diff --git a/include/polarssl/ecdh.h b/include/polarssl/ecdh.h
index 2184ab9..d91aea5 100644
--- a/include/polarssl/ecdh.h
+++ b/include/polarssl/ecdh.h
@@ -70,12 +70,20 @@
  * \param z         Destination MPI (shared secret)
  * \param Q         Public key from other party
  * \param d         Our secret exponent
+ * \param f_rng     RNG function (see notes)
+ * \param p_rng     RNG parameter
  *
  * \return          0 if successful,
  *                  or a POLARSSL_ERR_ECP_XXX or POLARSSL_MPI_XXX error code
+ *
+ * \note            If f_rng is not NULL, it is used to implement
+ *                  countermeasures against potential elaborate timing
+ *                  attacks, see \c ecp_mul() for details.
  */
 int ecdh_compute_shared( const ecp_group *grp, mpi *z,
-                         const ecp_point *Q, const mpi *d );
+                         const ecp_point *Q, const mpi *d,
+                         int (*f_rng)(void *, unsigned char *, size_t),
+                         void *p_rng );
 
 /**
  * \brief           Initialize context
@@ -156,11 +164,15 @@
  * \param olen      number of bytes written
  * \param buf       destination buffer
  * \param blen      buffer length
+ * \param f_rng     RNG function, see notes for \c ecdh_compute_shared()
+ * \param p_rng     RNG parameter
  *
  * \return          0 if successful, or an POLARSSL_ERR_ECP_XXX error code
  */
 int ecdh_calc_secret( ecdh_context *ctx, size_t *olen,
-                      unsigned char *buf, size_t blen );
+                      unsigned char *buf, size_t blen,
+                      int (*f_rng)(void *, unsigned char *, size_t),
+                      void *p_rng );
 
 /**
  * \brief          Checkup routine
diff --git a/include/polarssl/ecp.h b/include/polarssl/ecp.h
index ad31bff..5942231 100644
--- a/include/polarssl/ecp.h
+++ b/include/polarssl/ecp.h
@@ -411,17 +411,29 @@
  * \param R         Destination point
  * \param m         Integer by which to multiply
  * \param P         Point to multiply
+ * \param f_rng     RNG function (see notes)
+ * \param p_rng     RNG parameter
  *
  * \return          0 if successful,
  *                  POLARSSL_ERR_MPI_MALLOC_FAILED if memory allocation failed
- *                  POLARSSL_ERR_ECP_GENERIC if m < 0 of m has greater bit
- *                  length than N, the number of points in the group.
+ *                  POLARSSL_ERR_ECP_BAD_INPUT_DATA if m < 0 of m has greater
+ *                  bit length than N, the number of points in the group.
  *
- * \note            This function executes a constant number of operations
- *                  for random m in the allowed range.
+ * \note            In order to prevent simple timing attacks, this function
+ *                  executes a constant number of operations (that is, point
+ *                  doubling and addition of distinct points) for random m in
+ *                  the allowed range.
+ *
+ * \note            If f_rng is not NULL, it is used to randomize projective
+ *                  coordinates of indermediate results, in order to prevent
+ *                  more elaborate timing attacks relying on intermediate
+ *                  operations. (This is a prophylactic measure since so such
+ *                  attack has been published yet.)
  */
 int ecp_mul( const ecp_group *grp, ecp_point *R,
-             const mpi *m, const ecp_point *P );
+             const mpi *m, const ecp_point *P,
+             int (*f_rng)(void *, unsigned char *, size_t), void *p_rng );
+
 
 /**
  * \brief           Check that a point is a valid public key on this curve