Adapt cipher and MD layer with _init() and _free()

include/polarssl/md.h

@@ -172,10 +172,26 @@ const md_info_t *md_info_from_string( const char *md_name );
  */
 const md_info_t *md_info_from_type( md_type_t md_type );
 
+/**
+ * \brief               Initialize a md_context (as NONE)
+ */
+void md_init( md_context_t *ctx );
+
+/**
+ * \brief               Free and clear the message-specific context of ctx.
+ *                      Freeing ctx itself remains the responsibility of the
+ *                      caller.
+ */
+void md_free( md_context_t *ctx );
+
 /**
  * \brief          Initialises and fills the message digest context structure
  *                 with the appropriate values.
  *
+ * \note           Currently also clears structure. In future versions you
+ *                 will be required to call md_init() on the structure
+ *                 first.
+ *
  * \param ctx      context to initialise. May not be NULL. The
  *                 digest-specific context (ctx->md_ctx) must be NULL. It will
  *                 be allocated, and must be freed using md_free_ctx() later.
@@ -191,10 +207,11 @@ int md_init_ctx( md_context_t *ctx, const md_info_t *md_info );
  * \brief          Free the message-specific context of ctx. Freeing ctx itself
  *                 remains the responsibility of the caller.
  *
+ * \note           Deprecated: Redirects to md_free()
+ *
  * \param ctx      Free the message-specific context
  *
- * \returns        0 on success, POLARSSL_ERR_MD_BAD_INPUT_DATA if parameter
- *                 verification fails.
+ * \returns        0
  */
 int md_free_ctx( md_context_t *ctx );