After the latest updates I got a message from Synaptic to the effect that I was missing a public key. My research yielded the following remedy:
gpg --keyserver --recv-key  A8AA1FAA3F055C03
But when I tried it, I got this:
gpg: requesting key 3F055C03 from hkp server
gpgkeys: HTTP fetch error 6: Couldn't resolve host ''
gpg: no valid OpenPGP data found.
gpg: Total number processed: 0
(Same result with I encountered this problem a couple of years ago and found a solution, but I no longer remember what it was. Can anyone help?

The answer to this belongs in some FAQ or other.