ecc: Improve gcry_mpi_ec_curve_point

* mpi/ec.c (_gcry_mpi_ec_curve_point): Check range of coordinates.
* tests/t-mpi-point.c (point_on_curve): New.
--

Due to the conversion to affine coordinates we didn't detected points
with values >= P.  The solution here might not be the best according
to the NIST standard (it is done there at an earlier opportunity) but
it reliably detects points we do not expect to receive.

The new test vectors have been compared against gnutls/nettle.

Reported-by: Stephan Müller
Signed-off-by: Werner Koch <wk@gnupg.org>

1 files changed, 9 insertions, 0 deletions

diff --git a/mpi/ec.c b/mpi/ec.c
index 2c396a74..97afbfed 100644
--- a/mpi/ec.c
+++ b/mpi/ec.c
@@ -1731,6 +1731,15 @@ _gcry_mpi_ec_curve_point (gcry_mpi_point_t point, mpi_ec_t ctx)
   y = mpi_new (0);
   w = mpi_new (0);
 
+  /* Check that the point is in range.  This needs to be done here and
+   * not after conversion to affine coordinates.  */
+  if (mpi_cmpabs (point->x, ctx->p) >= 0)
+    goto leave;
+  if (mpi_cmpabs (point->y, ctx->p) >= 0)
+    goto leave;
+  if (mpi_cmpabs (point->z, ctx->p) >= 0)
+    goto leave;
+
   switch (ctx->model)
     {
     case MPI_EC_WEIERSTRASS: