Workaround if apt-get still works behind the proxy
- add sources manually to
/etc/apt/sources.list
- add gpg key
Adding sources manually
I think on launchpad.net every ppa still contains a small description how to add sources manually. The launchpad site for your mentioned ppa ppa:nilarimogard/webupd8 is https://launchpad.net/~nilarimogard/+archive/webupd8. If you scroll down you see an expandable label Technical details about this PPA. If you expand it you find the description how to add sources manually. Add the following lines to mentioned /etc/apt/sources.list
deb http://ppa.launchpad.net/nilarimogard/webupd8/ubuntu quantal main
deb-src http://ppa.launchpad.net/nilarimogard/webupd8/ubuntu quantal main
Of course you have to adjust quantal to whatever version you are currently using.
Adding the Signing Key
The description also contains a signing key. This is important, so that your system can always verify that you actually access a trustworthy ppa-address. In case of your ppa that is 1024R/4C9D234C(can also be found under Technical details about this PPA), where the number behind the slash is important.
You can add the fingerprint via the apt-key program. Typically you would perform the following command
sudo apt-key adv --recv-keys --keyserver keyserver.ubuntu.com 4C9D234C
Adding if apt-key doesn't work through proxy
Since you already had problems with the add-apt-repository program this might not work either. So instead you can download and add the 1024 Bit key manually. If wget works you can do that in one step.
wget "http://keyserver.ubuntu.com:11371/pks/lookup?op=get&search=0x531EE72F4C9D234C" -O out && sudo apt-key add out && rm out
Otherwise safe "http://keyserver.ubuntu.com:11371/pks/lookup?op=get&search=0x531EE72F4C9D234C" in e.g. /path/key and use sudo apt-key add /path/key to add it.
Close with the usual
Afterwards you have to update the repository information apt-get update and then you should be able to download the packages.
Resources
My personal favorite on how to use the packaging mechanism (sadly in German):
http://wiki.ubuntuusers.de/Paketquellen_freischalten/PPA
The launchpad version also mentions all the important points:
https://help.launchpad.net/Packaging/PPA/InstallingSoftware
Since it is untypical to use apt-key in the described way I only found the information in the man pages, man apt-key.
Related answer that pretty much describes the standard way to install ppa's manually:
https://askubuntu.com/a/38029/128349
There are a couple things you can try. First off, have you tried creating a new ethereum-ethereum-<version>.list file for ethereum in /etc/apt/sources.list.d/ manually? I doubt that will make any difference if it's truly a verifcation error, but if apt is just being screwy that could do it. The line you would want to put in that file is something like:
deb http://ppa.launchpad.net/ethereum/ethereum/ubuntu trusty main
Another thing you could try is, if you trust ethereum, you can pass in the allow-unauthenticated flag to apt-get, that may do it. This is NOT a particularly good idea.
apt-get --allow-unauthenticated add-apt-repository ppa:ethereum/ethereum
You could also try and download a certificate from ethereum, but a cursory glance at their blog suggests they're not too hot on signing authorities, so I don't know if that approach is viable.
Best Answer
The cert should be added by add-apt automatically. if you are sure that there is no typo in the PPA name, maybe try without the --update. I am using the syntax without --update and it worked fine. You could also check the normal url in a browser:
It should list the structure with the files. If not check that address. Using /etc/apt/sources.list you could also add the path manually, but then need to download the certificate and install it in /etc/apt/trusted.d folder.
CatMan