rsa_exptmod.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.
 *
 * Tom St Denis, tomstdenis@iahu.ca, http://libtomcrypt.org
 */

/* RSA Code by Tom St Denis */
#include "mycrypt.h"

#ifdef MRSA

/* compute an RSA modular exponentiation */
int rsa_exptmod(const unsigned char *in,   unsigned long inlen,
                      unsigned char *out,  unsigned long *outlen, int which,
                      prng_state    *prng, int           prng_idx,
                      rsa_key *key)
{
   mp_int        tmp, tmpa, tmpb;
   unsigned long x;
   int           err;

   _ARGCHK(in     != NULL);
   _ARGCHK(out    != NULL);
   _ARGCHK(outlen != NULL);
   _ARGCHK(key    != NULL);
   
   /* valid prng? */
   if ((err = prng_is_valid(prng_idx)) != CRYPT_OK) {
      return err;
   }

   /* is the key of the right type for the operation? */
   if (which == PK_PRIVATE && (key->type != PK_PRIVATE)) {
      return CRYPT_PK_NOT_PRIVATE;
   }

   /* must be a private or public operation */
   if (which != PK_PRIVATE && which != PK_PUBLIC) {
      return CRYPT_PK_INVALID_TYPE;
   }

   /* init and copy into tmp */
   if ((err = mp_init_multi(&tmp, &tmpa, &tmpb, NULL)) != MP_OKAY)                     { return mpi_to_ltc_error(err); }
   if ((err = mp_read_unsigned_bin(&tmp, (unsigned char *)in, (int)inlen)) != MP_OKAY) { goto error; }

   /* sanity check on the input */
   if (mp_cmp(&key->N, &tmp) == MP_LT) {
      err = CRYPT_PK_INVALID_SIZE;
      goto done;
   }

   /* are we using the private exponent and is the key optimized? */
   if (which == PK_PRIVATE) {
      /* tmpa = tmp^dP mod p */
      if ((err = tim_exptmod(prng, prng_idx, &tmp, &key->e, &key->dP, &key->p, &tmpa)) != MP_OKAY)    { goto error; }

      /* tmpb = tmp^dQ mod q */
      if ((err = tim_exptmod(prng, prng_idx, &tmp, &key->e,  &key->dQ, &key->q, &tmpb)) != MP_OKAY)    { goto error; }

      /* tmp = (tmpa - tmpb) * qInv (mod p) */
      if ((err = mp_sub(&tmpa, &tmpb, &tmp)) != MP_OKAY)                    { goto error; }
      if ((err = mp_mulmod(&tmp, &key->qP, &key->p, &tmp)) != MP_OKAY)      { goto error; }

      /* tmp = tmpb + q * tmp */
      if ((err = mp_mul(&tmp, &key->q, &tmp)) != MP_OKAY)                   { goto error; }
      if ((err = mp_add(&tmp, &tmpb, &tmp)) != MP_OKAY)                     { goto error; }
   } else {
      /* exptmod it */
      if ((err = mp_exptmod(&tmp, &key->e, &key->N, &tmp)) != MP_OKAY) { goto error; }
   }

   /* read it back */
   x = (unsigned long)mp_unsigned_bin_size(&key->N);
   if (x > *outlen) {
      err = CRYPT_BUFFER_OVERFLOW;
      goto done;
   }
   *outlen = x;

   /* convert it */
   zeromem(out, x);
   if ((err = mp_to_unsigned_bin(&tmp, out+(x-mp_unsigned_bin_size(&tmp)))) != MP_OKAY) { goto error; }

   /* clean up and return */
   err = CRYPT_OK;
   goto done;
error:
   err = mpi_to_ltc_error(err);
done:
   mp_clear_multi(&tmp, &tmpa, &tmpb, NULL);
   return err;
}

#endif