diff --git a/include/polarssl/rsa.h b/include/polarssl/rsa.h
index 7e6d19f..f9d02bd 100644
--- a/include/polarssl/rsa.h
+++ b/include/polarssl/rsa.h
@@ -210,7 +210,8 @@
  * \return         0 if successful, or an POLARSSL_ERR_RSA_XXX error code
  *
  * \note           This function does NOT take care of message
- *                 padding. Also, be sure to set input[0] = 0.
+ *                 padding. Also, be sure to set input[0] = 0 or assure that
+ *                 input is smaller than N.
  *
  * \note           The input and output buffers must be large
  *                 enough (eg. 128 bytes if RSA-1024 is used).