Q. I’m trying to install a package using apt-get command. I’m getting warning message that read as follows:

Need to get 15.1MB of archives.
After unpacking 50.2MB of additional disk space will be used.
Do you want to continue [Y/n]? y
WARNING: The following packages cannot be authenticated!
libmono-sharpzip0.84-cil autopano-sift libpano12-0 libwxbase2.8-0 libwxgtk2.8-0 libpano12-bin libplot2c2 enblend
Install these packages without verification [y/N]?

How do I solve this warning related issue under Debian or Ubuntu Linux 7.10?

A. Just run the following command to get rid of this problem:

Open terminal

Type command:
$ sudo apt-get update

You must always install package using following format to get latest packages:
$ sudo apt-get update && sudo apt-get install {package-name}

apt-key utility

apt-key is used to manage the list of keys used by apt to authenticate packages. Packages which have been authenticated using these keys will be considered trusted. To update the local keyring with the keyring of Debian / Ubuntu archive keys and removes from the keyring the archive keys which are no longer valid.
$ sudo apt-key update
To list keys, enter:
$ sudo apt-key list
Output:

/etc/apt/trusted.gpg
--------------------
pub   1024D/437D05B5 2004-09-12
uid                  Ubuntu Archive Automatic Signing Key 
sub   2048g/79164387 2004-09-12

pub   1024D/FBB75451 2004-12-30
uid                  Ubuntu CD Image Automatic Signing Key 

pub   1024D/E23C5FC3 2007-03-15
uid                  Arnav Ghosh (Automatix Team Lead) 
sub   2048g/C2D84CF8 2007-03-15

pub   1024D/9072870B 2007-08-13 [expires: 2009-08-12]
uid                  jre-phoenix (moblock-deb maintainer) 
sub   1024g/1E5C7A1D 2007-08-13 [expires: 2009-08-12]

To delete a key i.e. rmove a key from the list of trusted keys., enter:
$ sudo apt-key del keyid
$ sudo apt-key remove 1E5C7A1D
Output:

OK

Updated for accuracy.