Hardcore missions/operations chests, called 'Medium ARES Chest', requires 2 Security Keys to be opened;
Raid chests, called 'Large ARES Chest', requires 5 Security Keys to be opened;
Any squad member may use as many keys as he'd like. Once all keys are inserted, everyone still in the instance gets the reward;
Opening a Raid's Chest will also grant an extra Titan Token. The weekly limit does not apply to this, so while you'll only get 1 normal reward and Titan Token per week, you can get as many Titan Tokens as you'd like by completing Raids and opening it's chests.