diff --git a/cipher/elgamal.c b/cipher/elgamal.c index ae7a631..eead450 100644 --- a/cipher/elgamal.c +++ b/cipher/elgamal.c @@ -510,8 +510,9 @@ do_encrypt(gcry_mpi_t a, gcry_mpi_t b, gcry_mpi_t input, ELG_public_key *pkey ) static void decrypt (gcry_mpi_t output, gcry_mpi_t a, gcry_mpi_t b, ELG_secret_key *skey ) { - gcry_mpi_t t1, t2, r; + gcry_mpi_t t1, t2, r, r1, h; unsigned int nbits = mpi_get_nbits (skey->p); + gcry_mpi_t x_blind; mpi_normalize (a); mpi_normalize (b); @@ -522,20 +523,33 @@ decrypt (gcry_mpi_t output, gcry_mpi_t a, gcry_mpi_t b, ELG_secret_key *skey ) t2 = mpi_snew (nbits); r = mpi_new (nbits); + r1 = mpi_new (nbits); + h = mpi_new (nbits); + x_blind = mpi_snew (nbits); /* We need a random number of about the prime size. The random number merely needs to be unpredictable; thus we use level 0. */ _gcry_mpi_randomize (r, nbits, GCRY_WEAK_RANDOM); + /* Also, exponent blinding: x_blind = x + (p-1)*r1 */ + _gcry_mpi_randomize (r1, nbits, GCRY_WEAK_RANDOM); + mpi_set_highbit (r1, nbits - 1); + mpi_sub_ui (h, skey->p, 1); + mpi_mul (x_blind, h, r1); + mpi_add (x_blind, skey->x, x_blind); + /* t1 = r^x mod p */ - mpi_powm (t1, r, skey->x, skey->p); + mpi_powm (t1, r, x_blind, skey->p); /* t2 = (a * r)^-x mod p */ mpi_mulm (t2, a, r, skey->p); - mpi_powm (t2, t2, skey->x, skey->p); + mpi_powm (t2, t2, x_blind, skey->p); mpi_invm (t2, t2, skey->p); /* t1 = (t1 * t2) mod p*/ mpi_mulm (t1, t1, t2, skey->p); + mpi_free (x_blind); + mpi_free (h); + mpi_free (r1); mpi_free (r); mpi_free (t2);