Error getting access to LLVM Debian/Ubuntu nightly packages

Each one of these repositories has it's own key which you should add before updating your lists. See https://apt.llvm.org/

E.g: if you add this one to your sources:

deb http://apt.llvm.org/trusty/ llvm-toolchain-trusty main

Then you should also add the related key to this component, which is:

wget -O - https://apt.llvm.org/llvm-snapshot.gpg.key|sudo apt-key add -

So, if you add all lines for trusty, then you should add all three related keys to them, It will make your error goes away.

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For stable brunch (V3), source:

 deb http://apt.llvm.org/trusty/ llvm-toolchain-trusty-3.9 main

 wget -O - https://apt.llvm.org/llvm-snapshot.gpg.key|sudo apt-key add -

For qualification branch (V4), source:

deb http://apt.llvm.org/trusty/ llvm-toolchain-trusty-4.0 main

wget -O - https://apt.llvm.org/llvm-snapshot.gpg.key|sudo apt-key add -

I was getting the same error. I fixed it by installing launchpad-getkeys:

sudo add-apt-repository ppa:nilarimogard/webupd8
sudo apt-get update
sudo apt-get install launchpad-getkeys

Once it's installed, run:

sudo launchpad-getkeys
sudo apt-get update

Your key errors should be gone.