apt-get signature

[aschuetz]

Dear all,

since a few weaks

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apt-get update

yields the following error:

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W: GPG error: http://old-releases.ubuntu.com raring Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 40976EAF437D05B5 NO_PUBKEY 3B4FE6ACC0B21F32
redpitaya>

I was wondering that this comes from an Ubuntu repository. What is the best possible solution?

Best,
Alex