File 'repomd.xml' from repository is unsigned

For all those years I've been running SUSE-Linux, I've never bumped into this one while running a trivial zypper update:

Warning: File 'repomd.xml' from repository 'Update repository with updates from SUSE Linux Enterprise 15' is unsigned.

    Note: Signing data enables the recipient to verify that no modifications occurred
    after the data were signed. Accepting data with no, wrong or unknown signature can
    lead to a corrupted system and in extreme cases even to a system compromise.
    Note: File 'repomd.xml' is the repositories master index file. It ensures the
    integrity of the whole repo.

    Warning: We can't verify that no one meddled with this file, so it might not be
    trustworthy anymore! You should not continue unless you know it's safe.

continue? [yes/no] (no):

This error slash warning being weird and potentially dangerous, my obvious reaction was to hit ctrl-c and go investigate. First, my package verification mechanism should be intact and be able to verify if updates downloaded are unaltered or not. Second, there should have not been any breaking changes into my system, at least I didn't make any. As my system didn't seem to breached, I assumed a system malfunction and went investigating.

Quite soon, I learned this is less than rare event. It has happeded multiple times for other people. According to article Signature verification failed for file ‘repomd.xml’ from repository ‘openSUSE-Leap-42.2-Update’ there exists a simple fix.

By running two command zypper clean --all and zypper ref, the problem should dissolve.

Yes, that is the case. After a simple wash/clean/rinse -cycle zypper update worked again.
It was just weird to bump into that for the first time I'd assume this would have occurred some time earlier.