Errors

valid_signature
gpg: Signature made Thu 09 May 2019 11:24:13 PM JST
gpg:                using RSA key F726AA58136E97D97B4C41F06C94FD7F46B7F3C8
gpg: requesting key 6C94FD7F46B7F3C8 from hkp server keyserver.ubuntu.com
gpg: Good signature from "Michael Kröll <michael.kroell@geizhals.at>" [unknown]
gpg:                 aka "Michael Kröll <michael.kroell@uibk.ac.at>" [unknown]
gpg:                 aka "Michael Kröll <michael.kroell@gmail.com>" [unknown]
gpg: error getting main key 1C38FB1C576C4B45 of subkey 6C94FD7F46B7F3C8: No public key
gpg: WARNING: This key is not certified with a trusted signature!
gpg:          There is no indication that the signature belongs to the owner.
Primary key fingerprint: FF14 3D8E 6CC5 AD6E 39B2  4498 40CB 0F47 8A5F 913A
     Subkey fingerprint: F726 AA58 136E 97D9 7B4C  41F0 6C94 FD7F 46B7 F3C8
==> BAD/TAMPERED signature detected! <==