[arch-general] mksh-52.1-1-x86_64.pkg.tar.xz is corrupted (invalid or corrupted package (PGP signature)?

[David C. Rankin]

Attempting update today, I received:

error: mksh: signature from "Thorsten Töpper <........ at freethoughts.de>" is
unknown trust
:: File /var/cache/pacman/pkg/mksh-52.1-1-x86_64.pkg.tar.xz is corrupted
(invalid or corrupted package (PGP signature)).
Do you want to delete it? [Y/n] n
error: failed to commit transaction (invalid or corrupted package)
Errors occurred, no packages were upgraded.

Is this a know issue? How to work around (correctly)?

-- 
David C. Rankin, J.D.,P.E.