All Zip / DMG files state 'is damaged and can't be opened'

Hi Everyone,

I have just bought a new MacBook Pro and upgraded to macOS High Sierra. I have used Migration Assistant to transfer my files over from my old MacBook pro (running Sierra).

On my new MacBook every time I download a zip/dmg file when i try to open it it states //Filename// 'is damaged and can't be opened. you should move it to the trash.'

I have tried downloading several different files (Firefox, Spotify etc) and they all state the same thing. I have tried downloading and using Stuffit and The Unarchiver and the same thing happens.

Any help anyone could offer would be greatly appreciated.

Thanks.