sxt: duplication/convertation API desc. added;

include/sxt/sxtkey.h

@@ -90,6 +90,16 @@ int sxt_key_public(const sxtkey_t *);
 /* return 0 if key isn't private, 1 otherwise */
 int sxt_key_private(const sxtkey_t *);
 
+/* duplication/convertion API */
+/* will duplicate a key depends on it's kind */
+int sxt_key_dup(const sxtkey_t *, sxtkey_t **);
+
+/* will duplicate public key, if key was private or pair - it becomes public */
+int sxt_key_dup_public(const sxtkey_t *, sxtkey_t **);
+
+/* will duplicate private key, if key public error will returns */
+int sxt_key_dup_private(const sxtkey_t *, sxtkey_t **);
+
 /* export key API */
 /** this function will pack the private key to sxt key container and
  * write it to the file.