ct_utils.h

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/*
* Functions for constant time operations on data and testing of
* constant time annotations using valgrind.
*
* For more information about constant time programming see
* Wagner, Molnar, et al "The Program Counter Security Model"
*
* (C) 2010 Falko Strenzke
* (C) 2015,2016 Jack Lloyd
*
* Botan is released under the Simplified BSD License (see license.txt)
*/
 
#ifndef BOTAN_TIMING_ATTACK_CM_H__
#define BOTAN_TIMING_ATTACK_CM_H__
 
#include <botan/secmem.h>
#include <vector>
 
namespace Botan {
 
namespace CT {
 
/*
* T should be an unsigned machine integer type
* Expand to a mask used for other operations
* @param in an integer
* @return If n is zero, returns zero. Otherwise
* returns a T with all bits set for use as a mask with
* select.
*/
template<typename T>
inline T expand_mask(T x)
   {
   T r = x;
   // First fold r down to a single bit
   for(size_t i = 1; i != sizeof(T)*8; i *= 2)
      r |= r >> i;
   r &= 1;
   r = ~(r - 1);
   return r;
   }
 
template<typename T>
inline T select(T mask, T from0, T from1)
   {
   return (from0 & mask) | (from1 & ~mask);
   }
 
template<typename PredT, typename ValT>
inline ValT val_or_zero(PredT pred_val, ValT val)
   {
   return select(CT::expand_mask<ValT>(pred_val), val, static_cast<ValT>(0));
   }
 
template<typename T>
inline T is_zero(T x)
   {
   return ~expand_mask(x);
   }
 
template<typename T>
inline T is_equal(T x, T y)
   {
   return is_zero(x ^ y);
   }
 
template<typename T>
inline T is_less(T x, T y)
   {
   /*
   This expands to a constant time sequence with GCC 5.2.0 on x86-64
   but something more complicated may be needed for portable const time.
   */
   return expand_mask<T>(x < y);
   }
 
template<typename T>
inline T is_lte(T x, T y)
   {
   return expand_mask<T>(x <= y);
   }
 
template<typename T>
inline void conditional_copy_mem(T value,
                                 T* to,
                                 const T* from0,
                                 const T* from1,
                                 size_t elems)
   {
   const T mask = CT::expand_mask(value);
 
   for(size_t i = 0; i != elems; ++i)
      {
      to[i] = CT::select(mask, from0[i], from1[i]);
      }
   }
 
template<typename T>
inline void cond_zero_mem(T cond,
                          T* array,
                          size_t elems)
   {
   const T mask = CT::expand_mask(cond);
   const T zero(0);
 
   for(size_t i = 0; i != elems; ++i)
      {
      array[i] = CT::select(mask, zero, array[i]);
      }
   }
 
template<typename T>
inline T expand_top_bit(T a)
   {
   return expand_mask<T>(a >> (sizeof(T)*8-1));
   }
 
template<typename T>
inline T max(T a, T b)
   {
   const T a_larger = b - a; // negative if a is larger
   return select(expand_top_bit(a), a, b);
   }
 
template<typename T>
inline T min(T a, T b)
   {
   const T a_larger = b - a; // negative if a is larger
   return select(expand_top_bit(b), b, a);
   }
 
}
 
}
 
#endif