This is a new feature of the apt-get system that guarantees the authenticity of servers for updating Debian. 

Solution Simply type the following commands, taking care to replace the number below with that of the key that was displayed in the error message:  gpg --keyserver pgpkeys.mit.edu --recv-key  64AA94D00B849883       gpg -a --export 64AA94D00B849883 | sudo apt-key add -