From f544094d5e1ee627ebd34542aead7bf1241fae19 Mon Sep 17 00:00:00 2001
From: Pieter Wuille <pieter.wuille@gmail.com>
Date: Tue, 2 May 2017 18:21:33 -0700
Subject: [PATCH 1/3] Use hardware timestamps in RNG seeding

---
 src/random.cpp | 22 +++++++++++++++-------
 1 file changed, 15 insertions(+), 7 deletions(-)

diff --git a/src/random.cpp b/src/random.cpp
index 6187f1629..40879e873 100644
--- a/src/random.cpp
+++ b/src/random.cpp
@@ -16,6 +16,7 @@
 
 #include <stdlib.h>
 #include <limits>
+#include <chrono>
 
 #ifndef WIN32
 #include <sys/time.h>
@@ -43,15 +44,22 @@ static void RandFailure()
 
 static inline int64_t GetPerformanceCounter()
 {
-    int64_t nCounter = 0;
-#ifdef WIN32
-    QueryPerformanceCounter((LARGE_INTEGER*)&nCounter);
+    // Read the hardware time stamp counter when available.
+    // See https://en.wikipedia.org/wiki/Time_Stamp_Counter for more information.
+#if defined(_MSC_VER) && (defined(_M_IX86) || defined(_M_X64))
+    return __rdtsc();
+#elif !defined(_MSC_VER) && defined(__i386__)
+    uint64_t r = 0;
+    __asm__ volatile ("rdtsc" : "=A"(r)); // Constrain the r variable to the eax:edx pair.
+    return r;
+#elif !defined(_MSC_VER) && (defined(__x86_64__) || defined(__amd64__))
+    uint64_t r1 = 0, r2 = 0;
+    __asm__ volatile ("rdtsc" : "=a"(r1), "=d"(r2)); // Constrain r1 to rax and r2 to rdx.
+    return (r2 << 32) | r1;
 #else
-    timeval t;
-    gettimeofday(&t, NULL);
-    nCounter = (int64_t)(t.tv_sec * 1000000 + t.tv_usec);
+    // Fall back to using C++11 clock (usually microsecond or nanosecond precision)
+    return std::chrono::high_resolution_clock::now().time_since_epoch().count();
 #endif
-    return nCounter;
 }
 
 void RandAddSeed()

From 33f853d8d88917b145e2793bf8eb1b4e3790e7e7 Mon Sep 17 00:00:00 2001
From: Pieter Wuille <pieter.wuille@gmail.com>
Date: Fri, 5 May 2017 11:32:06 -0700
Subject: [PATCH 2/3] Test that GetPerformanceCounter() increments

---
 src/random.cpp | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/src/random.cpp b/src/random.cpp
index 40879e873..8107cb310 100644
--- a/src/random.cpp
+++ b/src/random.cpp
@@ -17,6 +17,7 @@
 #include <stdlib.h>
 #include <limits>
 #include <chrono>
+#include <thread>
 
 #ifndef WIN32
 #include <sys/time.h>
@@ -262,6 +263,8 @@ FastRandomContext::FastRandomContext(const uint256& seed) : requires_seed(false)
 
 bool Random_SanityCheck()
 {
+    uint64_t start = GetPerformanceCounter();
+
     /* This does not measure the quality of randomness, but it does test that
      * OSRandom() overwrites all 32 bytes of the output given a maximum
      * number of tries.
@@ -288,7 +291,14 @@ bool Random_SanityCheck()
 
         tries += 1;
     } while (num_overwritten < NUM_OS_RANDOM_BYTES && tries < MAX_TRIES);
-    return (num_overwritten == NUM_OS_RANDOM_BYTES); /* If this failed, bailed out after too many tries */
+    if (num_overwritten != NUM_OS_RANDOM_BYTES) return false; /* If this failed, bailed out after too many tries */
+
+    // Check that GetPerformanceCounter increases at least during a GetOSRand() call + 1ms sleep.
+    std::this_thread::sleep_for(std::chrono::milliseconds(1));
+    uint64_t stop = GetPerformanceCounter();
+    if (stop == start) return false;
+
+    return true;
 }
 
 FastRandomContext::FastRandomContext(bool fDeterministic) : requires_seed(!fDeterministic), bytebuf_size(0), bitbuf_size(0)

From 2c0a6f157da3c6bb3b0a1e77f003caf0d9cb9d6c Mon Sep 17 00:00:00 2001
From: Pieter Wuille <pieter.wuille@gmail.com>
Date: Fri, 5 May 2017 11:45:37 -0700
Subject: [PATCH 3/3] Use sanity check timestamps as entropy

---
 src/random.cpp | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/src/random.cpp b/src/random.cpp
index 8107cb310..805d7d387 100644
--- a/src/random.cpp
+++ b/src/random.cpp
@@ -298,6 +298,10 @@ bool Random_SanityCheck()
     uint64_t stop = GetPerformanceCounter();
     if (stop == start) return false;
 
+    // We called GetPerformanceCounter. Use it as entropy.
+    RAND_add((const unsigned char*)&start, sizeof(start), 1);
+    RAND_add((const unsigned char*)&stop, sizeof(stop), 1);
+
     return true;
 }