When performing apt-get update, I get the following error: root@ADS3-Debian6:/home/aluno# apt-get updateAtingido http://sft.if.usp.br squeeze Release.gpgIgn http://sft.if.usp.br/debian/ squeeze/contrib Translation-enIgn http://sft.if.usp.br/debian/ squeeze/contrib Translation-ptIgn http://sft.if.usp.br/debian/ squeeze/contrib Translation-pt_BR(...)Obter:10 http://security.debian.org squeeze/updates/non-free i386 Packages [14 B]Baixados 612 kB em 4s (125 kB/s)                    Lendo listas de pacotes... ProntoThere is no public key available for the following key IDs: 8B48AD6246925553
no public key available on apt-get update
debian
the other answers will work, or not, depending on whether or not the key '8B48AD6246925553' is present in the packages they indicate.if you need a key, you have to get that key, and where to find it, it's in a key server (very probably any key server will do):sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 8B48AD6246925553