GPG error - no public key available when updating package list - new install

Hello all I'm completely new to Linux, but love the philosophy so I have decided to give this a go. I am getting an error message when using Synaptic to reload my package list. I have the following repositories enabled: 64studio - main etch - main contrib non free etch/updates - main (cd rom) 64 studio - main When I press reload it starts to reload the package list and then the following error occurs: W: GPG error: http://ftp.uk.debian.org etch Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 9AA38DCD55BE302B W: There is no public key available for the following key IDs: 9AA38DCD55BE302B I would be very grateful for any help on this. Thanks Bruce