Then maybe the downloaded packages are actually corrupted. You could check if they have plausible file sizes. IIRC pacman will ask you if you want to delete the non-matching files but I’m not entirely sure. They should end up in /var/cache/pacman/pkg.

Update only archlinux-keyring and try again.

# pacman -S archlinux-keyring
# pacman -Syu

In some cases you may need to re-populate the keyring.

# pacman-key --init
# pacman-key --populate

Adding to this: individual package updates are not supported. Always update your whole system, unless you know what you are doing.

I know it sounds dumb, but lying still and thinking the same word over and over seems to help me if nothing else does the trick. If you notice yourself thinking something different just get back to it.