How to add a third-party repo. and key in Debian?

I looked at another similar question about adding third-party repos. I am trying to add a third-party desktop IM client called riot . While the site gives link to the third-party it gives no instructions as how to add third-party sources or keyring in Debian. I went through https://riot.im/packages/debian/pool/main/ and made the following additions in my /etc/apt/sources.list –

torbrowser signature verification fails – a glitch or an “attack”?

I wanted to try using TOR on my new Linux Mint 18.1 installation. So I apt-get installed torbrowser-launcher and tor, then ran torbrowser-launcher. It opened a dialog box and showed me it was downloading the TOR browser; but when it was done, it said it had failed the signature check and that I may be “under attack” (oh my!).