utils.c

/*
 *  SPDX-License-Identifier: MIT
 */

#if defined(HAVE_CONFIG_H)
#include <config.h>
#endif

#include "utils.h"

#include <assert.h>
#include <string.h>

static inline uint16_t num_rec_2(const uint8_t* v) {
  uint16_t r;
  memcpy(&r, v, sizeof(r));
  return le16toh(r);
}

// DecodeChall_3
static bool decode_chall_3(uint8_t* decoded_chall, const uint8_t* chall, unsigned int i,
                           const faest_paramset_t* params) {
  if (i >= params->tau) {
    return false;
  }

  const unsigned int t1 = params->tau1;
  const unsigned int k  = params->k;

  unsigned int lo;
  unsigned int hi;
  if (i < t1) {
    lo = i * k;
    hi = (i + 1) * k;
  } else {
    const unsigned int t = i - t1;

    lo = (t1 * k) + (t * (k - 1));
    hi = (t1 * k) + ((t + 1) * (k - 1));
  }

  assert(hi - lo == k || hi - lo == k - 1);
  // TODO: this could be implemented more efficiently using bit shifts
  for (unsigned int j = lo; j < hi; ++j) {
    ptr_set_bit(decoded_chall, j - lo, ptr_get_bit(chall, j));
  }
  return true;
}

bool decode_all_chall_3(uint16_t* decoded_chall, const uint8_t* chall,
                        const faest_paramset_t* params) {
  for (unsigned int i = 0; i != params->tau; ++i) {
    uint8_t tmp[2] = {0};
    if (!decode_chall_3(tmp, chall, i, params)) {
      return false;
    }
    decoded_chall[i] = num_rec_2(tmp);
  }
  return true;
}