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When updating the Debian based system, it may happens that the apt-get displays an error message like:

W: GPG error: ftp://ftp.debian.org/ testing Release: 
The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 010908312D230C5F

W: There is no public key available for the following key IDs:
010908312D230C5F

This is a new feature of the apt-get system that guarantee the authenticity of servers for updating Debian.

Solution:
Simply type the following commands, taking care to replace the number of the key that displayed in the error message:

gpg --keyserver pgpkeys.mit.edu --recv-key  010908312D230C5F
gpg -a --export 010908312D230C5F | sudo apt-key add -