Arch Linux

Trying to install jdk8-openjdk leads to an invalid signature error, despite the time being accurate (using ntpd):

$ LANG=C sudo pacman -S jdk8-openjdk
resolving dependencies...
looking for conflicting packages...

Packages (4) java-environment-common-2-2 jre8-openjdk-8.u121-1 jre8-openjdk-headless-8.u121-1
jdk8-openjdk-8.u121-1

Total Download Size: 25.75 MiB
Total Installed Size: 133.09 MiB

:: Proceed with installation? [Y/n]
:: Retrieving packages...
jre8-openjdk-headless-8.u121... 25.8 MiB 3.96M/s 00:07 [################################] 100%
(4/4) checking keys in keyring [################################] 100%
(4/4) checking package integrity [################################] 100%
error: jre8-openjdk-headless: signature from "Guillaume ALAUX <guillaume@archlinux.org>" is invalid
:: File /var/cache/pacman/pkg/jre8-openjdk-headless-8.u121-1-x86_64.pkg.tar.xz is corrupted (invalid or corrupted package (PGP signature)).
Do you want to delete it? [Y/n]
error: failed to commit transaction (invalid or corrupted package)
Errors occurred, no packages were upgraded.