Move Entropy_Estimator to utils/entropy.h (from anon namespace in HMAC_RNG

implementation), remove freestanding estimate_entropy function, change
Randpool to use entropy estimator.

1 files changed, 80 insertions, 0 deletions

diff --git a/src/utils/entropy.cpp b/src/utils/entropy.cpp
new file mode 100644
index 000000000..1562eb0d2
--- /dev/null
+++ b/src/utils/entropy.cpp
@@ -0,0 +1,80 @@
+/*************************************************
+* Entropy_Estimator Source File                  *
+* (C) 2008 Jack Lloyd                            *
+*************************************************/
+
+#include <botan/entropy.h>
+#include <botan/bit_ops.h>
+
+namespace Botan {
+
+/**
+Update the estimate
+*/
+void Entropy_Estimator::update(const byte buffer[], u32bit length,
+                               u32bit upper_limit)
+   {
+   u32bit this_buf_estimate = 0;
+
+   /*
+   This is pretty naive
+   */
+   for(u32bit j = 0; j != length; ++j)
+      {
+      byte delta = last ^ buffer[j];
+      last = buffer[j];
+
+      byte delta2 = delta ^ last_delta;
+      last_delta = delta;
+
+      byte delta3 = delta2 ^ last_delta2;
+      last_delta2 = delta2;
+
+      byte min_delta = delta;
+      if(min_delta > delta2) min_delta = delta2;
+      if(min_delta > delta3) min_delta = delta3;
+
+      this_buf_estimate += hamming_weight(min_delta);
+      }
+
+   this_buf_estimate /= 2;
+
+   if(upper_limit)
+      estimate += std::min(upper_limit, this_buf_estimate);
+   else
+      estimate += this_buf_estimate;
+   }
+
+/*************************************************
+* Estimate the entropy of the buffer             *
+*************************************************/
+u32bit entropy_estimate(const byte buffer[], u32bit length)
+   {
+   if(length <= 4)
+      return 0;
+
+   u32bit estimate = 0;
+   byte last = 0, last_delta = 0, last_delta2 = 0;
+
+   for(u32bit j = 0; j != length; ++j)
+      {
+      byte delta = last ^ buffer[j];
+      last = buffer[j];
+
+      byte delta2 = delta ^ last_delta;
+      last_delta = delta;
+
+      byte delta3 = delta2 ^ last_delta2;
+      last_delta2 = delta2;
+
+      byte min_delta = delta;
+      if(min_delta > delta2) min_delta = delta2;
+      if(min_delta > delta3) min_delta = delta3;
+
+      estimate += hamming_weight(min_delta);
+      }
+
+   return (estimate / 2);
+   }
+
+}