What is a replay attack in the context of network security?

A replay attack is a form of network security breach where an attacker captures and then retransmits valid data transmissions—often credentials or session tokens—to gain unauthorized access to a system or service. This is typically done without any alteration to the original data, which makes it seem legitimate to the receiving end. In essence, the attacker is "replaying" the captured data at a later time, thus deceiving the system into believing it is receiving a valid request from an authorized user.

In contrast, the other options provided do not accurately capture the essence of a replay attack. For example, methods of encrypting communications between two endpoints focus on securing data rather than exploiting it; techniques to bypass firewall restrictions concern circumventing security measures rather than reusing valid information; and processes to refresh data packets during transmission relate to ensuring data integrity rather than replaying old data. Understanding the nature of replay attacks is crucial for implementing effective security measures such as using timestamps, unique session tokens, or nonces (numbers used once) to defend against these types of exploits.