I've recently tried to update my install of linux mint MATE 17.3 but have had a warning about being unable to authenticate updates, hence it can't be proven that they are from a trusted source. See the screenshot for the error and a list of the updates I was trying to apply at the time. I have not installed these updates and assume I should not do so until I can authenticate them. I had tried checking for updates a week or so ago, but got this error then, I didn't install them then and resolved to wait a while and see if, when I tried to update later, authentication would work, now is later and it hasn't started working again. Please can anyone advise on how to get authentication working again so I can safely update linux.
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