All Chests when opened give items based on luck. Sometimes items which come up are not of great value to the Player, which is where Reroll Keys come into play, which are used to reshuffle the contents of the Chests.
Currently following are know ReRoll Keys costs for Chests:
Free Chest: 1 ReRoll Key
Daily Contract Case: 1 ReRoll Key
Red Ad Chest: 1 ReRoll Key
Gold Chest: 2 ReRoll Keys
Pink Ad Chest: 3 ReRoll Keys
Clan Chests: 1-2 Reroll Keys (WIP)
Steel Chest: 1 ReRoll Key
Bronze Chest:
Silver Chest:
Gold Chest:
Legendary Chest: 8 ReRoll Key
When you open Chest, you will get option of ReRolling contents with displayed cost required. ReRoll will consume Keys as per Cost. There are maximum of 3 ReRolls for each Chest.