Apt trusted.gpg file problems

[Miltiades Vasiliades <miltosv@outlook.com.gr>]

I tried switching the mirrors through synaptic then synaptic as usual asked to refresh however the refresh ended with the following errors

W: http://repo.steampowered.com/steam/dists/precise/InRelease: The key(s) in the keyring /etc/apt/trusted.gpg are ignored as the file is not readable by user '_apt' executing apt-key.
W: http://security.debian.org/…/dists/stretch/updates/InRelease: The key(s) in the keyring /etc/apt/trusted.gpg are ignored as the file is not readable by user '_apt' executing apt-key.
W: http://ftp.de.debian.org/debian/dists/stretch/InRelease: The key(s) in the keyring /etc/apt/trusted.gpg are ignored as the file is not readable by user '_apt' executing apt-key.

Any ideas how to resolve that issue?

Thank you!