"no public key available" on apt-get update

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

I recommend that you follow @mariotomo's answer rather than the steps below!

According to There is no public key available for the following key ID, this will fix it:

sudo aptitude install debian-archive-keyring

I recommend:

$ sudo apt-get install debian-archive-keyring
$ sudo apt-key update

This is better than other approaches because it does not install debian-keyring, which is big and 99% of the time unnecessary.