Having added my user (pi) to the debian-tor group, I try to run 'arm' and get the following:

Failed to read authentication cookie (permission denied): /var/run/tor/control.authcookie

I've tried reinstalling arm, relogged and checked that I'm in the right group still using 'groups' (which I am) - no joy.

I've checked my torrc and found the following lines:

CookieAuthentication wasn't initially enabled, turning it on makes no difference, I get the same error. 

I am able to get it working when setting up HashedControlPassword by generating a has using tor --hash-password <password> and editing torrc accordingly.

Am I missing something when it comes to setting up CookieAuthentication? I guess I must be...although it seems strange that I was getting that error even with CookieAuthentication disabled.

Any insight appreciated :)

Chris