There is implicit cost as well as "initial cost" and "renewalcost" in the severance of core employees in enterprises.
企业核心员工的离职除了“初始成本”和“更替成本”，还存在“隐性成本”。
2
Proper key renewal period might control the trade-off between security and renewalcost in key management, and thus key exposure was first studied to estimate key life.