Recently tried to update emacs packages and got this.
Failed to verify signature archive-contents.sig:
No public key for 066DAFCB81E42C40 created at 2019-10-02T10:10:02+0100 using RSA
Command output:
gpg: Signature made Wed 02 Oct 2019 10:10:02 AM BST
gpg: using RSA key C433554766D3DDC64221BFAA066DAFCB81E42C40
gpg: Can't check signature: No public key
Any ideas why?
See https://emacs.stackexchange.com/questions/233/ for the same question.
The signature check failed because you don't have the new key (the old signature key expired on Sep 23). The new key is available from the usual GPG key-servers, comes with Emacs≥26.3, and can also be obtained by installing the package gnu-elpa-keyring-update.
Try renaming your ~/.emacs.d/elpa/gnupg file to something else as a backup and then run M-x package-refresh-contents. This is what worked for me.