Good day. I think after $ sudo apt-get update i have got a message on $ sudo -t stable apt-get upgrade stating that some packages can not be authenticated. Before - all was all right: i was updating and upgrading - no problems, but now I have the problem. I have tried $ sudo apt-get -t stable install debian-archive-keyring it states that it is up to date. $ sudo apt-key update no any key changed. How I can fix it? Thank You for Your time.