Replay In a replay attack, an attacker captures network traffic and then replays (or retransmits) the captured traffic at a later time, in order to gain unauthorized access to a system. This type of attack may succeed in spite of encryption because even though the messages may be encrypted, and the attacker may not know what the actual keys and passwords are, the retransmission of valid logon messages may be sufficient to gain access to the network. This is the reason most certificates contain unique session identifiers and time stamps. Packet sequencing, time stamps, digital signatures and session tokens (or hash) are countermeasures used against replay attacks: Packet sequencing ensures that any packet received that is not in the proper order is dropped. Time stamps ensure that any packet received outside a specified time window is dropped. A session token is a one-time token or hash used to computationally transform a message such that it cannot be duplicated wi