sxt: added key export API description, some misc functions added as well;

include/sxt/sxtkey.h

@@ -81,8 +81,39 @@ void sxt_key_free(sxtkey_t *);
  */
 int sxt_key_generate(sxtkey_t *, int , int );
 
+/* generic functions useful to deal with keys */
+/* get 8bit ID from key name */
+uint8_t sxt_key_type_fname(const char *);
+
+/* return 0 if key isn't public, 1 otherwise */
+int sxt_key_public(const sxtkey_t *);
+/* return 0 if key isn't private, 1 otherwise */
+int sxt_key_private(const sxtkey_t *);
+
+/* export key API */
+/** this function will pack the private key to sxt key container and
+ * write it to the file.
+ * @key - private key
+ * @file - file name to write
+ * NOTE: the following parameters required in case if you want to encrypt the key
+ * with passkey (password)
+ * @passkey - plaintext passkey
+ * or
+ * @ask_passkey with:
+ *  @@pkbuf - passkey buffer (allocated)
+ *  @@length - length of pkbuf
+ *  @@confirm - if not nil - function should force passkey confirmation
+ *  @@priv - some custom pointer to the some custom data
+ * @priv - the data coming to ask_passkey function
+ */
+int sxt_key_export_priv_file(const sxtkey_t *key, const char *file, const char *passkey,
+                             int (*ask_passkey)(char *pkbuf, size_t length, int confirm, void *priv),
+                             void *priv);
+
 /* key custom hash ops */
+/* assign a 64bit hash, 0 mean there are no hash */
 int sxt_key_assign_hash(sxtkey_t *, uint64_t);
+/* get a 64bit hash from a key */
 uint64_t sxt_key_hash(const sxtkey_t *);
 
 #endif /* __SXT_SXTKEY_H__ */