/* Check if N is a prime and G a generator of the
* group. This is check only done if N is big enough.
* Otherwise only the included parameters must be used.
*/
static int
group_check_g_n (mpi_t g, mpi_t n)
{
mpi_t q = NULL, two = NULL, w = NULL;
int ret;
if (_gnutls_mpi_get_nbits (n) < 2048)
{
gnutls_assert ();
return GNUTLS_E_RECEIVED_ILLEGAL_PARAMETER;
}
/* N must be of the form N=2q+1
* where q is also a prime.
*/
if (_gnutls_prime_check (n, 0) != 0)
{
_gnutls_dump_mpi ("no prime N: ", n);
gnutls_assert ();
return GNUTLS_E_RECEIVED_ILLEGAL_PARAMETER;
}
two = _gnutls_mpi_new (4);
if (two == NULL)
{
gnutls_assert ();
return GNUTLS_E_MEMORY_ERROR;
}
q = _gnutls_mpi_alloc_like (n);
if (q == NULL)
{
gnutls_assert ();
ret = GNUTLS_E_MEMORY_ERROR;
goto error;
}
/* q = n-1
*/
_gnutls_mpi_sub_ui (q, n, 1);
/* q = q/2, remember that q is divisible by 2 (prime - 1)
*/
_gnutls_mpi_set_ui (two, 2);
_gnutls_mpi_div (q, NULL, q, two, 0);
if (_gnutls_prime_check (q, 0) != 0)
{
/* N was not on the form N=2q+1, where q = prime
*/
_gnutls_dump_mpi ("no prime Q: ", q);
gnutls_assert ();
return GNUTLS_E_RECEIVED_ILLEGAL_PARAMETER;
}
/* We also check whether g is a generator,
*/
/* check if g < q < N
*/
if (_gnutls_mpi_cmp (g, q) >= 0)
{
gnutls_assert ();
ret = GNUTLS_E_RECEIVED_ILLEGAL_PARAMETER;
goto error;
}
w = _gnutls_mpi_alloc_like (q);
if (w == NULL)
{
gnutls_assert ();
ret = GNUTLS_E_MEMORY_ERROR;
goto error;
}
/* check if g^q mod N == N-1
* w = g^q mod N
*/
_gnutls_mpi_powm (w, g, q, n);
/* w++
*/
_gnutls_mpi_add_ui (w, w, 1);
if (_gnutls_mpi_cmp (w, n) != 0)
{
gnutls_assert ();
ret = GNUTLS_E_RECEIVED_ILLEGAL_PARAMETER;
goto error;
}
ret = 0;
error:
_gnutls_mpi_release (&q);
_gnutls_mpi_release (&two);
_gnutls_mpi_release (&w);
return ret;
}
/* Check if N is a prime and G a generator of the
* group. This check is only done if N is big enough.
* Otherwise only the included parameters must be used.
*/
static int
group_check_g_n(gnutls_session_t session, bigint_t g, bigint_t n)
{
bigint_t q = NULL, two = NULL, w = NULL;
int ret;
if (_gnutls_mpi_get_nbits(n) < (session->internals.srp_prime_bits
? session->internals.srp_prime_bits
: 2048)) {
gnutls_assert();
return GNUTLS_E_RECEIVED_ILLEGAL_PARAMETER;
}
/* N must be of the form N=2q+1
* where q is also a prime.
*/
if (_gnutls_prime_check(n) != 0) {
_gnutls_mpi_log("no prime N: ", n);
gnutls_assert();
return GNUTLS_E_RECEIVED_ILLEGAL_PARAMETER;
}
ret = _gnutls_mpi_init_multi(&two, &q, &w, NULL);
if (ret < 0) {
gnutls_assert();
return ret;
}
/* q = n-1
*/
ret = _gnutls_mpi_sub_ui(q, n, 1);
if (ret < 0) {
gnutls_assert();
goto error;
}
/* q = q/2, remember that q is divisible by 2 (prime - 1)
*/
ret = _gnutls_mpi_set_ui(two, 2);
if (ret < 0) {
gnutls_assert();
goto error;
}
ret = _gnutls_mpi_div(q, q, two);
if (ret < 0) {
gnutls_assert();
goto error;
}
if (_gnutls_prime_check(q) != 0) {
/* N was not on the form N=2q+1, where q = prime
*/
_gnutls_mpi_log("no prime Q: ", q);
gnutls_assert();
ret = GNUTLS_E_RECEIVED_ILLEGAL_PARAMETER;
goto error;
}
/* We also check whether g is a generator,
*/
/* check if g < q < N
*/
if (_gnutls_mpi_cmp(g, q) >= 0) {
gnutls_assert();
ret = GNUTLS_E_RECEIVED_ILLEGAL_PARAMETER;
goto error;
}
/* check if g^q mod N == N-1
* w = g^q mod N
*/
ret = _gnutls_mpi_powm(w, g, q, n);
if (ret < 0) {
gnutls_assert();
goto error;
}
/* w++
*/
ret = _gnutls_mpi_add_ui(w, w, 1);
if (ret < 0) {
gnutls_assert();
goto error;
}
if (_gnutls_mpi_cmp(w, n) != 0) {
gnutls_assert();
ret = GNUTLS_E_RECEIVED_ILLEGAL_PARAMETER;
goto error;
}
ret = 0;
error:
_gnutls_mpi_release(&q);
_gnutls_mpi_release(&two);
_gnutls_mpi_release(&w);
return ret;
}
