Improve comments on parity trick
diff --git a/library/ecp.c b/library/ecp.c
index 454820a..41bf03d 100644
--- a/library/ecp.c
+++ b/library/ecp.c
@@ -1751,6 +1751,9 @@
* this wrapper ensures that by replacing m by N - m if necessary, and
* informs the caller that the result of multiplication will be negated.
*
+ * This works because we only support large prime order for Short Weierstrass
+ * curves, so N is always odd hence either m or N - m is.
+ *
* See ecp_comb_recode_core() for background.
*/
static int ecp_comb_recode_scalar( const mbedtls_ecp_group *grp,
@@ -1766,7 +1769,7 @@
mbedtls_mpi_init( &M );
mbedtls_mpi_init( &mm );
- /* N is odd with all real-world curves, just make extra sure */
+ /* N is always odd (see above), just make extra sure */
if( mbedtls_mpi_get_bit( &grp->N, 0 ) != 1 )
return( MBEDTLS_ERR_ECP_BAD_INPUT_DATA );
