diff --git a/src/salt.c b/src/salt.c
index 0957415..892047c 100644
--- a/src/salt.c
+++ b/src/salt.c
@@ -614,7 +614,7 @@ salt_ret_t salt_write_begin(uint8_t *p_buffer,
     return SALT_SUCCESS;
 }
 
-salt_ret_t salt_write_next(salt_msg_t *p_msg, void *p_buffer, uint32_t size)
+salt_ret_t salt_write_next(salt_msg_t *p_msg, const void *p_buffer, const uint32_t size)
 {
 
     /*
diff --git a/src/salt.h b/src/salt.h
index ba3c243..e774518 100644
--- a/src/salt.h
+++ b/src/salt.h
@@ -642,8 +642,8 @@ salt_ret_t salt_write_begin(uint8_t *p_buffer,
  *
  */
 salt_ret_t salt_write_next(salt_msg_t *p_msg,
-                           void *p_buffer,
-                           uint32_t size);
+                           const void *p_buffer,
+                           const uint32_t size);
 
 /**
  * @brief Add a clear text message to be encrypted to next encrypted package.