I did mine last weekend . . .
its a normal 32mm nut - that has been done up with an Airgun - needs to be "cracked" before it moves.
190nM is the setting - ferkin tight!
There is obviously a tab washer there as well.
A big bar and a second pair of hands helps - however you will be hard pushed to get the chain/sprocket off without cutting the chain first - I did a new chain and front sprocket (down to 14 teeth now at the front) last Saturday.
Makes a hell of a difference.
Khushy
Threaded Mode