The safe contains multiple rewards packages that first need to be unlock by completing certain requirements (e.g. use X energy points or spend X amount of gems).
After all the rewards are unlocked, the safe key can be obtained via certain methods:
1. Purchase the safe key
2. Collect rare key drops from daily rewards (then need to be merged to get the final key)
3. Complete missions that reward players with key drops (also need to be merged to get the finally key)