[Comm-en] Location of ALT Sisyphus GPG keys?

[David]

I've just finished installing the TDE build of ALT (from 2017) and I would like to try to do an apt-get update on the system.  However, when I try to do this, the response is that I do not have the appropriate GPG keys for Sisyphus in my keyring and the update fails.

Can anyone direct me as to where the GPG keys for Sisyphus i586 Classic are located and how I can get them appended to Apt?

David.