From my reading of the standard, random_device::entropy() should return 0.0 if a software engine is used. However, in VS2010 it returns 32.0, which in my understanding requires hardware to produce non-deterministic random numbers.
How does VS2010's random_device generate the number sequence?
According to a comment left on this question by Hans Passant, random_device uses advapi32:SystemFunction036, which according to MSDN is an alias for RtlGenRandom. This is verified by the runtime library source provided with VC++ 2010:
random_device::operator()() in <random> calls the following chain of functions:
_Random_device() // in xrngdev.cpp
rand_s() // in rand_s.c
RtlGenRandom()/SystemFunction036() // in advapi32.dll
According to a comment left by Michael Howard on one of his blog articles, "Cryptographically Secure Random number on Windows without using CryptoAPI", RtlGenRandom uses the following:
The RNG generates as specified in FIPS 186-2 appendix 3.1 with SHA-1 as the G function. With entropy from:
The current process ID (GetCurrentProcessID).
The current thread ID (GetCurrentThreadID).
The ticks since boot (GetTickCount).
The current time (GetLocalTime).
Various high-precision performance counters (QueryPerformanceCounter).
An MD4 hash of the user's environment block, which includes username, computer name, and search path. MD4 is a hashing algorithm that creates a 128-bit message digest from input data to verify data integrity.
High-precision internal CPU counters, such as RDTSC, RDMSR, RDPMC
Low-level system information: Idle Process Time, Io Read Transfer Count, I/O Write Transfer Count, I/O Other Transfer Count, I/O Read Operation Count, I/O Write Operation Count, I/O Other Operation Count, Available Pages, Committed Pages, Commit Limit, Peak Commitment, Page Fault Count, Copy On Write Count, Transition Count, Cache Transition Count, Demand Zero Count, Page Read Count, Page Read I/O Count, Cache Read Count, Cache I/O Count, Dirty Pages Write Count, Dirty Write I/O Count, Mapped Pages Write Count, Mapped Write I/O Count, Paged Pool Pages, Non Paged Pool Pages, Paged Pool Allocated space, Paged Pool Free space, Non Paged Pool Allocated space, Non Paged Pool Free space, Free System page table entry, Resident System Code Page, Total System Driver Pages, Total System Code Pages, Non Paged Pool Lookaside Hits, Paged Pool Lookaside Hits, Available Paged Pool Pages, Resident System Cache Page, Resident Paged Pool Page, Resident System Driver Page, Cache manager Fast Read with No Wait, Cache manager Fast Read with Wait, Cache manager Fast Read Resource Missed, Cache manager Fast Read Not Possible, Cache manager Fast Memory Descriptor List Read with No Wait, Cache manager Fast Memory Descriptor List Read with Wait, Cache manager Fast Memory Descriptor List Read Resource Missed, Cache manager Fast Memory Descriptor List Read Not Possible, Cache manager Map Data with No Wait, Cache manager Map Data with Wait, Cache manager Map Data with No Wait Miss, Cache manager Map Data Wait Miss, Cache manager Pin-Mapped Data Count, Cache manager Pin-Read with No Wait, Cache manager Pin Read with Wait, Cache manager Pin-Read with No Wait Miss, Cache manager Pin-Read Wait Miss, Cache manager Copy-Read with No Wait, Cache manager Copy-Read with Wait, Cache manager Copy-Read with No Wait Miss, Cache manager Copy-Read with Wait Miss, Cache manager Memory Descriptor List Read with No Wait, Cache manager Memory Descriptor List Read with Wait, Cache manager Memory Descriptor List Read with No Wait Miss, Cache manager Memory Descriptor List Read with Wait Miss, Cache manager Read Ahead IOs, Cache manager Lazy-Write IOs, Cache manager Lazy-Write Pages, Cache manager Data Flushes, Cache manager Data Pages, Context Switches, First Level Translation buffer Fills, Second Level Translation buffer Fills, and System Calls.
System exception information consisting of Alignment Fix up Count, Exception Dispatch Count, Floating Emulation Count, and Byte Word Emulation Count.
System lookaside information consisting of Current Depth, Maximum Depth, Total Allocates, Allocate Misses, Total Frees, Free Misses, Type, Tag, and Size.
System interrupt information consisting of context switches, deferred procedure call count, deferred procedure call rate, time increment, deferred procedure call bypass count, and asynchronous procedure call bypass count.
System process information consisting of Next Entry Offset, Number Of Threads, Create Time, User Time, Kernel Time, Image Name, Base Priority, Unique Process ID, Inherited from Unique Process ID, Handle Count, Session ID, Page Directory Base, Peak Virtual Size, Virtual Size, Page Fault Count, Peak Working Set Size, Working Set Size, Quota Peak Paged Pool Usage, Quota Paged Pool Usage, Quota Peak Non Paged Pool Usage, Quota Non Paged Pool Usage, Page file Usage, Peak Page file Usage, Private Page Count, Read Operation Count, Write Operation Count, Other Operation Count, Read Transfer Count, Write Transfer Count, and Other Transfer Count.
There's a full explanation (including diagrams) in Chapter 8 of Writing Secure Code, 2nd Edition.
