When using symmetric key cryptography for securing communication among a large number of nodes, provisioning and management of pair-wise key is often infeasible. As a convenient solution, a system administrator often decides to use a single group key that is shared among all authorized users (or devices) in the system to protect the communication among authorized members. Please list two (or more) security drawbacks of group-key-based system, compared to a system using pair-wise shared secret keys.