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Manjaro and Arch users have a security keyring on their machine that allows them to safely download packages from the Manjaro and Arch repositories without those packages getting tampered with by a man-in-the-middle-attack.
Sometimes if a user doesn’t update their system for a long time, this keyring might expire. The keyring can also get corrupted for some reason.
To solve basic keyring issues do the following:
Resynchronise with the Manjaro repository servers to ensure that everything is up to date - by entering the command:
sudo pacman -Syy
Refresh and update the signature keys by entering the command:
sudo pacman-key --refresh-keys
Reload the signature keys by entering the command:
sudo pacman-key --populate archlinux manjaro
The three commands above should solve most (basic) keyring issues.
If they don’t, then you can also try more drastic measures.
Warning: The following commands only work if your system time is set correctly!
Remove all keys
sudo rm -rf /etc/pacman.d/gnupg
Reinitialize files & folders for keys
sudo pacman-key --init
Repopulate keys
sudo pacman-key --populate archlinux manjaro
Reinstall latest keyrings
sudo pacman -Sy gnupg archlinux-keyring manjaro-keyring
Refresh the signature keys
sudo pacman-key --refresh-keys
Clear out the software packages downloaded during aborted installations (optional):
sudo pacman -Sc
Warning: The above command clears the pacman cache completely, and you will not be able to downgrade to a previous version of a package until you’ve updated the packages at least two times.
Finally, try updating your system to see if the errors were resolved.
sudo pacman -Syu
Imported & adapted from @Heart-Of-A-Lion’s and @arvind’s Posts on the archived forum.
Creating this as a wiki post so that experts can make the necessary changes 
