tlmgr bad signature

Thank you for your comment! The reason why I was using www.ctan.org, was because when I began managing my TeX system with tlmgr it worked without specifying a mirror. When I specify a mirror, for example:

sudo tlmgr info tcolorbox --repository http://ftp.fau.de/ctan/systems/texlive/tlnet/

it works without problems. I still don't know the problem, but it's solved!