Scientists are this world’s wizards

The repository included the private portion of the platform key in encrypted form. The encrypted file, however, was protected by a four-character password,

How do you fuck up this badly?

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