,pBuffer
invoke CryptReleaseContext,hProvider, 0
ret
RandomBytes endp