dh_check_pubkey.c

/* LibTomCrypt, modular cryptographic library -- Tom St Denis
 *
 * LibTomCrypt is a library that provides various cryptographic
 * algorithms in a highly modular and flexible manner.
 *
 * The library is free for all purposes without any express
 * guarantee it works.
 */

#include "tomcrypt.h"

#ifdef LTC_MDH

/**
  Check DH public key (INTERNAL ONLY, not part of public API)
  @param key    The key you wish to test
  @return CRYPT_OK if successful
*/
int dh_check_pubkey(dh_key *key)
{
   void *p_minus1;
   ltc_mp_digit digit;
   int i, digit_count, bits_set = 0, err;

   LTC_ARGCHK(key != NULL);

   if ((err = mp_init(&p_minus1)) != CRYPT_OK) {
      return err;
   }

   /* avoid: y <= 1 OR y >= p-1 */
   if ((err = mp_sub_d(key->prime, 1, p_minus1)) != CRYPT_OK) {
      goto error;
   }
   if (mp_cmp(key->y, p_minus1) != LTC_MP_LT || mp_cmp_d(key->y, 1) != LTC_MP_GT) {
      err = CRYPT_INVALID_ARG;
      goto error;
   }

   /* public key must have more than one bit set */
   digit_count = mp_get_digit_count(key->y);
   for (i = 0; i < digit_count && bits_set < 2; i++) {
      digit = mp_get_digit(key->y, i);
      while (digit > 0) {
         if (digit & 1) bits_set++;
         digit >>= 1;
      }
   }
   if (bits_set > 1) {
      err = CRYPT_OK;
   }
   else {
      err = CRYPT_INVALID_ARG;
   }

error:
   mp_clear(p_minus1);
   return err;
}

#endif /* LTC_MDH */