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/*************************************************
* 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);
}
}
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