Simplify usage of DHM blinding
diff --git a/include/polarssl/dhm.h b/include/polarssl/dhm.h
index 0152dc9..75dff19 100644
--- a/include/polarssl/dhm.h
+++ b/include/polarssl/dhm.h
@@ -230,13 +230,11 @@
*
* \return 0 if successful, or an POLARSSL_ERR_DHM_XXX error code
*
- * \note If f_rng is not NULL, it is used to blind the input as
- * countermeasure against timing attacks. This is only useful
- * when this function is called repeatedly with the same
- * secret value (X field), eg when using DH key exchange as
- * opposed to DHE. It is recommended to use a non-NULL f_rng
- * only when needed, since otherwise this countermeasure has
- * high overhead.
+ * \note If non-NULL, f_rng is used to blind the input as
+ * countermeasure against timing attacks. Blinding is
+ * automatically used if and only if our secret value X is
+ * re-used and costs nothing otherwise, so it is recommended
+ * to always pass a non-NULL f_rng argument.
*/
int dhm_calc_secret( dhm_context *ctx,
unsigned char *output, size_t *olen,