/*

* (C) 2018 Jack Lloyd

*

* Botan is released under the Simplified BSD License (see license.txt)

*/

#include <botan/internal/ct_utils.h>

namespace Botan {

namespace CT {

secure_vector<uint8_t> copy_output(CT::Mask<uint8_t> bad_input,

                                   const uint8_t input[],

                                   size_t input_length,

                                   size_t offset)

   {

   if(input_length == 0)

      return secure_vector<uint8_t>();

   /*

   * Ensure at runtime that offset <= input_length. This is an invalid input,

   * but we can't throw without using the poisoned value. Instead, if it happens,

   * set offset to be equal to the input length (so output_bytes becomes 0 and

   * the returned vector is empty)

   */

   const auto valid_offset = CT::Mask<size_t>::is_lte(offset, input_length);

   offset = valid_offset.select(offset, input_length);

   const size_t output_bytes = input_length - offset;

   secure_vector<uint8_t> output(input_length);

   /*

   Move the desired output bytes to the front using a slow (O^n)

   but constant time loop that does not leak the value of the offset

   */

   for(size_t i = 0; i != input_length; ++i)

      {

      /*

      start index from i rather than 0 since we know j must be >= i + offset

      to have any effect, and starting from i does not reveal information

      */

      for(size_t j = i; j != input_length; ++j)

         {

         const uint8_t b = input[j];

         const auto is_eq = CT::Mask<size_t>::is_equal(j, offset + i);

         output[i] |= is_eq.if_set_return(b);

         }

      }

   bad_input.if_set_zero_out(output.data(), output.size());

   CT::unpoison(output.data(), output.size());

   CT::unpoison(output_bytes);

   /*

   This is potentially not const time, depending on how std::vector is

   implemented. But since we are always reducing length, it should

   just amount to setting the member var holding the length.

   */

   output.resize(output_bytes);

   return output;

   }

secure_vector<uint8_t> strip_leading_zeros(const uint8_t in[], size_t length)

   {

   size_t leading_zeros = 0;

   auto only_zeros = Mask<uint8_t>::set();

   for(size_t i = 0; i != length; ++i)

      {

      only_zeros &= CT::Mask<uint8_t>::is_zero(in[i]);

      leading_zeros += only_zeros.if_set_return(1);

      }

   return copy_output(CT::Mask<uint8_t>::cleared(), in, length, leading_zeros);

   }

}

}