internal static extern int indy_prep_anonymous_msg(int command_handle, string recipient_vk, byte[] msg_data, int msg_len, PrepareMsgCompletedDelegate cb);
internal static extern int indy_prep_msg(int command_handle, IntPtr wallet_handle, string sender_pk, string recipient_vk, byte[] msg_data, int msg_len, PrepareMsgCompletedDelegate cb);