big_rand.cpp

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/*
* BigInt Random Generation
* (C) 1999-2007 Jack Lloyd
*
* Botan is released under the Simplified BSD License (see license.txt)
*/
 
#include <botan/bigint.h>
 
#include <botan/exceptn.h>
#include <botan/rng.h>
#include <botan/internal/rounding.h>
 
namespace Botan {
 
/*
* Randomize this number
*/
void BigInt::randomize(RandomNumberGenerator& rng, size_t bitsize, bool set_high_bit) {
   set_sign(Positive);
 
   if(bitsize == 0) {
      clear();
   } else {
      secure_vector<uint8_t> array = rng.random_vec(round_up(bitsize, 8) / 8);
 
      // Always cut unwanted bits
      if(bitsize % 8 > 0) {
         array[0] &= 0xFF >> (8 - (bitsize % 8));
      }
 
      // Set the highest bit if wanted
      if(set_high_bit) {
         array[0] |= 0x80 >> ((bitsize % 8) > 0 ? (8 - bitsize % 8) : 0);
      }
 
      assign_from_bytes(array);
   }
}
 
/*
* Generate a random integer within given range
*/
BigInt BigInt::random_integer(RandomNumberGenerator& rng, const BigInt& min, const BigInt& max) {
   if(min.is_negative() || max.is_negative() || max <= min) {
      throw Invalid_Argument("BigInt::random_integer invalid range");
   }
 
   /*
   If min is > 1 then we generate a random number `r` in [0,max-min)
   and return min + r.
 
   This same logic could also be reasonably chosen for min == 1, but
   that breaks certain tests which expect stability of this function
   when generating within [1,n)
   */
   if(min > 1) {
      const BigInt diff = max - min;
      // This call is recursive, but will not recurse further
      return min + BigInt::random_integer(rng, BigInt::zero(), diff);
   }
 
   BOTAN_DEBUG_ASSERT(min <= 1);
 
   const size_t bits = max.bits();
 
   for(;;) {
      BigInt r;
      r.randomize(rng, bits, false);
      if(r >= min && r < max) {
         return r;
      }
   }
}
 
}  // namespace Botan