diff --git a/src/or/relay_crypto.c b/src/or/relay_crypto.c
index 9d61ecd424..c42a4f9cca 100644
--- a/src/or/relay_crypto.c
+++ b/src/or/relay_crypto.c
@@ -170,6 +170,9 @@ relay_decrypt_cell(circuit_t *circ, cell_t *cell,
/**
* Encrypt a cell cell that we are creating, and sending outbound on
* circ until the hop corresponding to layer_hint.
+ *
+ * The integrity field and recognized field of cell's relay headers
+ * must be set to zero.
*/
void
relay_encrypt_cell_outbound(cell_t *cell,
@@ -193,6 +196,9 @@ relay_encrypt_cell_outbound(cell_t *cell,
/**
* Encrypt a cell cell that we are creating, and sending on
* circuit to the origin.
+ *
+ * The integrity field and recognized field of cell's relay headers
+ * must be set to zero.
*/
void
relay_encrypt_cell_inbound(cell_t *cell,