Salut
après avoir fait un apt-get update qui se passe bien je fais un apt-get upgrade et après avoir dis Y pour la mise à jour j'ai ça et puis plus rien :

Checking GPG signatures...
Unknown signature /var/cache/apt/archives/aalib_1.4.0-5.1.fc3.rf_i386.rpm: (SHA1) DSA sha1 md5 (GPG) NOT OK (MISSING KEYS: GPG#6b8d79e6)
Unknown signature /var/cache/apt/archives/wxGTK_2.4.2-5.1.fc3.rf_i386.rpm: (SHA1) DSA sha1 md5 (GPG) NOT OK (MISSING KEYS: GPG#6b8d79e6)
Unknown signature /var/cache/apt/archives/synaptic_0.55.3-1.1.fc3.rf_i386.rpm: (SHA1) DSA sha1 md5 (GPG) NOT OK (MISSING KEYS: GPG#6b8d79e6)
Unknown signature /var/cache/apt/archives/apt_0.5.15cnc6-4.1.fc3.rf_i386.rpm: (SHA1) DSA sha1 md5 (GPG) NOT OK (MISSING KEYS: GPG#6b8d79e6)
Unknown signature /var/cache/apt/archives/faad2_2.0-2.20040923.1.fc3.rf_i386.rpm: (SHA1) DSA sha1 md5 (GPG) NOT OK (MISSING KEYS: GPG#6b8d79e6)
Unknown signature /var/cache/apt/archives/id3lib_3.8.3-7.1.fc3.rf_i386.rpm: (SHA1) DSA sha1 md5 (GPG) NOT OK (MISSING KEYS: GPG#6b8d79e6)
Unknown signature /var/cache/apt/archives/imlib2_1.1.2-2.1.fc3.rf_i386.rpm: (SHA1) DSA sha1 md5 (GPG) NOT OK (MISSING KEYS: GPG#6b8d79e6)
Unknown signature /var/cache/apt/archives/lame_3.96.1-2.1.fc3.rf_i386.rpm: (SHA1) DSA sha1 md5 (GPG) NOT OK (MISSING KEYS: GPG#6b8d79e6)
Unknown signature /var/cache/apt/archives/libdvdcss_1.2.8-4.1.fc3.rf_i386.rpm: (SHA1) DSA sha1 md5 (GPG) NOT OK (MISSING KEYS: GPG#6b8d79e6)
Unknown signature /var/cache/apt/archives/libdvdread_0.9.4-5.1.fc3.rf_i386.rpm: (SHA1) DSA sha1 md5 (GPG) NOT OK (MISSING KEYS: GPG#6b8d79e6)
Unknown signature /var/cache/apt/archives/libmad_0.15.1b-3.1.fc3.rf_i386.rpm: (SHA1) DSA sha1 md5 (GPG) NOT OK (MISSING KEYS: GPG#6b8d79e6)
Unknown signature /var/cache/apt/archives/lzo_1.08-3.1.fc3.rf_i386.rpm: (SHA1) DSA sha1 md5 (GPG) NOT OK (MISSING KEYS: GPG#6b8d79e6)
Unknown signature /var/cache/apt/archives/mplayer-fonts_1.1-2.0.rf_noarch.rpm: (SHA1) DSA sha1 md5 (GPG) NOT OK (MISSING KEYS: GPG#6b8d79e6)
Unknown signature /var/cache/apt/archives/syslinux_3.06-1.1.fc3.rf_i386.rpm: (SHA1) DSA sha1 md5 (GPG) NOT OK (MISSING KEYS: GPG#6b8d79e6)
Unknown signature /var/cache/apt/archives/xine_0.99.3-2.1.fc3.rf_i386.rpm: (SHA1) DSA sha1 md5 (GPG) NOT OK (MISSING KEYS: GPG#6b8d79e6)
Unknown signature /var/cache/apt/archives/xmms_1%3a1.2.10-9.2.1.fc3.rf_i386.rpm: (SHA1) DSA sha1 md5 (GPG) NOT OK (MISSING KEYS: GPG#6b8d79e6)
Unknown signature /var/cache/apt/archives/xvidcore_1.0.2-2.1.fc3.rf_i386.rpm: (SHA1) DSA sha1 md5 (GPG) NOT OK (MISSING KEYS: GPG#6b8d79e6)
Unknown signature /var/cache/apt/archives/lftp_3.0.12-1.1.fc3.rf_i386.rpm: (SHA1) DSA sha1 md5 (GPG) NOT OK (MISSING KEYS: GPG#6b8d79e6)
Unknown signature /var/cache/apt/archives/mtr_2%3a0.65-1.1.fc3.rf_i386.rpm: (SHA1) DSA sha1 md5 (GPG) NOT OK (MISSING KEYS: GPG#6b8d79e6)
Unknown signature /var/cache/apt/archives/rsync_2.6.3-1.1.fc3.rf_i386.rpm: (SHA1) DSA sha1 md5 (GPG) NOT OK (MISSING KEYS: GPG#6b8d79e6)
Unknown signature /var/cache/apt/archives/xmms-mp3_1%3a1.2.10-9.2.1.fc3.rf_i386.rpm: (SHA1) DSA sha1 md5 (GPG) NOT OK (MISSING KEYS: GPG#6b8d79e6)
Unknown signature /var/cache/apt/archives/xmms-skins_1%3a1.2.10-9.2.1.fc3.rf_i386.rpm: (SHA1) DSA sha1 md5 (GPG) NOT OK (MISSING KEYS: GPG#6b8d79e6)
E: Error(s) while checking package signatures:
0 unsigned package(s)
22 package(s) with unknown signatures
0 package(s) with illegal/corrupted signatures


que faire ?
voici la réponse du moins je l'espère avec la faq non officielle de fedora :
http://perso.wanadoo.fr/fester/faq-fedora.html#gpgsig
sinon, je ne saurai que trop te conseiller d'aller jeter un oeil sur le tuto pour yum/apt sur ce site, car il te permettra de mettre à jour ton fichier de sources disponible et correct