src/pk/dsa/dsa_sign_hash.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@gmail.com, http://libtomcrypt.org
 */
#include "tomcrypt.h"

/**
   @file dsa_sign_hash.c
   DSA implementation, sign a hash, Tom St Denis
*/

#ifdef MDSA

/**
  Sign a hash with DSA
  @param in       The hash to sign
  @param inlen    The length of the hash to sign
  @param out      [out] Where to store the signature
  @param outlen   [in/out] The max size and resulting size of the signature
  @param prng     An active PRNG state
  @param wprng    The index of the PRNG desired
  @param key      A private DSA key
  @return CRYPT_OK if successful
*/
int dsa_sign_hash(const unsigned char *in,  unsigned long inlen,
                        unsigned char *out, unsigned long *outlen,
                        prng_state *prng, int wprng, dsa_key *key)
{
   mp_int         k, kinv, tmp, r, s;
   unsigned char *buf;
   int            err;
   unsigned long  out1, out2;

   LTC_ARGCHK(in     != NULL);
   LTC_ARGCHK(out    != NULL);
   LTC_ARGCHK(outlen != NULL);
   LTC_ARGCHK(key    != NULL);

   if ((err = prng_is_valid(wprng)) != CRYPT_OK) {
      return err;
   }
   if (key->type != PK_PRIVATE) {
      return CRYPT_PK_NOT_PRIVATE;
   }

   /* check group order size  */
   if (key->qord >= MDSA_MAX_GROUP) {
      return CRYPT_INVALID_ARG;
   }

   buf = XMALLOC(MDSA_MAX_GROUP);
   if (buf == NULL) {
      return CRYPT_MEM;
   }

   /* Init our temps */
   if ((err = mp_init_multi(&k, &kinv, &r, &s, &tmp, NULL)) != MP_OKAY)               { goto error; }

retry:

   do {
      /* gen random k */
      if (prng_descriptor[wprng].read(buf, key->qord, prng) != (unsigned long)key->qord) {
         err = CRYPT_ERROR_READPRNG;
         goto LBL_ERR;
      }

      /* read k */
      if ((err = mp_read_unsigned_bin(&k, buf, key->qord)) != MP_OKAY)                { goto error; }

      /* k > 1 ? */
      if (mp_cmp_d(&k, 1) != MP_GT)                                                   { goto retry; }

      /* test gcd */
      if ((err = mp_gcd(&k, &key->q, &tmp)) != MP_OKAY)                               { goto error; }
   } while (mp_cmp_d(&tmp, 1) != MP_EQ);

   /* now find 1/k mod q */
   if ((err = mp_invmod(&k, &key->q, &kinv)) != MP_OKAY)                              { goto error; }

   /* now find r = g^k mod p mod q */
   if ((err = mp_exptmod(&key->g, &k, &key->p, &r)) != MP_OKAY)                       { goto error; }
   if ((err = mp_mod(&r, &key->q, &r)) != MP_OKAY)                                    { goto error; }

   if (mp_iszero(&r) == MP_YES)                                                       { goto retry; }

   /* now find s = (in + xr)/k mod q */
   if ((err = mp_read_unsigned_bin(&tmp, (unsigned char *)in, inlen)) != MP_OKAY)     { goto error; }
   if ((err = mp_mul(&key->x, &r, &s)) != MP_OKAY)                                    { goto error; }
   if ((err = mp_add(&s, &tmp, &s)) != MP_OKAY)                                       { goto error; }
   if ((err = mp_mulmod(&s, &kinv, &key->q, &s)) != MP_OKAY)                          { goto error; }

   if (mp_iszero(&s) == MP_YES)                                                       { goto retry; }

   /* now store em both */
   
   /* first check that we have enough room */
   if ((err = der_length_integer(&s, &out1)) != CRYPT_OK)                             { goto LBL_ERR; }
   if ((err = der_length_integer(&r, &out2)) != CRYPT_OK)                             { goto LBL_ERR; }
   if (*outlen < (out1+out2)) {
      err = CRYPT_BUFFER_OVERFLOW;
      goto LBL_ERR;
   }

   /* store ints */
   err = der_put_multi_integer(out, outlen, &r, &s, NULL);
   goto LBL_ERR;

error: 
   err = mpi_to_ltc_error(err);
LBL_ERR: 
   mp_clear_multi(&k, &kinv, &r, &s, &tmp, NULL);
#ifdef LTC_CLEAN_STACK
   zeromem(buf, MDSA_MAX_GROUP);
#endif
   XFREE(buf);
   return err;
}

#endif