There is no public key available for the following key IDs

Mengatasi masalah public key saat melakukan update paket di debian

root@risetpro:/etc/apt# apt-get update
Hit http://kambing.ui.ac.id squeeze Release.gpg
Hit http://kambing.ui.ac.id squeeze-updates Release.gpg
Hit http://kambing.ui.ac.id squeeze/updates Release.gpg
Hit http://kambing.ui.ac.id squeeze Release
Hit http://kambing.ui.ac.id squeeze-updates Release
Hit http://kambing.ui.ac.id squeeze/updates Release
Hit http://kambing.ui.ac.id squeeze/main amd64 Packages
Hit http://kambing.ui.ac.id squeeze/contrib amd64 Packages
Hit http://kambing.ui.ac.id squeeze/non-free amd64 Packages
Hit http://kambing.ui.ac.id squeeze-updates/main amd64 Packages
Hit http://kambing.ui.ac.id squeeze-updates/contrib amd64 Packages
Hit http://kambing.ui.ac.id squeeze-updates/non-free amd64 Packages
Hit http://kambing.ui.ac.id squeeze/updates/main amd64 Packages
Hit http://kambing.ui.ac.id squeeze/updates/contrib amd64 Packages
Hit http://kambing.ui.ac.id squeeze/updates/non-free amd64 Packages
Ign http://kambing.ui.ac.id squeeze-updates/contrib Translation-en_US
Ign http://kambing.ui.ac.id squeeze-updates/contrib Translation-en
Ign http://kambing.ui.ac.id squeeze-updates/main Translation-en_US
Ign http://kambing.ui.ac.id squeeze-updates/main Translation-en
Ign http://kambing.ui.ac.id squeeze-updates/non-free Translation-en_US
Ign http://kambing.ui.ac.id squeeze-updates/non-free Translation-en
Ign http://kambing.ui.ac.id squeeze/updates/contrib Translation-en_US
Ign http://kambing.ui.ac.id squeeze/updates/contrib Translation-en
Ign http://kambing.ui.ac.id squeeze/updates/main Translation-en_US
Ign http://kambing.ui.ac.id squeeze/updates/main Translation-en
Ign http://kambing.ui.ac.id squeeze/updates/non-free Translation-en_US
Ign http://kambing.ui.ac.id squeeze/updates/non-free Translation-en
Hit http://security.debian.org wheezy/updates Release.gpg
Hit http://security.debian.org wheezy/updates Release
Hit http://security.debian.org wheezy/updates/main Sources
Hit http://security.debian.org wheezy/updates/contrib Sources
Hit http://security.debian.org wheezy/updates/main amd64 Packages
Hit http://security.debian.org wheezy/updates/contrib amd64 Packages
Hit http://security.debian.org wheezy/updates/contrib Translation-en
Hit http://security.debian.org wheezy/updates/main Translation-en
Reading package lists... Done
W: There is no public key available for the following key IDs:
9D6D8F6BC857C906

Jalankan :

root@risetpro:/etc/apt# apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 9D6D8F6BC857C906
Executing: gpg --ignore-time-conflict --no-options --no-default-keyring --secret-keyring /tmp/tmp.7bnhJrNQAh --trustdb-name /etc/apt//trustdb.gpg --keyring /etc/apt/trusted.gpg --primary-keyring /etc/apt/trusted.gpg --keyring /etc/apt/trusted.gpg.d//debian-archive-squeeze-automatic.gpg --keyring /etc/apt/trusted.gpg.d//debian-archive-squeeze-stable.gpg --keyring /etc/apt/trusted.gpg.d//debian-archive-wheezy-automatic.gpg --keyring /etc/apt/trusted.gpg.d//debian-archive-wheezy-stable.gpg --keyserver keyserver.ubuntu.com --recv-keys 9D6D8F6BC857C906
gpg: requesting key C857C906 from hkp server keyserver.ubuntu.com
gpg: key C857C906: public key "Debian Security Archive Automatic Signing Key (8/jessie) " imported
gpg: no ultimately trusted keys found
gpg: Total number processed: 1
gpg:               imported: 1  (RSA: 1)

Sekarang coba lagi :

root@risetpro:/etc/apt# apt-get update
Hit http://security.debian.org wheezy/updates Release.gpg
Hit http://security.debian.org wheezy/updates Release
Hit http://security.debian.org wheezy/updates/main Sources
Hit http://security.debian.org wheezy/updates/contrib Sources
Hit http://security.debian.org wheezy/updates/main amd64 Packages
Hit http://security.debian.org wheezy/updates/contrib amd64 Packages
Hit http://security.debian.org wheezy/updates/contrib Translation-en
Hit http://security.debian.org wheezy/updates/main Translation-en
Hit http://kambing.ui.ac.id squeeze Release.gpg
Hit http://kambing.ui.ac.id squeeze-updates Release.gpg
Hit http://kambing.ui.ac.id squeeze/updates Release.gpg
Hit http://kambing.ui.ac.id squeeze Release
Hit http://kambing.ui.ac.id squeeze-updates Release
Hit http://kambing.ui.ac.id squeeze/updates Release
Hit http://kambing.ui.ac.id squeeze/main amd64 Packages
Hit http://kambing.ui.ac.id squeeze/contrib amd64 Packages
Hit http://kambing.ui.ac.id squeeze/non-free amd64 Packages
Hit http://kambing.ui.ac.id squeeze-updates/main amd64 Packages
Hit http://kambing.ui.ac.id squeeze-updates/contrib amd64 Packages
Hit http://kambing.ui.ac.id squeeze-updates/non-free amd64 Packages
Hit http://kambing.ui.ac.id squeeze/updates/main amd64 Packages
Hit http://kambing.ui.ac.id squeeze/updates/contrib amd64 Packages
Hit http://kambing.ui.ac.id squeeze/updates/non-free amd64 Packages
Ign http://kambing.ui.ac.id squeeze-updates/contrib Translation-en_US
Ign http://kambing.ui.ac.id squeeze-updates/contrib Translation-en
Ign http://kambing.ui.ac.id squeeze-updates/main Translation-en_US
Ign http://kambing.ui.ac.id squeeze-updates/main Translation-en
Ign http://kambing.ui.ac.id squeeze-updates/non-free Translation-en_US
Ign http://kambing.ui.ac.id squeeze-updates/non-free Translation-en
Ign http://kambing.ui.ac.id squeeze/updates/contrib Translation-en_US
Ign http://kambing.ui.ac.id squeeze/updates/contrib Translation-en
Ign http://kambing.ui.ac.id squeeze/updates/main Translation-en_US
Ign http://kambing.ui.ac.id squeeze/updates/main Translation-en
Ign http://kambing.ui.ac.id squeeze/updates/non-free Translation-en_US
Ign http://kambing.ui.ac.id squeeze/updates/non-free Translation-en
Reading package lists... Done

Sebenernya bisa juga dengan meng-edit reponya ke kambing seperti link di bawah ini.

Sumber :
http://unix.stackexchange.com/questions/75807/no-public-key-available-on-apt-get-update
https://www.idroot.com/mengubah-repository-debian-ke-repository-local-indonesia