Add comment on internal function API
diff --git a/library/ecdh.c b/library/ecdh.c
index fb46845..eaad62c 100644
--- a/library/ecdh.c
+++ b/library/ecdh.c
@@ -41,6 +41,10 @@
#if !defined(MBEDTLS_ECDH_GEN_PUBLIC_ALT)
/*
* Generate public key (restartable version)
+ *
+ * Note: this internal function relies on its caller preserving the value of
+ * the output parameter 'd' accross continuation calls. This would not be
+ * acceptable for a public function but is OK here as we control call sites.
*/
static int ecdh_gen_public_restartable( mbedtls_ecp_group *grp,
mbedtls_mpi *d, mbedtls_ecp_point *Q,
