Everyone in Greece will need to sign an authorization at some point. You need it in order to authorize someone to act on your behalf when issuing documents without your presence or connecting your utilities to your new home or so many more actions.
Until now, you could visit a police station, citizens center or administrative authority and verify your signature so the authorization would be ready for use.
Police stations do not do this any more. According to the latest amendment of Law, the verification of signature specimens will now only take place at administrative authorities or any KEP.
Let us know if you need any help!