ECP: Prevent freeing a buffer on stack
The function ecp_mod_koblitz computed the space for the result of a
multiplication optimally for that specific case, but unfortunately
the function mbedtls_mpi_mul_mpi performs a generic, suboptimal
calculation and needs one more limb for the result. Since the result's
buffer is on the stack, the best case scenario is that the program
stops.
This only happened on 64 bit platforms.
Fixes #569
diff --git a/library/ecp_curves.c b/library/ecp_curves.c
index 9a6e8eb..a2a5495 100644
--- a/library/ecp_curves.c
+++ b/library/ecp_curves.c
@@ -1213,7 +1213,7 @@
int ret;
size_t i;
mbedtls_mpi M, R;
- mbedtls_mpi_uint Mp[P_KOBLITZ_MAX + P_KOBLITZ_R];
+ mbedtls_mpi_uint Mp[P_KOBLITZ_MAX + P_KOBLITZ_R + 1];
if( N->n < p_limbs )
return( 0 );
@@ -1235,7 +1235,7 @@
memcpy( Mp, N->p + p_limbs - adjust, M.n * sizeof( mbedtls_mpi_uint ) );
if( shift != 0 )
MBEDTLS_MPI_CHK( mbedtls_mpi_shift_r( &M, shift ) );
- M.n += R.n - adjust; /* Make room for multiplication by R */
+ M.n += R.n; /* Make room for multiplication by R */
/* N = A0 */
if( mask != 0 )
@@ -1257,7 +1257,7 @@
memcpy( Mp, N->p + p_limbs - adjust, M.n * sizeof( mbedtls_mpi_uint ) );
if( shift != 0 )
MBEDTLS_MPI_CHK( mbedtls_mpi_shift_r( &M, shift ) );
- M.n += R.n - adjust; /* Make room for multiplication by R */
+ M.n += R.n; /* Make room for multiplication by R */
/* N = A0 */
if( mask != 0 )