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/*************************************************
* Utility Functions Source File *
* (C) 1999-2007 Jack Lloyd *
*************************************************/
#include <botan/util.h>
#include <botan/bit_ops.h>
#include <algorithm>
#include <cmath>
namespace Botan {
/*************************************************
* Round up n to multiple of align_to *
*************************************************/
u32bit round_up(u32bit n, u32bit align_to)
{
if(n % align_to || n == 0)
n += align_to - (n % align_to);
return n;
}
/*************************************************
* Round down n to multiple of align_to *
*************************************************/
u32bit round_down(u32bit n, u32bit align_to)
{
return (n - (n % align_to));
}
/*************************************************
* Choose the exponent size for a DL group
*************************************************/
u32bit dl_work_factor(u32bit bits)
{
#if 0
/*
These values were taken from RFC 3526
*/
if(bits <= 1536)
return 90;
else if(bits <= 2048)
return 110;
else if(bits <= 3072)
return 130;
else if(bits <= 4096)
return 150;
else if(bits <= 6144)
return 170;
else if(bits <= 8192)
return 190;
return 256;
#else
const u32bit MIN_ESTIMATE = 64;
const double log_x = bits / 1.44;
const double strength =
2.76 * std::pow(log_x, 1.0/3.0) * std::pow(std::log(log_x), 2.0/3.0);
if(strength > MIN_ESTIMATE)
return static_cast<u32bit>(strength);
return MIN_ESTIMATE;
#endif
}
/*************************************************
* 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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