diff --git a/library/ecp.c b/library/ecp.c
index 66cf58e..8d0f49a 100644
--- a/library/ecp.c
+++ b/library/ecp.c
@@ -830,7 +830,7 @@
         mbedtls_mpi_free( &pt->Y );
 
         if( grp->id == MBEDTLS_ECP_DP_CURVE25519 )
-            /* Set most significant bit to 0 */
+            /* Set most significant bit to 0 as prescribed in RFC7748 §5 */
             MBEDTLS_MPI_CHK( mbedtls_mpi_set_bit( &pt->X, plen * 8 - 1, 0 ) );
     }
 #endif