/*

* (C) 2018,2021 Jack Lloyd

*

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

*/

#include <botan/internal/ct_utils.h>

namespace Botan::CT {

secure_vector<uint8_t> copy_output(CT::Mask<uint8_t> bad_input_u8,

                                   const uint8_t input[],

                                   size_t input_length,

                                   size_t offset) {

   /*

   * We do not poison the input here because if we did we would have

   * to unpoison it at exit. We assume instead that callers have

   * already poisoned the input and will unpoison it at their own

   * time.

   */

   CT::poison(&offset, sizeof(size_t));

   secure_vector<uint8_t> output(input_length);

   auto bad_input = CT::Mask<size_t>::expand(bad_input_u8);

   /*

   * If the offset is greater than input_length then the arguments are

   * invalid. Ideally we would through an exception but that leaks

   * information about the offset. Instead treat it as if the input

   * was invalid.

   */

   bad_input |= CT::Mask<size_t>::is_gt(offset, input_length);

   /*

   * If the input is invalid, then set offset == input_length as a result

   * at the end we will set output_bytes == 0 causing the final result to

   * be an empty vector.

   */

   offset = bad_input.select(input_length, offset);

   /*

   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) {

      /*

      * If bad_input was set then we modified offset to equal the input_length.

      * In that case, this_loop will be greater than input_length, and so is_eq

      * mask will always be false. As a result none of the input values will be

      * written to output.

      *

      * This is ignoring the possibility of integer overflow of offset + i. But

      * for this to happen the input would have to consume nearly the entire

      * address space, and we just allocated an output buffer of equal size.

      */

      const size_t this_loop = offset + 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, this_loop);

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

      }

   }

   const size_t output_bytes = input_length - offset;

   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);

}

}  // namespace Botan::CT