Question

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?

Answer 1

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.